Post by volkerboehme on Aug 10, 2008 12:09:43 GMT -5
Realistic operation of propliners:
This keeps coming up so I need to start with the simple stuff.
The dynamics of jet engines and piston engines are not just dissimilar, they are totally different. What ATC would like the pilot to do is always the same anyway. They always want you to descend when they are good and ready, not a moment sooner, and then as steeply as possible. They are planning to manage your energy state in fine detail. Ignore ATC for the moment. I will get back to them.
Miles per gallon achieved in a jet depend on altitude. Any jet has double the fuel economy and therefore double the range at 41,000 feet. It must get up there as fast as possible, stay up there as long as possible and therefore plans to descend in a high drag, steep, power off, dive. It's not about money, any jet will run out of fuel as little as half way to destination if it flies too low or descends too soon. Regardless of the speed it flies at.
*For a jet late descent is a flight safety requirement*.
Piston engines have neither the benefit nor the problem. They achieve about the same fuel economy (range) at any altitude. Things are marginally worse below rated altitude or with a low speed supercharger blower selected. However even though fuel economy varies little the higher they fly the less air resistance they encounter, at constant MAP and rpm, and the higher the TAS (velocity) they achieve without any loss of range or economy of operation.
The time it takes a propliner to get from A to B depends on altitude, but unlike a jet the fuel burned does not. Piston engined aircraft are therefore inefficient for long range flying. However the only way to get from A to B in the minimum time in any aeroplane is to operate it at its operational ceiling. The operational ceiling depends on the current weight. You must climb to the initial operational ceiling and as weight reduces through the flight you step climb to new higher operational ceilings.
The practical definition of operational ceiling when using MSFS is the maximum level to which you can climb *using only climb MAP and rpm* without the VSI falling below 500ft/min *and* without the IAS falling below the mandated climb IAS.
During a short haul flight a propliner, (or bomber), may never reach operational ceiling, and will never achieve the cruising speed you see quoted in references. Cruising speed can only be achieved at operational ceiling.
In real life the navigator will instruct the pilot to fly at a level that maximises ground speed, not TAS. 4D navigation taking into account the wind vector is well beyond the scope of this post so it assumes nil wind throughout.
It might take a propliner 30+ minutes to reach its initial operational ceiling and more than 10 hours to reach the final cruising level in the flight plan after several step climbs.
Most MSFS users fail to understand that they will arrive at destination many hours later than necessary if they do not sustain operational ceiling throughout the flight.
The fuel used will not vary significantly with altitude at constant power. Piston engined aircraft can cruise slowly at low level without significant fuel penalty if required to do so. Jets cannot. Jets cannot adjust MAP at constant altitude.
The lower you fly the slower you fly in any aircraft. You are ramming more air molecules and they slow you down (a lot). Think about what a 34 KIAS wind, (called a gale), does to a flag or a tree at sea level. Every 34 KIAS you add to the ASI adds another gale of drag. The airspeed indicator (ASI) is just recording the number of molecules rammed per second, (collected in the pitot tube), and therefore displays your drag, not your velocity. Whenever you fly an aircraft you must work hard to maximise your velocity (KTAS) whilst restraining your drag (KIAS).
From the DC-6B handling notes.
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Econ Cruise (FL160 to FL225):
CALL for economy cruise power (about 1100hp)
COWL FLAPS - CLOSED
30 inches MAP
1800 RPM
Check CHT < 232C
Plan 370 USG per hour
Note - yields 258 KTAS at FL220 @ 83000lbs
*************
Econ cruise MAP and rpm delivers about 182 KIAS at any level but it depends on current weight and the weight of a DC-6B varies a lot over ten hours of flight. This is anyway a drag not a velocity. It is the most economical drag for cruising. The only altitude at which the drag and the velocity are equal is sea level. 182 KIAS = 182 KTAS at sea level but 182 KIAS = 258 KTAS at FL220.
At constant MAP the fuel burn per hour rises with altitude. The fuel burn per mile does not. Piston engined aircraft therefore have much more endurance at low level, but have about the same range at any level. Piston engined aircraft were retained for long patrol endurance long after they were abandoned for long range operation, but if you fly low you can only fly slow.
With piston engines fuel consumption per mile (range) is nearly constant versus altitude. It is the velocity at which you can traverse that range which differs (a lot).
You can fly 1820 miles at low level in a DC-6B and take ten hours, or you can do it at FL220 and take seven. Entirely your choice. You don't waste any fuel either way, but the airline does not pay you to arrive three hours late on every medium haul trip in a DC-6B. You are paid to fly the DC-6B with a drag of 182 KIAS at a velocity of 258 KTAS up at operational ceiling, not down at low level with a velocity of only 182 KTAS.
Nor are you paid to apply abusive power at low level to try to get the drag up to 258 KIAS. Abusive power forces an aircraft to fly noticeably nose down. Using the fuel to increase the drag (IAS) is not a substitute for using it to increase the velocity (TAS). All available excess power must be used to create climb power to reach the thinnest possible air.
The tail becomes stressed if you add too much drag. Vno is 251 KIAS. If you push the drag on the tail beyond 251 KIAS it will fatigue fail if you encounter turbulence. You must keep the drag below 251 KIAS or risk sudden death. To force the aircraft nose down enough in level flight to reach a drag of 251 KIAS you have to apply very abusive power and you will run out of fuel half way to destination because you used fuel to increase drag (IAS) not velocity (TAS). The only way to fly fast is to fly high. Sure you can fly lower than the operational ceiling, but you are wasting huge amounts of time. It will take many extra hours to complete even a medium haul.
Air molecules exert great drag on aeroplanes. Gale force upon gale force of drag. You must keep the IAS down and the TAS high by flying as high as possible in the thinnest possible air.
Anyone using a flight simulator needs to understand all that before they can use it realistically, but most MSFS users never quite grasp the difference between drag (IAS) and velocity (TAS). Consequently they end up trying to increase the wrong one, applying more and more power, at too low an altitude, achieving ever more nose down attitudes, as the gales of drag rise out of control due to the abusive power.
That extra power is only there so that you can climb into thinner air. It is not there to increase drag (IAS) at low level.
Now let's take a step nearer to realistic descent planning.
The optimum en route drag in a DC-6B at mid cruise weight is 182 KIAS. The resulting velocity (TAS) varies with altitude but you must use the optimum MAP and rpm anyway. The engine overhaul costs for piston engines outweigh consideration of fuel costs. You must operate the aircraft to minimise engine wear and overhaul costs, not to minimise fuel costs. That is why the handling notes tell you to apply a particular MAP and a particular rpm and do not tell you to fly at a particular drag (KIAS). The airframe and wing do indeed have one particular drag at which their efficiency maximises and miles per gallon maximise, but it is irrelevant. En route, fuel economy must be sacrificed to the needs of the hugely expensive, maintenance hungry, engines.
For jet aircraft with low maintenance, long life turbines the economics are reversed. If necessary they are cruised at variable power to achieve a Mach number target. Mach drag can be fearsome in more ways than one.
For a propliner when not en route, e.g. when holding (or loitering / patrolling in military aircraft), it may be appropriate to vary the MAP to sustain a target drag. From the DC-6B handling notes again.
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Holding:
FLAP - UP
COWL FLAPS - CRACKED
2000 RPM
Slowly REDUCE MAP
To obtain 160 KIAS
Check CHT < 232C
Plan 230 USG per hour
****************************
Obviously we reduce drag to hold, but en route, including en route descent, the engine settings are more important to economic operation than sustaining a particular drag.
However before I explain minimum cost descent profiles it is also necessary for anyone who hopes to use a flight simulator realistically to grasp the concept of energy state management. Unfortunately understanding energy state management of aircraft is tough. Fortunately in the real world it is only really a problem for air traffic controllers and combat pilots and not for commercial pilots.
Most MSFs users have never flown an aircraft, but have operated terrestrial vehicles. Everything ythey have ever 'learned' about terrestrial vehicles leads ythem to believe that any vehicle is easier to accelerate going downhill than going uphill. The whole point about aircraft, and the only reason airliners exist, is that aircraft are incredibly easy to accelerate when going uphill and almost impossible to accelerate when going downhill.
If that sounds unlikely then you are bound to be flying unrealistically.
It takes MSFS users, (and many pilots), a long time to understand that if a fighter pilot power dives his fighter from 250 KIAS at 40,000 feet to 400 KIAS at low level he has decelerated from about 500 KTAS to about 400 KTAS. As the pilot dives hard and watches the ASI needle proceed from 250 to 400 he is watching the drag rise, he hears the wind noise screaming ever louder as he decelerates a hundred knots in no time at all.
A drag of 400 KIAS at low level ensures that the fighter is much slower than it is with a drag of 250 KIAS at high level. Its just a lot more drag, so you hear much more wind noise. Wind noise isn't an indicator of velocity, it's just an indicator of drag. IAS isn't an indicator of velocity, it's just an indicator of drag.
Until you can grasp that IAS is drag and TAS is a velocity it is impossible to understand how to plan the climb and descent of aircraft. It is impossible to flight plan, and it is impossible to understand why you must follow a 4D flight plan.
Now we need to consider manoeuvrability.
Aircraft energy state = momentum = mass * TAS^2.
Energy state (momentum) has nothing to do with drag and hence nothing to do with IAS. Energy state controls manoeuvrability. Manoeuvrability depends on TAS and is nothing to do with IAS.
A jet airliner descending from FL410 to sea level at a constant drag of 250 KIAS halves its velocity, from around 500 KTAS to 250 KTAS, and consequently loses three quarters of its energy state. The fuel burn per hour will not change and so the range will halve. It must descend as late as possible or it may crash.
Since radius of turn depends inversely on energy state that same jet quarters its turn radius at any applied bank angle.
R = TAS^2 / G
In order to fly the approach procedures you must reduce your energy state. You must fly the turns at rate 1 (turn 180 degrees in 1 minute) = 3 deg /sec.
It is legal, (and just about safe in most weather), to allow a DC-6B to reach a drag of 250 KIAS, but at approach altitudes a drag of 250 KIAS is also a velocity of about 250 KTAS. You would have about double the turn radius that you would have at 182 KTAS and you would have to apply massive bank angles to fly the rate 1 turns of the approach and holding procedures. Your classic era passengers would hate you.
258 squared is twice as much as 182 squared. At any given bank angle the radius of turn is doubled at 258 KTAS. To fly the same radius at 258 KTAS as at 182 KTAS you have to double the G load. 251 KIAS is when the tail will fail at 1G, not 2G or 3G. Turning at Vno is not an option.
There is another problem. In a DC-6B you not only do not want to arrive near the initial approach fix at 250 KIAS, you don't even want to be at 182 KIAS. You want to be all the way back to a drag less than 160 KIAS so that you can extend FLAP 1, without ballooning the aircraft, and before you attempt the first rate 1 turn mandated by ATC, else you will need huge bank angles.
The AP in the DC-6B cannot fly approach procedures at velocities of more than 145 KTAS because it is limited, (in both real life and FS9), to applying 25 degrees of bank. If you want to use the AP during the approach you must get back to a drag of about 140 KIAS before you reach the initial approach fix, or (in the modern era only) ATC start to give you approach radar vectors to final. In other words about ten minutes (25 miles) before touchdown.
Forget the runway. You need to be down to a drag of 140 KIAS (usually a velocity of about 145 KTAS) 25 miles before the runway. Having grasped that the DC-6B must be down to a drag of 140 KIAS at least 25 miles from touchdown I can explain a minimum cost descent profile.
Because engine costs are paramount the minimum cost profile is an econ cruise power profile. When cleared for descent you set econ cruise MAP and rpm. Often they are set already. Just pitch the nose down to sustain a minimum of 500 VSI and perhaps as much as 700 VSI in practice. The drag (IAS) will of course rise in this power dive.
You must ensure that this technique does not exceed the drag on the tail associated with either Mno or Vno so this technique is generally unwise above FL170. From the DC-6B handling notes.
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Descent:
ABOVE FL170 DO NOT EXCEED Mno = 220 KIAS
DO NOT EXCEED Vno = 251 KIAS
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Neither Mno nor Vno is a target speed in a propliner. You should avoid them. Both are pretty much target speeds in a jet, and will be much higher too.
In a DC-6B you can allow the drag to rise to 220 KIAS initially and 251 KIAS later, so long as you do not turn (apply G), but you still need to reduce drag to 140 KIAS when you are 25 miles from the runway. If you fly this profile you will arrive at low level at a drag well in excess of 140 KIAS. You must therefore reach your cleared ATC level at least 40 miles from the runway leaving 15 miles to reduce from over 200 KIAS to 140 KIAS in level flight at your cleared level. In real life these are the 15 or so miles approaching the stack (initial approach) fix whether or not you are going to hold. You will average about 180 KIAS as you slow down in level flight, which is about three miles per minute at approach altitudes, allowing five minutes to bleed the drag from economic descent drag to approach drag and extending FLAP 1 before attempting any Rate 1 turns.
So if you fly an economic descent profile finishing at well over 200 KIAS you must finish the high drag descent at least 40 miles and therefore 15 minutes short of the runway.
It would take you 40 minutes to descend from FL 220 to 2000 feet at 500 ft/min, but you will probably prefer 700 VSI subject to Mno and Vno. The descent phase would then last only 29 minutes. Add back the 15 minutes of the initial, intermediate and final approach phases and normal DC-6B top of descent is 44 minutes before the runway.
This is 2 x the thousands of feet from your cruising level to the runway in minutes. In the example above 44 minutes from FL 220 to a sea level, coastal, runway.
Try to think in minutes not speed and distance. The VSI is based on time not distance. However if you intend to average about 240 KTAS (4 miles per minute) during a propliner descent then that will be 8 times your cruising level in miles. It is however more appropriate to flight plan by time rather than distance. You will be targeting the same VSI rate in all propliners. The time is therefore the same in all propliners. The distance varies with speed so don't think about distance and speed. Planned top of descent is at a time in the flightplan not a place. Even if ATC reroute you TOD is at the same time. In the era of radio ranges, before DME, it was a hell of a lot easier to monitor time than place. Even in a glass cockpit it still is, especially if you get rerouted, or vectored all over the sky.
If you short haul a DC-6B at FL 150 then you will be bitching for descent from ATC only 30 minutes short of destination. If the flight was longer than 90 minutes, i.e. 300 miles airport to airport, you should have been cruising higher than FL150 because you will take hours longer if you stay down there ramming all those high density molecules for no good reason.
The only reason it was worth paying incredible prices for a DC-6B with a pressure hull was so that it did not have to ram the same number of molecules as a dirt cheap war surplus C-54. Forget the marketing drivel about passenger comfort, pressurisation is all about velocity (TAS). It's all about getting more miles per day out of the aircraft. You cannot do that by flying too low and adding drag, or indeed by short hauling.
The 2 minutes per flight level rule allows the descent profile that your airline wants you to fly whenever ATC permit it. You could of course descend 5 minutes later, 39 minutes before the runway, reduce power, descend with less drag (IAS), arrive later at destination, and nail 140 KIAS at 25 miles out, but it takes more skill than most of us have.
If you want to try that descent profile it is called a 'cruise descent'. You manipulate MAP to sustain cruise drag, (normally 182 KIAS in a DC-6B), and then reduce MAP to 20 inches in time to nail 140 KIAS at 25 miles. What makes the cruise descent tricky is the requirement to reduce MAP at no more than 3 inches per minute. Cruise descents are easier to manage in a turboprop.
Now consider the real world where ATC are in charge.
FS9 has a poor implementation of modern US ATC, but as it happens the 2 minutes per thousand feet rule will work well with FS9 ATC.
FS9 assumes that ATC are using radar and so it assumes that ATC can construct the approach sequence using lateral separation. In the classic era they could not. The sequencing was entirely vertical. The first aircraft to badger a clearance out of ATC to the lowest level in the stack (sequence) landed first. All pilots bitched for early descent, but they got a descent clearance based on their number in the stack sequence anyway. There is always a stack sequence whether or not aircraft are actually stacking.
In real life you don't make a continuous descent to the runway elevation. You hold. You fly approach procedures, or you get vectored all over the sky by ATC. It makes no difference. The two minute rule works in all cases. It works for any piston engined aircraft, including general aviation puddle jumpers, because it removes speed and distance from the equation.
All the turns in those procedures should be rate 1 turns flown using the rate one turn co-ordinator on the panel. That's what it's for and it's on every IFR panel and almost all VFR panels. It may bear the legend '2 minutes' because it takes 2 minutes to fly 360 degrees at rate 1 (3 deg/sec). From reports of difficulty in capturing LOC beams or NAV radials I conclude that most MSFS users fail to turn at the correct rate and turn much slower than Rate 1 applying an inadequate bank angle for their current energy state. The AP will fly a sinusoidal capture unless you control your energy state and slow to <= 145 KTAS for the capture. This is not a bug.
ATC will control your altitude to suit them because it controls your energy state and therefore your turn rate. If they intend to start turning you with vectors or give you a clearance to fly an approach procedure they will force you to descend to kill your energy state first.
Suppose ATC instruct a DC-6B to maintain FL 220 and to reduce (drag) by ten knots from 190 KIAS to 180 KIAS. This decelerates the aircraft by 14 knots from 269 KTAS to 255 KTAS. But ATC can only tell an aircraft in the cruise to reduce drag (IAS) a fairly small amount before it might become unsafe.
Instructing the same aircraft to increase (drag) to 200 KIAS and descend to FL150 reduces its velocity from 269 KTAS at FL220 to 252 KTAS at FL150. Increasing drag by 10 knots while power diving 7000 feet with increasing IAS slows the aircraft 17 knots. The higher the IAS in a dive, the more the drag, the steeper the dive, and the faster the deceleration.
On reaching FL150 the pilot can now be instructed to reduce (drag) 20 knots to 180 KIAS and TAS will fall by a further 25 knots to 227 KTAS. The aircraft will have decelerated 42 knots for the 10 knot drag reduction from the original 190 KIAS to 180 KIAS losing almost 16% of its velocity (TAS) and a quarter of its energy state. The 16% deceleration and 25% reduced energy state are mostly due to the ATC mandated descent.
At any bank angle its turn radius will now be 25% reduced when radar vectored. ATC will force it much lower and kill its energy state much further before vectoring it hard for approach sequencing else it will exit the protected airspace of the airway or terminal area when turning. That's why terminal ATC airspace has to look like a series of inverted wedding cake tiers.
The sky is crowded. ATC cannot afford to do most of the early approach sequencing by dog legging subsonic aircraft all over the sky. Inbounds are selectively decelerated by instructing them to descend in the appropriate sequence. Telling a pilot to reduce altitude and drag at the same time is indeed stupid. An aircraft can go down and slow down very easily, but it cannot easily go down and reduce drag (IAS) at the same time. A descent with drag lower than cruise drag would be very shallow. The pilot needs to target higher drag than econ cruise IAS to dive steeply to decelerate the aircraft quickly.
In real life you can pregnant dog at ATC for descent in accordance with your airline's fuel saving policy all you like, but you get clearance according to your position in the approach sequence. At a busy airport today there are never fewer than 30 aircraft in the queue for each landing runway, often there are over fifty. In the classic era more like a dozen. Either way you are being approach sequenced by ATC before you get descent clearance from FL 220. When ATC have killed your energy state to their satisfaction they will start to vector you hard in low radius turns that do not endanger other aircraft and don't take 2 minutes to turn 60 degrees.
ATC may sometimes force you to descend your propliner with less than econ cruise power applied and you may have to descend at more than 700 VSI, but given a free hand you would not normally choose to do so in a propliner because you risk exceeding Mno in particular and Vno later. Of course some propliners have more drag than others and some are stronger than others. Some run little risk of exceeding Mno, even at more than 700 VSI, even in econ cruise power. The DC-6B is pretty slippery and tends to have an energy state problem that you have to manage with both care and foresight. That's what makes it interesting to operate.
Back in the classic era you would have been approach sequenced entirely by when you were given descent clearance, from cruising level, and to each successive level. Remember you are not entitled to descent clearance at all. You do not have an approach clearance. In real life you may have to maintain cruising level into the stack and make all of your descent winding down in the hold, round and round until it is your turn to have approach clearance. For a DC-3 down at FL100 this would happen frequently, for a DC-6B up at FL220 hardly ever.
However inbound to a busy airfield you are always in the ATC approach sequence at least 20 minutes before you get an approach clearance and normally before top of descent. In real life, when and how much an airliner descends is not an aircrew problem, but they can always ask politely if ATC have forgotten them.
In FS9 the ATC is generally too dumb to cope with any of this and you will have to tell it when you need to descend. Two minutes per thousand feet from current level to airfield elevation will work pretty well for any piston engined aircraft. After vacating cruising level on that basis don't forget that you are targeting a continuous 700 VSI, not 500 VSI, because you also need to be down to the minimum stack level (typically about 7000 feet above the runway) with enough drag in place to get FLAP 1 down when you are still at least 10 minutes and at least 25 miles from the runway threshold so that you can start to make Rate 1 turns at reasonable bank angles (or use the AP) from there on in.
Now for some reason the question of flight rules keeps cropping up too.
VFR and IFR are two sets of rules, not two visibility states. In FS9 there is no separation from other AI generated traffic under any circumstances. Separation is a complete non issue. Position reporting is also a non issue. The issue is navigating a precise path at all times and knowing where you are at all times.
If you want to get from A to B in 7 hours rather than 10 in a DC-6B you have to climb to FL220. Whatever the visibility, whatever the cloud, over water, and at night. Airliners do not navigate from A to B by visual reference to the surface. This has nothing to do with whether they filed VFR or IFR plans or who is responsible for separation from which other aircraft in each of those circumstances. They navigate along the airways using radio beacons and instruments either way.
Provision of separation in accordance with the two sets of rules is never an issue in a flight simulator that provides no separation. In a detailed post here a few weeks ago concerning operation of airliners within the US between 1936 and 1969 I explained exactly how much and how little the presence of some aircraft on VFR plans affected operation of airliners whether they were themselves flying VFR or IFR. It makes almost no difference to how you operate an airliner whether you file VFR or IFR. VFR and IFR are not related to low level and high level operation. The FAA did not forbid VFR operation along the airways at high level in the classic era.
So let me try to explain normal airliner operation within the US once again. This time with some additional historical background. I claim copyright over the following and may repeat it elsewhere.
Britain and France introduced mandatory air traffic control regulations in 1919, and by 1926 much of Northern Europe had ATC. European airliners were large. Their crew included a wireless operator and a navigator. Over the sea, or above cloud the former obtained bearings from ATC and the latter plotted the course by triangulating the bearings. European airlines generally flew direct from A to B and could proceed without visual reference to the surface. There were no airways and navigation was imprecise. In poor visibility they flew above or around mountains (not between them). European airlines only flew routes that had a government subsidy, sometimes in the form of an air mail subsidy, but often a simple passenger subsidy. They had to attract passengers to fly in big aeroplanes.
In the US there was no ATC, but from 1923 there were airways marked by light beacons. The only government subsidies were for mail. The mail planes were very small, single crew, zig-zagged from beacon to beacon, at very low level, and for navigation relied on visual reference. They carried no passengers.
By 1929 several US airlines had purchased airliners with cabins as well as mail compartments and were flying passengers alongside the mail around the subsidised Contract Air Mail routes (CAMs). The number of passenger miles flown in the US had reached 78% of the European level, but certification procedures in the US were so lax and the safety record of the air mail operators so poor that passenger demand was muted. American aero engines and aircraft could not be sold outside the artificial domestic market.
In 1929 the Wall Street Crash changed the aviation world forever. The US Government acted swiftly to pump taxpayer funds into the emerging US aviation industry, initially in the form of enhanced air mail subsidies.
These had the desired stimulating effect on the frail American aviation industry, but the resulting safety record was still poor. This spawned a further round of massive US taxpayer investment in the world's first Federal integrated ATC and Airways system. As the thirties progressed this made ever more extensive use of newly emerging radio communication and point source radio navigation systems, all delivered to the US airlines free at the point of demand. These point source radio beacons were pilot, (rather than wireless operator and navigator), interpreted and so facilitated smaller flight deck crews, provided the crew zig-zagged from beacon to beacon and did not attempt to fly direct from A to B using area navigation.
With the US taxpayer now providing most of the investment capital for the aviation industry laissez faire, (not to say corrupt), practices gave way to ever tighter federal regulation of the US aviation industry involving, certification procedures, control over airline route licensing, and increasingly day to day operation. Over the space of no more than six years the US aviation industry progressed from producing small numbers of commercial aircraft and aero engines that no one outside America wanted, to world leader status in both commercial aero engines and airframes.
Before the introduction of the radio beacon based ATC and airways system the concept of instrument flying in the US existed only as a temporary expedient for climbing and descending through a layer of cloud. It was not a navigation concept. In bad weather passenger flights were either subject to cancellation, extreme delays or fatal crashes.
Design of the DC-3 coincided with both the latest and greatest Air Mail Act and promulgation of the Federal plan for formation of the new ATC and airways system based on point source 'Radio Ranges' defining the centrelines of the new medium level airways. All DC-3s were fully airways equipped, but more to the point they were the first airliners actually designed for operation on airways. The DC-3 could cruise above cloud, or even in cloud, at medium level following the flight plan in complete safety, navigating between mountains following the new airway on course radio signals from the Radio Ranges. Earlier aircraft were adapted to do this, but they were not designed to do this.
By the end of the thirties despite having only two crew a DC-3 could transition from the airways system defined by Radio ranges to fly a pilot interpreted instrument approach procedure using the newly developed automatic direction finding (ADF) receiver to tune the new non directional beacons (NDBs). At major airports they could make their final approach down a Lorenz locator beam (LOC). This new way of flying was a revolution in air transport that happened first in the US and would not be copied elsewhere in its entirety until the mid fifties.
Much of what is claimed for the DC-3 by its protagonists, in reality was due to the new safer, day/night and weather independent, way of flying that the new Federal ATC and airways system made possible. The improved safety levels, and on time departures, turned potential passengers into real passengers. The ability to fly long distances above cloud, and then locate the landing runway in rain, snow or fog, using only radio beacons, meant that far fewer flights were cancelled or delayed. Far, far fewer collided with terrain.
European and Asian governments did not act quickly enough to stimulate their fragile aviation industries. They failed to direct taxpayer funds to the creation of national commercial aviation infrastructures. In the wake of the Wall Street Crash and swift US Government reaction 1930 passenger miles flown in the US were suddenly more than double the number flown in Europe. When European and Asian Governments finally reacted with subsidies they came in the form of military procurement with predictable and well known results. In the wake of the Crash there was no investment capital available for commercial aviation outside the U.S. The economic flip-flop in commercial aviation activity between Europe and the U.S. was very sudden and final.
Air mail subsidies were vital to the survival of some airlines in Europe and Asia too of course, but in the US the air mail subsidies were now so high that airlines could make a profit just flying a few hundred pounds of air mail along the ever increasing Contract Air Mail routes. Any revenue received from passengers was pure profit.
The price of passenger airline travel in the US fell rapidly as the airlines slashed their ticket prices to attract ever more passengers. Passenger miles flown in the US increased by 172% between 1929 and 1930. By the end of 1930 a US air mail route licence was a federal licence to print money. Because the US taxpayer was now providing (or withholding) almost all of the funding for each airline the government could force airlines to do what it wanted without introducing mandatory procedures.
These huge inflows of pure profit allowed the newly empowered US airlines, to specify what should be built. They could afford to fund extensive R&D, and extensive new technology. There was only so much that airlines could do to compete on price. By the end of 1930 they had realised that in future they would have to compete in terms of both passenger comfort and travel times, especially on the trunk routes, which the federal authorities could now insist must have competition between airlines.
This new need for speed in commercial aviation brought to practical fruition, at a commercially affordable price, three key technologies. These were the retractable undercarriage, turbine superchargers and the constant speed airscrew.
Used in combination in aircraft like the DC-2 and L-10 Electra these allowed the new breed of airliners to climb to medium level within the new federal airway system where, (in the thinner air), they could achieve high TAS at modest IAS. The ability to navigate precisely at medium level, above cloud, and at night, was the key to both higher true air speed and much improved safety.
The mass produced autopilot was also developed within the US during the thirties, but from within the USAAC budget. The US airlines went from hedge hopping in vintage biplanes to modern airline travel, cruising on autopilot at medium level between radio beacons, in an integrated ATC airways environment, transitioning into instrument approaches, flying retractable gear monoplanes, with supercharged engines and constant speed airscrews within a single decade.
The old generation of Fokker and Ford Trimotors had neither the avionics, nor the supercharged engines, nor the variable pitch airscrews needed to utilise the new federal airway system and disappeared fast.
It was the improvement in safety derived from increased regulation as much as the subsidised ticket prices, that stimulated a huge growth in passenger demand that was not matched anywhere else in the world. Without the new possibility of medium level navigation using Radio Ranges en route, NDBs for instrument approaches, and autopilots to ease workload, the DC-3 would have needed four crew to navigate above cloud over the longer distances, which its four fuel tanks allowed. With the burden of four aircrew salaries it would not have been profitable. With fewer it would not have been safe.
As the new breed of airliners flew faster, by consistently flying higher in thinner air, they generated more route segments per day, and more passenger miles per year. The Airways system was the engine room of U.S. supremacy in air transport.
The DC-3 could fly all day with 21 passengers in seats and then fly all night with 14 passengers in bed. Unlike earlier airliners the entire package including avionics and Sperry AP were so expensive that these newer airliners had to fly through the night to pay their mortgage costs, but the DC-3 had absolutely everything the US airlines wanted. That was the problem!
It made every other airliner in the US redundant. The airlines had just invested huge amounts of their stockholders money in brand new fleets of Douglas DC-2s, Boeing 247s, Lockheed Electras and Curtiss Condors, but suddenly they could no longer persuade Americans to travel in them! These obsolete aircraft now had to be sold off to third level Mexican airlines for a few Pesos each. The dominance of the DC-3 was very much a two edged sword for the airlines.
It allowed Douglas to increase prices rapidly from $79,500 in 1936 to $100,000 in 1937 and $110,000 in 1939. A 40% price hike over three years. Every price hike required the DC-3 to fly more at night and in ever more marginal weather to pay the mortgage.
Airline stockholders were looking at airline balance sheets containing huge write downs for all the brand new DC-2s, Boeing 247s, L10s and Condors. Their airline was making more operating profit each month flying the DC-3, but its embedded asset value was diminishing fast whilst it borrowed more and more to purchase ever more expensive DC-3s
The DC-3 was a significant factor in the first ever year on year fall in US airline travel. Between its introduction in 1936 and 1937 the number of passenger miles being flown in the US actually fell by 2%. Far from adding capacity the airlines, who had just dumped their nearly new Trimotors for a few pesos each, were now retiring their nearly new narrow bodied Condors, Boeings and Lockheeds so fast that revenue miles actually fell. There were only so many heavily subsidised CAMs and none of the narrow bodies could make a profit on any other route.
The federal airways system, provided to US airlines free at the point of demand revolutionised their profitability, changed the aircraft they flew and the way they operated them.
Within a few years the airlines were cramming more passengers into their novel circular fuselage wide body DC-3s and consequently they could use the DC-3 outside the Contract Air Mail routes and for the first time make profits without a direct air mail subsidy from the taxpayer. The airways and ATC system created the first aircraft whose seat/mile costs allowed airlines to experiment with routes that had no overt subsidy. They could begin to build networks based on real consumer demand.
At first these new routes were branch lines from the CAMs or extensions beyond the end of existing CAMs, but eventually the major airfields along the CAM mainline had so many branch lines radiating like the spokes of a wheel that they became known as hubs.
Having fallen by 2% during 1937, in 1938 US passenger miles rose by almost 16%.
Six years after AAL first put the DC-3 into service the entire US airline fleet stood at 322 aircraft of which 260 were DC-3s. Those 260 were flying airways at medium level and transitioning to NDB approaches all day and all night, in all weathers. When the visibility near destination was good and they saw the runway early enough they converted to a visual approach, but a visual approach is an IFR procedure not a VFR procedure. The other 72 aircraft were mostly modern monoplane twins like the DC-2, Boeing 247 and L-10 Electra. Most of them flew airways day and night in all weathers as well. Sure they filed VFR on airways if the weather permitted, but it's not the point.
Trans continental competition and increased government subsidies drove the coast to coast fare down from $400 at the end of 1927 to $149.50 at the end of 1937, but airways flying at medium level in place of hedge hopping had reduced the accident rate even faster.
The US airline industry had never had a year without a fatal accident, usually there were many. The new Civil Aeronautics Authority (later CAB, later FAA) took firm regulatory control over the airlines in 1938. Suddenly in 1939 and 1940, despite swiftly rising traffic, there were no fatal accidents at all. The airways system had finally been completed on 1st May 1939 with all 231 Radio Ranges in use, roughly where the VHF Omni Ranges (VORs) now stand.
The airways system and air traffic controllers who increasingly imposed it were the cause of the increased safety. The increased safety was the cause of the huge surge in demand for airline travel, and that was the cause of falling prices as more airlines purchased more airliners and entered the competition for the rising consumer demand.
The whole virtuous cycle depended on airliners having the ability to navigate safely at the altitudes where those airliners could fly at high speed. Altitudes from which it is hard to pick out many ground features even in good visibility on a cloudless day and impossible in average weather or at night.
Airliners cannot fly fast at low level. They do not have enough power. To fly fast an airliner must accelerate for as long as possible, and the only way to accelerate an aircraft, for more than a couple of minutes, is to point it uphill and keep on going uphill for as long as possible.
In a DC-6B you must take care that the drag does not rise above 165 KIAS until you have finished accelerating the aircraft, which will be at least 30 minutes after take off. You must keep the drag low and point it up hill or it will not accelerate. So long as you keep going up hill it will accelerate so fast, that you can reduce MAP from 48 inches in the stage 1 climb to just 37 inches in stage 3 climb during the final stage of the acceleration. You start the acceleration burning 600 gallons per hour and finish it burning only 450 gallons per hour. You cannot accelerate a DC-6B by applying 37 inches and burning only 450 USG/hr at low level. You can only do it at the top of a long, long hill climb. In an aeroplane climbing enables acceleration and diving promotes deceleration. When climbing you need less and less power to go faster and faster. The aeroplane is the exact opposite of a terrestrial vehicle. That's the whole point.
At sea level a drag of 160 KIAS delivers a velocity of 160 KTAS, but after going up hill in a DC-6B at a drag of about 160 KIAS for 30 minutes you will have reached about FL160 and will have accelerated to 205 KTAS. If you departed at max gross in a DC-6B you will be around your current operational ceiling by then so you will reduce power further to 31 inches and allow the drag to rise to just over 180 KIAS allowing the aircraft to accelerate further to 231 KTAS. To go faster (accelerate) you must step climb again and again as weight reduces hour by hour. Many hours later you can cruise at 258 KTAS up at FL220, still with only 182 KIAS of drag. You will have turned a ten hour flight into a seven hour flight by climbing and sustaining operational ceiling. You will be flying above the weather in smooth air. In the U.S. you won't collide with any mountains.
To fly at even 231 KTAS at low level you have to apply abusive power to try to get the drag up to almost 231 KIAS. The aircraft will have been forced nose down passing a drag of about 190 KIAS and the fuel burn will be horrendous. You are trashing the engines at the same time confusing drag with velocity, confusing IAS with TAS.
Aeroplanes are not terrestrial vehicles. The closer they are to terra firma the worse they perform. Of course manufacturers like Cessna provide aircraft like the c172 with an operational ceiling of a couple of thousand feet or the C182 whose operational ceiling is a few thousand higher. They are efficient at low levels.
The original Cyclone engined DC-3 of 1936 was optimised to cruise in the new airways at eleven thousand feet. Each subsequent airliner was designed to fly in ever thinner air, and starting with the Boeing 307 Stratoliner of 1940 they soon needed pressure hulls. They needed the pressure hulls because they needed to fly high, and to fly high they needed radio navigation.
Whether they were avoiding the aeroplanes around them at FL 220 by looking out of the window on a VFR flight plan, or by following ATC separation instructions on an IFR plan, is not at all the point. That the crew were making position reports to ATC if the plan was IFR, but maybe not if it was VFR, is not at all the point. Flying airways at high level and high speed whilst being very certain of position all the time, without needing to pay a radio operator or a navigator was the point.
The rest of the world failed to institute an airways system that would permit high speed airliner flight, and consequently failed to build, high flying, high speed, pressurised airliners until the 1950s. They fell far, far behind the U.S. airline industry and airliner manufacturers in consequence.
From 1936 until after the classic era aviation in the U.S. was heavily regulated, by award of government contracts with strings attached, or after formation of the CAA / CAB / FAA by law. The aircraft flew from beacon to beacon, transitioned into published approach procedures, knew where they were all the time and consequently hardly anybody died in the process. Making up ad hoc direct routes, flying slowly at low level in all the turbulence and precipitation under the clouds, or at night, and colliding with the terrain was not the American way.
Sure there were a few airlines who ignored the huge taxpayer subsidies on offer, and the superior operating economics of flying at high level, and who scheduled 7 hour flights for 10 hours, but they either learned to be smarter or went broke. Of course today there are old propliners around whose pressure hulls are so fatigued that they cannot be pressurised. They are only good for slowly hauling low priority cargo or fighting fires at low level.
Normal airline flying within the US continued throughout the war. There were 16 US airlines in 1941 and 15 in 1945. Western had taken over Inland Air Lines. US airline passenger miles were 4% up in 1942. IFR rated airline pilots were needed to pilot airliners, at night, in all weathers, into Africa, Iceland and Alaska, around the clock. Novices would be trained to fly military aircraft in a VFR environment.
As the war progressed passengers on US airlines were increasingly travelling on government business, and over ever increasing distances. By 1945 US airline passenger miles stood at 246% of the 1941 level even though the number of passengers carried was almost identical. The DC-3 fleet was averaging 10 hours flying per day, and load factor was around 90% compared to 60% in 1941. There were fewer airliners flying even faster and higher as C-53s replaced DC-3s.
Increasingly the government insisted that scheduled flights carrying government personnel be conducted on airways. They did not care whether the plan was VFR or IFR. Collision with other aircraft was a tiny risk. Dozens of mail plane pilots, and airliner occupants, had been killed by trying to fly direct, under the weather. They lost track of their position and collided with terrain. Controlled flight into terrain remains the single greatest cause of death in airline flying. Flying the Ranges reduced the death toll to zero. If you tracked the on course signal from one to the next you and your passengers stayed alive because the on course signal went between the mountains and not into them.
When the war came to a close thousands of nearly new C-53s and C-47s recently purchased by the US tax payer for over 110,000 USD each were sold to the US airlines for around 14,000 USD each. In 1949 if you wanted to start an airline you could buy one nearly new DC-6, or more than forty nearly new C-47s, for the same price. You didn't fly that DC-6 around at DC-3 flight levels suffering DC-3 economics, but unlike any other pre war airliner the DC-3 was designed to operate in the modern ATC airways environment and so it endured long after all others disappeared.
Which brings us neatly back to ATC. The 1940s did not happen on a different planet. The US airways and ATC system of the 1940s were the modern system in use everywhere today. VORs had not yet replaced Radio Ranges but the NDBs were in place. I explained how to use 2 x VOR receivers in FS9 to simulate flying the g Radio Ranges at length in a recent post here. There was no radar and no DME in the Forties. Turn the DME receiver off. Turn the FS9 ATC and its radar vectors off. Fly the published procedures, unless you want to simulate the deregulated modern system of 2004, with flight following, area navigation and radar vectors.
Believe it or not there are days when DME and radar are off in 2004. There are places where they have not yet been installed. On those days and in those places the 2004 ATC system is the 1940 system. You don't have to wonder what it was like, you don't have to make up any routes or procedures, you just have to download the real ones from the FAA for free, because they are still in use every day.
The only thing about the 1940s system that I did not explain in detail in my last post here was how to transition from the airway to the initial approach fix and how to fly the procedural (non radar) NDB approach which was the only option in the 1940s. Since these are still in use there are books you can buy, and websites you can visit, which explain that in detail, but they are aimed at qualified pilots who are obtaining further instrument qualifications and take quite a lot for granted.
If Tom is willing to host a download of about 300Kb on his website I will provide a tutorial aimed at FS9 users which references an approach plate included in the download. Once you understand one such transition and procedural approach fully, and slowly train yourself to fly it, you can start to use FS9 as a flight simulator rather then a pilot role playing game. FS9 is a flight simulator, not a game. You can fly real 4D flight plans, appropriate to a particular aircraft type, and use the real procedures, in real time, rather then making them up and pretending. It takes some effort, but then there is no reward without effort.
Some of the NDBs extant in 1940 have been removed in real life and are not present in FS9. As Arch has explained you can add them back, but you would need to invent an approach procedure to use, so the place to start is with real 1940 procedures that are still in use today, when the radar is off for maintenance, or has never been installed.
FSAviator June 2004
This keeps coming up so I need to start with the simple stuff.
The dynamics of jet engines and piston engines are not just dissimilar, they are totally different. What ATC would like the pilot to do is always the same anyway. They always want you to descend when they are good and ready, not a moment sooner, and then as steeply as possible. They are planning to manage your energy state in fine detail. Ignore ATC for the moment. I will get back to them.
Miles per gallon achieved in a jet depend on altitude. Any jet has double the fuel economy and therefore double the range at 41,000 feet. It must get up there as fast as possible, stay up there as long as possible and therefore plans to descend in a high drag, steep, power off, dive. It's not about money, any jet will run out of fuel as little as half way to destination if it flies too low or descends too soon. Regardless of the speed it flies at.
*For a jet late descent is a flight safety requirement*.
Piston engines have neither the benefit nor the problem. They achieve about the same fuel economy (range) at any altitude. Things are marginally worse below rated altitude or with a low speed supercharger blower selected. However even though fuel economy varies little the higher they fly the less air resistance they encounter, at constant MAP and rpm, and the higher the TAS (velocity) they achieve without any loss of range or economy of operation.
The time it takes a propliner to get from A to B depends on altitude, but unlike a jet the fuel burned does not. Piston engined aircraft are therefore inefficient for long range flying. However the only way to get from A to B in the minimum time in any aeroplane is to operate it at its operational ceiling. The operational ceiling depends on the current weight. You must climb to the initial operational ceiling and as weight reduces through the flight you step climb to new higher operational ceilings.
The practical definition of operational ceiling when using MSFS is the maximum level to which you can climb *using only climb MAP and rpm* without the VSI falling below 500ft/min *and* without the IAS falling below the mandated climb IAS.
During a short haul flight a propliner, (or bomber), may never reach operational ceiling, and will never achieve the cruising speed you see quoted in references. Cruising speed can only be achieved at operational ceiling.
In real life the navigator will instruct the pilot to fly at a level that maximises ground speed, not TAS. 4D navigation taking into account the wind vector is well beyond the scope of this post so it assumes nil wind throughout.
It might take a propliner 30+ minutes to reach its initial operational ceiling and more than 10 hours to reach the final cruising level in the flight plan after several step climbs.
Most MSFS users fail to understand that they will arrive at destination many hours later than necessary if they do not sustain operational ceiling throughout the flight.
The fuel used will not vary significantly with altitude at constant power. Piston engined aircraft can cruise slowly at low level without significant fuel penalty if required to do so. Jets cannot. Jets cannot adjust MAP at constant altitude.
The lower you fly the slower you fly in any aircraft. You are ramming more air molecules and they slow you down (a lot). Think about what a 34 KIAS wind, (called a gale), does to a flag or a tree at sea level. Every 34 KIAS you add to the ASI adds another gale of drag. The airspeed indicator (ASI) is just recording the number of molecules rammed per second, (collected in the pitot tube), and therefore displays your drag, not your velocity. Whenever you fly an aircraft you must work hard to maximise your velocity (KTAS) whilst restraining your drag (KIAS).
From the DC-6B handling notes.
***********
Econ Cruise (FL160 to FL225):
CALL for economy cruise power (about 1100hp)
COWL FLAPS - CLOSED
30 inches MAP
1800 RPM
Check CHT < 232C
Plan 370 USG per hour
Note - yields 258 KTAS at FL220 @ 83000lbs
*************
Econ cruise MAP and rpm delivers about 182 KIAS at any level but it depends on current weight and the weight of a DC-6B varies a lot over ten hours of flight. This is anyway a drag not a velocity. It is the most economical drag for cruising. The only altitude at which the drag and the velocity are equal is sea level. 182 KIAS = 182 KTAS at sea level but 182 KIAS = 258 KTAS at FL220.
At constant MAP the fuel burn per hour rises with altitude. The fuel burn per mile does not. Piston engined aircraft therefore have much more endurance at low level, but have about the same range at any level. Piston engined aircraft were retained for long patrol endurance long after they were abandoned for long range operation, but if you fly low you can only fly slow.
With piston engines fuel consumption per mile (range) is nearly constant versus altitude. It is the velocity at which you can traverse that range which differs (a lot).
You can fly 1820 miles at low level in a DC-6B and take ten hours, or you can do it at FL220 and take seven. Entirely your choice. You don't waste any fuel either way, but the airline does not pay you to arrive three hours late on every medium haul trip in a DC-6B. You are paid to fly the DC-6B with a drag of 182 KIAS at a velocity of 258 KTAS up at operational ceiling, not down at low level with a velocity of only 182 KTAS.
Nor are you paid to apply abusive power at low level to try to get the drag up to 258 KIAS. Abusive power forces an aircraft to fly noticeably nose down. Using the fuel to increase the drag (IAS) is not a substitute for using it to increase the velocity (TAS). All available excess power must be used to create climb power to reach the thinnest possible air.
The tail becomes stressed if you add too much drag. Vno is 251 KIAS. If you push the drag on the tail beyond 251 KIAS it will fatigue fail if you encounter turbulence. You must keep the drag below 251 KIAS or risk sudden death. To force the aircraft nose down enough in level flight to reach a drag of 251 KIAS you have to apply very abusive power and you will run out of fuel half way to destination because you used fuel to increase drag (IAS) not velocity (TAS). The only way to fly fast is to fly high. Sure you can fly lower than the operational ceiling, but you are wasting huge amounts of time. It will take many extra hours to complete even a medium haul.
Air molecules exert great drag on aeroplanes. Gale force upon gale force of drag. You must keep the IAS down and the TAS high by flying as high as possible in the thinnest possible air.
Anyone using a flight simulator needs to understand all that before they can use it realistically, but most MSFS users never quite grasp the difference between drag (IAS) and velocity (TAS). Consequently they end up trying to increase the wrong one, applying more and more power, at too low an altitude, achieving ever more nose down attitudes, as the gales of drag rise out of control due to the abusive power.
That extra power is only there so that you can climb into thinner air. It is not there to increase drag (IAS) at low level.
Now let's take a step nearer to realistic descent planning.
The optimum en route drag in a DC-6B at mid cruise weight is 182 KIAS. The resulting velocity (TAS) varies with altitude but you must use the optimum MAP and rpm anyway. The engine overhaul costs for piston engines outweigh consideration of fuel costs. You must operate the aircraft to minimise engine wear and overhaul costs, not to minimise fuel costs. That is why the handling notes tell you to apply a particular MAP and a particular rpm and do not tell you to fly at a particular drag (KIAS). The airframe and wing do indeed have one particular drag at which their efficiency maximises and miles per gallon maximise, but it is irrelevant. En route, fuel economy must be sacrificed to the needs of the hugely expensive, maintenance hungry, engines.
For jet aircraft with low maintenance, long life turbines the economics are reversed. If necessary they are cruised at variable power to achieve a Mach number target. Mach drag can be fearsome in more ways than one.
For a propliner when not en route, e.g. when holding (or loitering / patrolling in military aircraft), it may be appropriate to vary the MAP to sustain a target drag. From the DC-6B handling notes again.
*****************************
Holding:
FLAP - UP
COWL FLAPS - CRACKED
2000 RPM
Slowly REDUCE MAP
To obtain 160 KIAS
Check CHT < 232C
Plan 230 USG per hour
****************************
Obviously we reduce drag to hold, but en route, including en route descent, the engine settings are more important to economic operation than sustaining a particular drag.
However before I explain minimum cost descent profiles it is also necessary for anyone who hopes to use a flight simulator realistically to grasp the concept of energy state management. Unfortunately understanding energy state management of aircraft is tough. Fortunately in the real world it is only really a problem for air traffic controllers and combat pilots and not for commercial pilots.
Most MSFs users have never flown an aircraft, but have operated terrestrial vehicles. Everything ythey have ever 'learned' about terrestrial vehicles leads ythem to believe that any vehicle is easier to accelerate going downhill than going uphill. The whole point about aircraft, and the only reason airliners exist, is that aircraft are incredibly easy to accelerate when going uphill and almost impossible to accelerate when going downhill.
If that sounds unlikely then you are bound to be flying unrealistically.
It takes MSFS users, (and many pilots), a long time to understand that if a fighter pilot power dives his fighter from 250 KIAS at 40,000 feet to 400 KIAS at low level he has decelerated from about 500 KTAS to about 400 KTAS. As the pilot dives hard and watches the ASI needle proceed from 250 to 400 he is watching the drag rise, he hears the wind noise screaming ever louder as he decelerates a hundred knots in no time at all.
A drag of 400 KIAS at low level ensures that the fighter is much slower than it is with a drag of 250 KIAS at high level. Its just a lot more drag, so you hear much more wind noise. Wind noise isn't an indicator of velocity, it's just an indicator of drag. IAS isn't an indicator of velocity, it's just an indicator of drag.
Until you can grasp that IAS is drag and TAS is a velocity it is impossible to understand how to plan the climb and descent of aircraft. It is impossible to flight plan, and it is impossible to understand why you must follow a 4D flight plan.
Now we need to consider manoeuvrability.
Aircraft energy state = momentum = mass * TAS^2.
Energy state (momentum) has nothing to do with drag and hence nothing to do with IAS. Energy state controls manoeuvrability. Manoeuvrability depends on TAS and is nothing to do with IAS.
A jet airliner descending from FL410 to sea level at a constant drag of 250 KIAS halves its velocity, from around 500 KTAS to 250 KTAS, and consequently loses three quarters of its energy state. The fuel burn per hour will not change and so the range will halve. It must descend as late as possible or it may crash.
Since radius of turn depends inversely on energy state that same jet quarters its turn radius at any applied bank angle.
R = TAS^2 / G
In order to fly the approach procedures you must reduce your energy state. You must fly the turns at rate 1 (turn 180 degrees in 1 minute) = 3 deg /sec.
It is legal, (and just about safe in most weather), to allow a DC-6B to reach a drag of 250 KIAS, but at approach altitudes a drag of 250 KIAS is also a velocity of about 250 KTAS. You would have about double the turn radius that you would have at 182 KTAS and you would have to apply massive bank angles to fly the rate 1 turns of the approach and holding procedures. Your classic era passengers would hate you.
258 squared is twice as much as 182 squared. At any given bank angle the radius of turn is doubled at 258 KTAS. To fly the same radius at 258 KTAS as at 182 KTAS you have to double the G load. 251 KIAS is when the tail will fail at 1G, not 2G or 3G. Turning at Vno is not an option.
There is another problem. In a DC-6B you not only do not want to arrive near the initial approach fix at 250 KIAS, you don't even want to be at 182 KIAS. You want to be all the way back to a drag less than 160 KIAS so that you can extend FLAP 1, without ballooning the aircraft, and before you attempt the first rate 1 turn mandated by ATC, else you will need huge bank angles.
The AP in the DC-6B cannot fly approach procedures at velocities of more than 145 KTAS because it is limited, (in both real life and FS9), to applying 25 degrees of bank. If you want to use the AP during the approach you must get back to a drag of about 140 KIAS before you reach the initial approach fix, or (in the modern era only) ATC start to give you approach radar vectors to final. In other words about ten minutes (25 miles) before touchdown.
Forget the runway. You need to be down to a drag of 140 KIAS (usually a velocity of about 145 KTAS) 25 miles before the runway. Having grasped that the DC-6B must be down to a drag of 140 KIAS at least 25 miles from touchdown I can explain a minimum cost descent profile.
Because engine costs are paramount the minimum cost profile is an econ cruise power profile. When cleared for descent you set econ cruise MAP and rpm. Often they are set already. Just pitch the nose down to sustain a minimum of 500 VSI and perhaps as much as 700 VSI in practice. The drag (IAS) will of course rise in this power dive.
You must ensure that this technique does not exceed the drag on the tail associated with either Mno or Vno so this technique is generally unwise above FL170. From the DC-6B handling notes.
****************************
Descent:
ABOVE FL170 DO NOT EXCEED Mno = 220 KIAS
DO NOT EXCEED Vno = 251 KIAS
*****************************
Neither Mno nor Vno is a target speed in a propliner. You should avoid them. Both are pretty much target speeds in a jet, and will be much higher too.
In a DC-6B you can allow the drag to rise to 220 KIAS initially and 251 KIAS later, so long as you do not turn (apply G), but you still need to reduce drag to 140 KIAS when you are 25 miles from the runway. If you fly this profile you will arrive at low level at a drag well in excess of 140 KIAS. You must therefore reach your cleared ATC level at least 40 miles from the runway leaving 15 miles to reduce from over 200 KIAS to 140 KIAS in level flight at your cleared level. In real life these are the 15 or so miles approaching the stack (initial approach) fix whether or not you are going to hold. You will average about 180 KIAS as you slow down in level flight, which is about three miles per minute at approach altitudes, allowing five minutes to bleed the drag from economic descent drag to approach drag and extending FLAP 1 before attempting any Rate 1 turns.
So if you fly an economic descent profile finishing at well over 200 KIAS you must finish the high drag descent at least 40 miles and therefore 15 minutes short of the runway.
It would take you 40 minutes to descend from FL 220 to 2000 feet at 500 ft/min, but you will probably prefer 700 VSI subject to Mno and Vno. The descent phase would then last only 29 minutes. Add back the 15 minutes of the initial, intermediate and final approach phases and normal DC-6B top of descent is 44 minutes before the runway.
This is 2 x the thousands of feet from your cruising level to the runway in minutes. In the example above 44 minutes from FL 220 to a sea level, coastal, runway.
Try to think in minutes not speed and distance. The VSI is based on time not distance. However if you intend to average about 240 KTAS (4 miles per minute) during a propliner descent then that will be 8 times your cruising level in miles. It is however more appropriate to flight plan by time rather than distance. You will be targeting the same VSI rate in all propliners. The time is therefore the same in all propliners. The distance varies with speed so don't think about distance and speed. Planned top of descent is at a time in the flightplan not a place. Even if ATC reroute you TOD is at the same time. In the era of radio ranges, before DME, it was a hell of a lot easier to monitor time than place. Even in a glass cockpit it still is, especially if you get rerouted, or vectored all over the sky.
If you short haul a DC-6B at FL 150 then you will be bitching for descent from ATC only 30 minutes short of destination. If the flight was longer than 90 minutes, i.e. 300 miles airport to airport, you should have been cruising higher than FL150 because you will take hours longer if you stay down there ramming all those high density molecules for no good reason.
The only reason it was worth paying incredible prices for a DC-6B with a pressure hull was so that it did not have to ram the same number of molecules as a dirt cheap war surplus C-54. Forget the marketing drivel about passenger comfort, pressurisation is all about velocity (TAS). It's all about getting more miles per day out of the aircraft. You cannot do that by flying too low and adding drag, or indeed by short hauling.
The 2 minutes per flight level rule allows the descent profile that your airline wants you to fly whenever ATC permit it. You could of course descend 5 minutes later, 39 minutes before the runway, reduce power, descend with less drag (IAS), arrive later at destination, and nail 140 KIAS at 25 miles out, but it takes more skill than most of us have.
If you want to try that descent profile it is called a 'cruise descent'. You manipulate MAP to sustain cruise drag, (normally 182 KIAS in a DC-6B), and then reduce MAP to 20 inches in time to nail 140 KIAS at 25 miles. What makes the cruise descent tricky is the requirement to reduce MAP at no more than 3 inches per minute. Cruise descents are easier to manage in a turboprop.
Now consider the real world where ATC are in charge.
FS9 has a poor implementation of modern US ATC, but as it happens the 2 minutes per thousand feet rule will work well with FS9 ATC.
FS9 assumes that ATC are using radar and so it assumes that ATC can construct the approach sequence using lateral separation. In the classic era they could not. The sequencing was entirely vertical. The first aircraft to badger a clearance out of ATC to the lowest level in the stack (sequence) landed first. All pilots bitched for early descent, but they got a descent clearance based on their number in the stack sequence anyway. There is always a stack sequence whether or not aircraft are actually stacking.
In real life you don't make a continuous descent to the runway elevation. You hold. You fly approach procedures, or you get vectored all over the sky by ATC. It makes no difference. The two minute rule works in all cases. It works for any piston engined aircraft, including general aviation puddle jumpers, because it removes speed and distance from the equation.
All the turns in those procedures should be rate 1 turns flown using the rate one turn co-ordinator on the panel. That's what it's for and it's on every IFR panel and almost all VFR panels. It may bear the legend '2 minutes' because it takes 2 minutes to fly 360 degrees at rate 1 (3 deg/sec). From reports of difficulty in capturing LOC beams or NAV radials I conclude that most MSFS users fail to turn at the correct rate and turn much slower than Rate 1 applying an inadequate bank angle for their current energy state. The AP will fly a sinusoidal capture unless you control your energy state and slow to <= 145 KTAS for the capture. This is not a bug.
ATC will control your altitude to suit them because it controls your energy state and therefore your turn rate. If they intend to start turning you with vectors or give you a clearance to fly an approach procedure they will force you to descend to kill your energy state first.
Suppose ATC instruct a DC-6B to maintain FL 220 and to reduce (drag) by ten knots from 190 KIAS to 180 KIAS. This decelerates the aircraft by 14 knots from 269 KTAS to 255 KTAS. But ATC can only tell an aircraft in the cruise to reduce drag (IAS) a fairly small amount before it might become unsafe.
Instructing the same aircraft to increase (drag) to 200 KIAS and descend to FL150 reduces its velocity from 269 KTAS at FL220 to 252 KTAS at FL150. Increasing drag by 10 knots while power diving 7000 feet with increasing IAS slows the aircraft 17 knots. The higher the IAS in a dive, the more the drag, the steeper the dive, and the faster the deceleration.
On reaching FL150 the pilot can now be instructed to reduce (drag) 20 knots to 180 KIAS and TAS will fall by a further 25 knots to 227 KTAS. The aircraft will have decelerated 42 knots for the 10 knot drag reduction from the original 190 KIAS to 180 KIAS losing almost 16% of its velocity (TAS) and a quarter of its energy state. The 16% deceleration and 25% reduced energy state are mostly due to the ATC mandated descent.
At any bank angle its turn radius will now be 25% reduced when radar vectored. ATC will force it much lower and kill its energy state much further before vectoring it hard for approach sequencing else it will exit the protected airspace of the airway or terminal area when turning. That's why terminal ATC airspace has to look like a series of inverted wedding cake tiers.
The sky is crowded. ATC cannot afford to do most of the early approach sequencing by dog legging subsonic aircraft all over the sky. Inbounds are selectively decelerated by instructing them to descend in the appropriate sequence. Telling a pilot to reduce altitude and drag at the same time is indeed stupid. An aircraft can go down and slow down very easily, but it cannot easily go down and reduce drag (IAS) at the same time. A descent with drag lower than cruise drag would be very shallow. The pilot needs to target higher drag than econ cruise IAS to dive steeply to decelerate the aircraft quickly.
In real life you can pregnant dog at ATC for descent in accordance with your airline's fuel saving policy all you like, but you get clearance according to your position in the approach sequence. At a busy airport today there are never fewer than 30 aircraft in the queue for each landing runway, often there are over fifty. In the classic era more like a dozen. Either way you are being approach sequenced by ATC before you get descent clearance from FL 220. When ATC have killed your energy state to their satisfaction they will start to vector you hard in low radius turns that do not endanger other aircraft and don't take 2 minutes to turn 60 degrees.
ATC may sometimes force you to descend your propliner with less than econ cruise power applied and you may have to descend at more than 700 VSI, but given a free hand you would not normally choose to do so in a propliner because you risk exceeding Mno in particular and Vno later. Of course some propliners have more drag than others and some are stronger than others. Some run little risk of exceeding Mno, even at more than 700 VSI, even in econ cruise power. The DC-6B is pretty slippery and tends to have an energy state problem that you have to manage with both care and foresight. That's what makes it interesting to operate.
Back in the classic era you would have been approach sequenced entirely by when you were given descent clearance, from cruising level, and to each successive level. Remember you are not entitled to descent clearance at all. You do not have an approach clearance. In real life you may have to maintain cruising level into the stack and make all of your descent winding down in the hold, round and round until it is your turn to have approach clearance. For a DC-3 down at FL100 this would happen frequently, for a DC-6B up at FL220 hardly ever.
However inbound to a busy airfield you are always in the ATC approach sequence at least 20 minutes before you get an approach clearance and normally before top of descent. In real life, when and how much an airliner descends is not an aircrew problem, but they can always ask politely if ATC have forgotten them.
In FS9 the ATC is generally too dumb to cope with any of this and you will have to tell it when you need to descend. Two minutes per thousand feet from current level to airfield elevation will work pretty well for any piston engined aircraft. After vacating cruising level on that basis don't forget that you are targeting a continuous 700 VSI, not 500 VSI, because you also need to be down to the minimum stack level (typically about 7000 feet above the runway) with enough drag in place to get FLAP 1 down when you are still at least 10 minutes and at least 25 miles from the runway threshold so that you can start to make Rate 1 turns at reasonable bank angles (or use the AP) from there on in.
Now for some reason the question of flight rules keeps cropping up too.
VFR and IFR are two sets of rules, not two visibility states. In FS9 there is no separation from other AI generated traffic under any circumstances. Separation is a complete non issue. Position reporting is also a non issue. The issue is navigating a precise path at all times and knowing where you are at all times.
If you want to get from A to B in 7 hours rather than 10 in a DC-6B you have to climb to FL220. Whatever the visibility, whatever the cloud, over water, and at night. Airliners do not navigate from A to B by visual reference to the surface. This has nothing to do with whether they filed VFR or IFR plans or who is responsible for separation from which other aircraft in each of those circumstances. They navigate along the airways using radio beacons and instruments either way.
Provision of separation in accordance with the two sets of rules is never an issue in a flight simulator that provides no separation. In a detailed post here a few weeks ago concerning operation of airliners within the US between 1936 and 1969 I explained exactly how much and how little the presence of some aircraft on VFR plans affected operation of airliners whether they were themselves flying VFR or IFR. It makes almost no difference to how you operate an airliner whether you file VFR or IFR. VFR and IFR are not related to low level and high level operation. The FAA did not forbid VFR operation along the airways at high level in the classic era.
So let me try to explain normal airliner operation within the US once again. This time with some additional historical background. I claim copyright over the following and may repeat it elsewhere.
Britain and France introduced mandatory air traffic control regulations in 1919, and by 1926 much of Northern Europe had ATC. European airliners were large. Their crew included a wireless operator and a navigator. Over the sea, or above cloud the former obtained bearings from ATC and the latter plotted the course by triangulating the bearings. European airlines generally flew direct from A to B and could proceed without visual reference to the surface. There were no airways and navigation was imprecise. In poor visibility they flew above or around mountains (not between them). European airlines only flew routes that had a government subsidy, sometimes in the form of an air mail subsidy, but often a simple passenger subsidy. They had to attract passengers to fly in big aeroplanes.
In the US there was no ATC, but from 1923 there were airways marked by light beacons. The only government subsidies were for mail. The mail planes were very small, single crew, zig-zagged from beacon to beacon, at very low level, and for navigation relied on visual reference. They carried no passengers.
By 1929 several US airlines had purchased airliners with cabins as well as mail compartments and were flying passengers alongside the mail around the subsidised Contract Air Mail routes (CAMs). The number of passenger miles flown in the US had reached 78% of the European level, but certification procedures in the US were so lax and the safety record of the air mail operators so poor that passenger demand was muted. American aero engines and aircraft could not be sold outside the artificial domestic market.
In 1929 the Wall Street Crash changed the aviation world forever. The US Government acted swiftly to pump taxpayer funds into the emerging US aviation industry, initially in the form of enhanced air mail subsidies.
These had the desired stimulating effect on the frail American aviation industry, but the resulting safety record was still poor. This spawned a further round of massive US taxpayer investment in the world's first Federal integrated ATC and Airways system. As the thirties progressed this made ever more extensive use of newly emerging radio communication and point source radio navigation systems, all delivered to the US airlines free at the point of demand. These point source radio beacons were pilot, (rather than wireless operator and navigator), interpreted and so facilitated smaller flight deck crews, provided the crew zig-zagged from beacon to beacon and did not attempt to fly direct from A to B using area navigation.
With the US taxpayer now providing most of the investment capital for the aviation industry laissez faire, (not to say corrupt), practices gave way to ever tighter federal regulation of the US aviation industry involving, certification procedures, control over airline route licensing, and increasingly day to day operation. Over the space of no more than six years the US aviation industry progressed from producing small numbers of commercial aircraft and aero engines that no one outside America wanted, to world leader status in both commercial aero engines and airframes.
Before the introduction of the radio beacon based ATC and airways system the concept of instrument flying in the US existed only as a temporary expedient for climbing and descending through a layer of cloud. It was not a navigation concept. In bad weather passenger flights were either subject to cancellation, extreme delays or fatal crashes.
Design of the DC-3 coincided with both the latest and greatest Air Mail Act and promulgation of the Federal plan for formation of the new ATC and airways system based on point source 'Radio Ranges' defining the centrelines of the new medium level airways. All DC-3s were fully airways equipped, but more to the point they were the first airliners actually designed for operation on airways. The DC-3 could cruise above cloud, or even in cloud, at medium level following the flight plan in complete safety, navigating between mountains following the new airway on course radio signals from the Radio Ranges. Earlier aircraft were adapted to do this, but they were not designed to do this.
By the end of the thirties despite having only two crew a DC-3 could transition from the airways system defined by Radio ranges to fly a pilot interpreted instrument approach procedure using the newly developed automatic direction finding (ADF) receiver to tune the new non directional beacons (NDBs). At major airports they could make their final approach down a Lorenz locator beam (LOC). This new way of flying was a revolution in air transport that happened first in the US and would not be copied elsewhere in its entirety until the mid fifties.
Much of what is claimed for the DC-3 by its protagonists, in reality was due to the new safer, day/night and weather independent, way of flying that the new Federal ATC and airways system made possible. The improved safety levels, and on time departures, turned potential passengers into real passengers. The ability to fly long distances above cloud, and then locate the landing runway in rain, snow or fog, using only radio beacons, meant that far fewer flights were cancelled or delayed. Far, far fewer collided with terrain.
European and Asian governments did not act quickly enough to stimulate their fragile aviation industries. They failed to direct taxpayer funds to the creation of national commercial aviation infrastructures. In the wake of the Wall Street Crash and swift US Government reaction 1930 passenger miles flown in the US were suddenly more than double the number flown in Europe. When European and Asian Governments finally reacted with subsidies they came in the form of military procurement with predictable and well known results. In the wake of the Crash there was no investment capital available for commercial aviation outside the U.S. The economic flip-flop in commercial aviation activity between Europe and the U.S. was very sudden and final.
Air mail subsidies were vital to the survival of some airlines in Europe and Asia too of course, but in the US the air mail subsidies were now so high that airlines could make a profit just flying a few hundred pounds of air mail along the ever increasing Contract Air Mail routes. Any revenue received from passengers was pure profit.
The price of passenger airline travel in the US fell rapidly as the airlines slashed their ticket prices to attract ever more passengers. Passenger miles flown in the US increased by 172% between 1929 and 1930. By the end of 1930 a US air mail route licence was a federal licence to print money. Because the US taxpayer was now providing (or withholding) almost all of the funding for each airline the government could force airlines to do what it wanted without introducing mandatory procedures.
These huge inflows of pure profit allowed the newly empowered US airlines, to specify what should be built. They could afford to fund extensive R&D, and extensive new technology. There was only so much that airlines could do to compete on price. By the end of 1930 they had realised that in future they would have to compete in terms of both passenger comfort and travel times, especially on the trunk routes, which the federal authorities could now insist must have competition between airlines.
This new need for speed in commercial aviation brought to practical fruition, at a commercially affordable price, three key technologies. These were the retractable undercarriage, turbine superchargers and the constant speed airscrew.
Used in combination in aircraft like the DC-2 and L-10 Electra these allowed the new breed of airliners to climb to medium level within the new federal airway system where, (in the thinner air), they could achieve high TAS at modest IAS. The ability to navigate precisely at medium level, above cloud, and at night, was the key to both higher true air speed and much improved safety.
The mass produced autopilot was also developed within the US during the thirties, but from within the USAAC budget. The US airlines went from hedge hopping in vintage biplanes to modern airline travel, cruising on autopilot at medium level between radio beacons, in an integrated ATC airways environment, transitioning into instrument approaches, flying retractable gear monoplanes, with supercharged engines and constant speed airscrews within a single decade.
The old generation of Fokker and Ford Trimotors had neither the avionics, nor the supercharged engines, nor the variable pitch airscrews needed to utilise the new federal airway system and disappeared fast.
It was the improvement in safety derived from increased regulation as much as the subsidised ticket prices, that stimulated a huge growth in passenger demand that was not matched anywhere else in the world. Without the new possibility of medium level navigation using Radio Ranges en route, NDBs for instrument approaches, and autopilots to ease workload, the DC-3 would have needed four crew to navigate above cloud over the longer distances, which its four fuel tanks allowed. With the burden of four aircrew salaries it would not have been profitable. With fewer it would not have been safe.
As the new breed of airliners flew faster, by consistently flying higher in thinner air, they generated more route segments per day, and more passenger miles per year. The Airways system was the engine room of U.S. supremacy in air transport.
The DC-3 could fly all day with 21 passengers in seats and then fly all night with 14 passengers in bed. Unlike earlier airliners the entire package including avionics and Sperry AP were so expensive that these newer airliners had to fly through the night to pay their mortgage costs, but the DC-3 had absolutely everything the US airlines wanted. That was the problem!
It made every other airliner in the US redundant. The airlines had just invested huge amounts of their stockholders money in brand new fleets of Douglas DC-2s, Boeing 247s, Lockheed Electras and Curtiss Condors, but suddenly they could no longer persuade Americans to travel in them! These obsolete aircraft now had to be sold off to third level Mexican airlines for a few Pesos each. The dominance of the DC-3 was very much a two edged sword for the airlines.
It allowed Douglas to increase prices rapidly from $79,500 in 1936 to $100,000 in 1937 and $110,000 in 1939. A 40% price hike over three years. Every price hike required the DC-3 to fly more at night and in ever more marginal weather to pay the mortgage.
Airline stockholders were looking at airline balance sheets containing huge write downs for all the brand new DC-2s, Boeing 247s, L10s and Condors. Their airline was making more operating profit each month flying the DC-3, but its embedded asset value was diminishing fast whilst it borrowed more and more to purchase ever more expensive DC-3s
The DC-3 was a significant factor in the first ever year on year fall in US airline travel. Between its introduction in 1936 and 1937 the number of passenger miles being flown in the US actually fell by 2%. Far from adding capacity the airlines, who had just dumped their nearly new Trimotors for a few pesos each, were now retiring their nearly new narrow bodied Condors, Boeings and Lockheeds so fast that revenue miles actually fell. There were only so many heavily subsidised CAMs and none of the narrow bodies could make a profit on any other route.
The federal airways system, provided to US airlines free at the point of demand revolutionised their profitability, changed the aircraft they flew and the way they operated them.
Within a few years the airlines were cramming more passengers into their novel circular fuselage wide body DC-3s and consequently they could use the DC-3 outside the Contract Air Mail routes and for the first time make profits without a direct air mail subsidy from the taxpayer. The airways and ATC system created the first aircraft whose seat/mile costs allowed airlines to experiment with routes that had no overt subsidy. They could begin to build networks based on real consumer demand.
At first these new routes were branch lines from the CAMs or extensions beyond the end of existing CAMs, but eventually the major airfields along the CAM mainline had so many branch lines radiating like the spokes of a wheel that they became known as hubs.
Having fallen by 2% during 1937, in 1938 US passenger miles rose by almost 16%.
Six years after AAL first put the DC-3 into service the entire US airline fleet stood at 322 aircraft of which 260 were DC-3s. Those 260 were flying airways at medium level and transitioning to NDB approaches all day and all night, in all weathers. When the visibility near destination was good and they saw the runway early enough they converted to a visual approach, but a visual approach is an IFR procedure not a VFR procedure. The other 72 aircraft were mostly modern monoplane twins like the DC-2, Boeing 247 and L-10 Electra. Most of them flew airways day and night in all weathers as well. Sure they filed VFR on airways if the weather permitted, but it's not the point.
Trans continental competition and increased government subsidies drove the coast to coast fare down from $400 at the end of 1927 to $149.50 at the end of 1937, but airways flying at medium level in place of hedge hopping had reduced the accident rate even faster.
The US airline industry had never had a year without a fatal accident, usually there were many. The new Civil Aeronautics Authority (later CAB, later FAA) took firm regulatory control over the airlines in 1938. Suddenly in 1939 and 1940, despite swiftly rising traffic, there were no fatal accidents at all. The airways system had finally been completed on 1st May 1939 with all 231 Radio Ranges in use, roughly where the VHF Omni Ranges (VORs) now stand.
The airways system and air traffic controllers who increasingly imposed it were the cause of the increased safety. The increased safety was the cause of the huge surge in demand for airline travel, and that was the cause of falling prices as more airlines purchased more airliners and entered the competition for the rising consumer demand.
The whole virtuous cycle depended on airliners having the ability to navigate safely at the altitudes where those airliners could fly at high speed. Altitudes from which it is hard to pick out many ground features even in good visibility on a cloudless day and impossible in average weather or at night.
Airliners cannot fly fast at low level. They do not have enough power. To fly fast an airliner must accelerate for as long as possible, and the only way to accelerate an aircraft, for more than a couple of minutes, is to point it uphill and keep on going uphill for as long as possible.
In a DC-6B you must take care that the drag does not rise above 165 KIAS until you have finished accelerating the aircraft, which will be at least 30 minutes after take off. You must keep the drag low and point it up hill or it will not accelerate. So long as you keep going up hill it will accelerate so fast, that you can reduce MAP from 48 inches in the stage 1 climb to just 37 inches in stage 3 climb during the final stage of the acceleration. You start the acceleration burning 600 gallons per hour and finish it burning only 450 gallons per hour. You cannot accelerate a DC-6B by applying 37 inches and burning only 450 USG/hr at low level. You can only do it at the top of a long, long hill climb. In an aeroplane climbing enables acceleration and diving promotes deceleration. When climbing you need less and less power to go faster and faster. The aeroplane is the exact opposite of a terrestrial vehicle. That's the whole point.
At sea level a drag of 160 KIAS delivers a velocity of 160 KTAS, but after going up hill in a DC-6B at a drag of about 160 KIAS for 30 minutes you will have reached about FL160 and will have accelerated to 205 KTAS. If you departed at max gross in a DC-6B you will be around your current operational ceiling by then so you will reduce power further to 31 inches and allow the drag to rise to just over 180 KIAS allowing the aircraft to accelerate further to 231 KTAS. To go faster (accelerate) you must step climb again and again as weight reduces hour by hour. Many hours later you can cruise at 258 KTAS up at FL220, still with only 182 KIAS of drag. You will have turned a ten hour flight into a seven hour flight by climbing and sustaining operational ceiling. You will be flying above the weather in smooth air. In the U.S. you won't collide with any mountains.
To fly at even 231 KTAS at low level you have to apply abusive power to try to get the drag up to almost 231 KIAS. The aircraft will have been forced nose down passing a drag of about 190 KIAS and the fuel burn will be horrendous. You are trashing the engines at the same time confusing drag with velocity, confusing IAS with TAS.
Aeroplanes are not terrestrial vehicles. The closer they are to terra firma the worse they perform. Of course manufacturers like Cessna provide aircraft like the c172 with an operational ceiling of a couple of thousand feet or the C182 whose operational ceiling is a few thousand higher. They are efficient at low levels.
The original Cyclone engined DC-3 of 1936 was optimised to cruise in the new airways at eleven thousand feet. Each subsequent airliner was designed to fly in ever thinner air, and starting with the Boeing 307 Stratoliner of 1940 they soon needed pressure hulls. They needed the pressure hulls because they needed to fly high, and to fly high they needed radio navigation.
Whether they were avoiding the aeroplanes around them at FL 220 by looking out of the window on a VFR flight plan, or by following ATC separation instructions on an IFR plan, is not at all the point. That the crew were making position reports to ATC if the plan was IFR, but maybe not if it was VFR, is not at all the point. Flying airways at high level and high speed whilst being very certain of position all the time, without needing to pay a radio operator or a navigator was the point.
The rest of the world failed to institute an airways system that would permit high speed airliner flight, and consequently failed to build, high flying, high speed, pressurised airliners until the 1950s. They fell far, far behind the U.S. airline industry and airliner manufacturers in consequence.
From 1936 until after the classic era aviation in the U.S. was heavily regulated, by award of government contracts with strings attached, or after formation of the CAA / CAB / FAA by law. The aircraft flew from beacon to beacon, transitioned into published approach procedures, knew where they were all the time and consequently hardly anybody died in the process. Making up ad hoc direct routes, flying slowly at low level in all the turbulence and precipitation under the clouds, or at night, and colliding with the terrain was not the American way.
Sure there were a few airlines who ignored the huge taxpayer subsidies on offer, and the superior operating economics of flying at high level, and who scheduled 7 hour flights for 10 hours, but they either learned to be smarter or went broke. Of course today there are old propliners around whose pressure hulls are so fatigued that they cannot be pressurised. They are only good for slowly hauling low priority cargo or fighting fires at low level.
Normal airline flying within the US continued throughout the war. There were 16 US airlines in 1941 and 15 in 1945. Western had taken over Inland Air Lines. US airline passenger miles were 4% up in 1942. IFR rated airline pilots were needed to pilot airliners, at night, in all weathers, into Africa, Iceland and Alaska, around the clock. Novices would be trained to fly military aircraft in a VFR environment.
As the war progressed passengers on US airlines were increasingly travelling on government business, and over ever increasing distances. By 1945 US airline passenger miles stood at 246% of the 1941 level even though the number of passengers carried was almost identical. The DC-3 fleet was averaging 10 hours flying per day, and load factor was around 90% compared to 60% in 1941. There were fewer airliners flying even faster and higher as C-53s replaced DC-3s.
Increasingly the government insisted that scheduled flights carrying government personnel be conducted on airways. They did not care whether the plan was VFR or IFR. Collision with other aircraft was a tiny risk. Dozens of mail plane pilots, and airliner occupants, had been killed by trying to fly direct, under the weather. They lost track of their position and collided with terrain. Controlled flight into terrain remains the single greatest cause of death in airline flying. Flying the Ranges reduced the death toll to zero. If you tracked the on course signal from one to the next you and your passengers stayed alive because the on course signal went between the mountains and not into them.
When the war came to a close thousands of nearly new C-53s and C-47s recently purchased by the US tax payer for over 110,000 USD each were sold to the US airlines for around 14,000 USD each. In 1949 if you wanted to start an airline you could buy one nearly new DC-6, or more than forty nearly new C-47s, for the same price. You didn't fly that DC-6 around at DC-3 flight levels suffering DC-3 economics, but unlike any other pre war airliner the DC-3 was designed to operate in the modern ATC airways environment and so it endured long after all others disappeared.
Which brings us neatly back to ATC. The 1940s did not happen on a different planet. The US airways and ATC system of the 1940s were the modern system in use everywhere today. VORs had not yet replaced Radio Ranges but the NDBs were in place. I explained how to use 2 x VOR receivers in FS9 to simulate flying the g Radio Ranges at length in a recent post here. There was no radar and no DME in the Forties. Turn the DME receiver off. Turn the FS9 ATC and its radar vectors off. Fly the published procedures, unless you want to simulate the deregulated modern system of 2004, with flight following, area navigation and radar vectors.
Believe it or not there are days when DME and radar are off in 2004. There are places where they have not yet been installed. On those days and in those places the 2004 ATC system is the 1940 system. You don't have to wonder what it was like, you don't have to make up any routes or procedures, you just have to download the real ones from the FAA for free, because they are still in use every day.
The only thing about the 1940s system that I did not explain in detail in my last post here was how to transition from the airway to the initial approach fix and how to fly the procedural (non radar) NDB approach which was the only option in the 1940s. Since these are still in use there are books you can buy, and websites you can visit, which explain that in detail, but they are aimed at qualified pilots who are obtaining further instrument qualifications and take quite a lot for granted.
If Tom is willing to host a download of about 300Kb on his website I will provide a tutorial aimed at FS9 users which references an approach plate included in the download. Once you understand one such transition and procedural approach fully, and slowly train yourself to fly it, you can start to use FS9 as a flight simulator rather then a pilot role playing game. FS9 is a flight simulator, not a game. You can fly real 4D flight plans, appropriate to a particular aircraft type, and use the real procedures, in real time, rather then making them up and pretending. It takes some effort, but then there is no reward without effort.
Some of the NDBs extant in 1940 have been removed in real life and are not present in FS9. As Arch has explained you can add them back, but you would need to invent an approach procedure to use, so the place to start is with real 1940 procedures that are still in use today, when the radar is off for maintenance, or has never been installed.
FSAviator June 2004