Post by volkerboehme on Dec 29, 2011 4:04:45 GMT -5
Hi,
this is a post from a thread originally posted at the SOH. The orignal link is here:
www.sim-outhouse.com/sohforums/showthread.php?60721-WEIGHT-WAIT-WEIGHT!-IN-FS2004
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Greetings Vonernsk,
Yes you need to take Zero Fuel Weight (ZFW) and Maximum Structural Payload (MSP) into account in FS9 if you hope to have a realistic experience on short haul flights. Depending on the type of aircraft a flight of 3000 miles and eleven hours can be a short haul flight. the flight must be trated as short haul if Maximum Landing Weight (MLW), or ZFW restrict payload.
The weights you ask about are exactly what they say they are.
Once the aeroplane reaches ZFW you must load only fuel, you cannot load payload, or crew, or catering, or booze, or anything else. However some applicable aircraft have an exemption allowing water methanol coolant to be loaded as part of the 'Fuel Weight', but in others it must be included in the Zero Fuel Weight.
In some cases ZFW is what limits the maximum payload you can carry on a 'short haul' flight, but you must never load more than the MSP regardless. Different airlines equip the same airliner differently and they may have very different empty equipped weights (EEW). With many seats in all economy class, toilets and galleys the difference between EEW and ZFW may be modest and much less than the MSP, in which case ZFW will be payload limiting. But if a decade later the same aeroplane hauls cargo EEW + MPS may be less than ZFW. The airline still must not load more payload than MPS, even if more payload does not infringe ZFW.
This arises because there is a complex safety interaction between ZFW and Maximum Landing Weight (MLW). Essentially the regulatory authority sets ZFW so that employers cannot coerce their aircrew employees into squeezing down diversion and holding fuel in order to load extra payload when total payload is below MSP. The owner / employer has no discretion. The captain must legally insist that all weight above ZFW is fuel, and that is how passengers are protected from employers, or aircraft owners, who might put profit before safety.
If you ignore ZFW and MSP you will end up flying 'short hauls' in FS9 at weights much higher than are allowed in real life and what you experience will be unrealistic. Here is a worked example from my L-1649A 'read before flight' text.
BEGIN EXTRACT >>>>>>>>>
**************************
L-1649A Read Before Flight
**************************
The Lockheed L-1649A Starliner was one of the most complicated propliners ever created. In was the last of the great dinosaurs weighing in a full 15,000lbs heavier than the Boeing 377 or Douglas DC-7C. To allow it to fly at all it needed a wing of exceptional span and aspect ratio. The spar of that extended wing was weak. It could depart with a great deal of fuel aboard, enough for San Francisco direct London which is around 4700 miles, but it could not land with significant fuel remaining. The bending moment on the main spar at the moment of touchdown was too great.
Whenever we fly the L-1649A fuel planning is essential. It has the potential to depart at 160,000lbs, yet it must land at no more than 123,000lbs. Of course if we fly KSFO - EGLL we will use over 55,000lbs of fuel and we will be down to around 105,000lbs before we land, but shorter distances require careful planning. The TWA London service began in Los Angeles. We cannot depart Los Angeles at 160,000lbs since we must be down to 123,000lbs when we begin the approach to San Francisco.
KLAX - KSFO is only 300 miles. We must fuel plan accordingly. Fuel planning is explained in detail within Part 6 of the Propliner Tutorial available from www.calclassic.com/tutorials. Our holding reserve will be 45 minutes, our diversion reserve will be 45 minutes, and our headwind reserve will be 15%. To discover the route fuel required we consult the L-1649A handling notes;
***********************************
Normal Cruise:
''''
MAP = 37 inches
RPM = 2200
''''
PLAN 3200 PPH
Note: - Yields 290 KTAS at FL220
When WEIGHT <= 123,000lbs
Begin ECON CRUISE
***********************************
On any short haul flight we intend to land at 123,000lbs so we will never call for econ cruise on a short haul flight. Normal cruise must be planned at 3200 PPH and 290 KTAS nil wind. The Propliner Tutorial explains why we will never reach 290 KTAS and why that is irrelevant.
The route fuel is 300 miles @ 290 KTAS @ 3200 PPH = 3300lbs
Headwind reserve = 3300 * 15% = 500lbs
Holding and diversion reserve (0.75 + 0.75) * 3200 = 4800lbs
So we will load only 8600lbs of AVGAS for KLAX - KSFO regardless of our payload.
How much payload can we carry?
Although we load fuel reserves for every flight we hope we will not use them. We plan to land with them still aboard. When we begin the approach at 123,000lbs we hope to have all 5,300lbs of reserve fuel still in the tanks so that we can use it as reserve fuel over and over again.
We consult the aircraft.cfg to discover our empty weight.
empty_weight=94700 ;APS inc galley, toilets, catering etc
Now we add the fuel we hope to land with. 94700 + 5300 = 100,000lbs.
Since maximum take off weight (MTOW) is 160,000lbs it appears we could add 60,000lbs of payload, but this is false. To protect the main spar payload must never exceed MTOW minus zero fuel weight (ZFW). We consult the aircraft.cfg again.
ZFW = 116000
So maximum payload (including any crew and bags) is 160000 - 116,000 = 44,000lbs
We must never depart KLAX for SFO with more than 44,000lbs of payload or more than 8600lbs of fuel or we will fracture the main spar when we land at KSFO. Our departure weight from KLAX cannot exceed 94700 + 44000 + 8600 = 147,300lbs and anyway our fuel (loaded in the wings and presenting the maximum threat to the main spar) must never exceed 8600lbs on a 300 mile trip.
Trips this short in a propliner always require very careful fuel planning or we will end the flight flying the approach at a dangerous weight. What makes the L-1649A special is the fact that it was designed to fly 4700 miles non stop and *any trip under 3000 miles is a short haul* which prevents us from loading full fuel! We would be dangerously heavy on the approach and would fracture the main spar on landing. Since we will usually intend to fly less than 3000 miles (less than 11 hours) in FS9 we must always conduct careful fuel planning before we fly the L1649A in FS9.
It should come as no surprise that it makes a huge difference to performance whether we depart with 8,600lbs of fuel for KSFO or 59,000lbs of fuel for EGLL. We must not climb to an altitude that we cannot sustain in normal cruise power. We must use the technique described in the handling notes to ensure that we do not climb above our operational ceiling, however light or heavy we depart.
***********************************
Climb Power:
Plan 3600 PPH
COWL FLAPS = CLIMB
MAP = 40 inches
RPM = 2500
VSI = 500
IAS will increase then decay
WHEN IAS DECAYS THROUGH 175 KIAS
Begin NORMAL CRUISE and step climb
see www.calclassic.com/tutorials
>>>>>>>>>>>>>>>>>>>>>>>>>>
*POLAR FLIGHTS ONLY*
Reaching FL150/160 (eastbound/westbound)
Begin ECON CRUISE and step climbs
***********************************
END EXCTRACT>>>>>>>>>>>
It is operational ceiling which determine velocity (TAS) whether we employ a constant power cruise technique, or a constant drag (IAS) cruise technique, (and jetliners may insted use a constant Mach technique). If we fail to limit payload, or we load unnecessary fuel, our induced drag rises in parallel and our TAS is limited both for that reason and because our operational ceiling is limited by being overweight. It becomes impossible to replicate real world flight plan schedules, or realistic flight plan cruising levels, if we operate over weight because we become trapped at low altitude, crawling along in thick air.
In the real example above if you ignore ZFW you may load 16,000lbs (80 passengers plus bags) of illegal payload !
Then, having restricted payload versus ZFW, if we fail to fuel plan and load excessive fuel versus MLW we will be massively overweight during the approach, which will require a very high IAS approach, which at a different destination would render the landing runway too short, because when we are massively overweight we must land too fast to stop safely since momentum = mass * velocity squared. The runway length reuired to land is an exponential function of our mass on approach and so we must plan our approach weight carefully, taking into account both MLW and ZFW when hauling virtual passengers, or perhaps instead MLW amd MSP when hauling virtual cargo.
Many FS9 users fail to plan approach weight and operate many / most flights in an illegally overweight condition, trapped at low altitude and therefore at low velocity (TAS).
Here is a further extract, this time from the 2008 Calclassic Propliner Tutorial, which explains how to evaluate ZFW if it is not known for a particular aeroplane. Some handling notes have been revised at Calclassic.com since publication of the 2008 PT, but the principle is illustrated. This extract refers back to earlier sections on holding and diversion fuel planning which are not replicated below.
BEGIN EXTRACT >>>>>>>>>>>>>>>>>
PAYLOAD PLANNING - ZERO FUEL WEIGHT
The parameter that limits payload in real life is zero fuel weight (ZFW). This is the maximum weight to which the aircraft may be loaded before fuel is introduced to the fuel tanks. It is set by the certification authority mostly by reference to the strength of the wing spar. For obvious reasons ZFW is always less than the Maximum Landing Weight (MLW) since even when carrying maximum payload an airliner always lands with some fuel remaining.
The difference between ZFW and MLW correlates to the maximum fuel that the captain may specify as the holding and diversion fuel. When carrying maximum payload, if the aircraft neither holds nor diverts, it may land with all that fuel still aboard, but no more. Although other factors intervene the certification authority will usually set ZFW around 94% of MLW allowing the captain to carry around 6% of the maximum landing weight as holding and diversion fuel, even when airline management require the aircraft to carry its maximum payload.
For the purposes of flight simulation we normally gloss over ZFW, but pay careful attention to MLW. I normally state MLW in the aircraft.cfg and if the relationship between MLW and ZFW was abnormal I usually state ZFW too. I always state MLW in the handling notes or the aircraft.cfg. For instance the relevant section of the DC-6B handling notes says;
******************
REDUCE < 120 KIAS
FLAP - STAGE 5
Cross airfield boundary 105 KIAS (@ 88,000lbs)
FLARE and LAND
******************
It is the 88,000lb MLW that determines how many passengers can be aboard a DC-6B. The Maximum Take Off Weight (MTOW) only determines how much fuel may be loaded in addition. Consequently MTOW is the variable that controls maximum range. However odd it may seem MTOW has no relevance to maximum payload at all.
Cabin volume is irrelevant as well since it does not predict the strength of the wing main spar which is what limits MLW. The shock of (a heavy) landing is what matters. Taking off imposes little load on the wing spar. MTOW (fuel load and range) are limited by horsepower to lift the fuel whilst MLW (payload) is limited only by structural weakness.
AIRCRAFT PREPARED FOR SERVICE WEIGHT
Now at this point we must remember that Empty Weight (EW) does NOT include the crew, their bags, hundreds of pounds of oil in the oil tanks, the galley, catering, and so on. Nor does it even include the seats, the number of which varies over time. The weight that includes all these extras is the Aircraft Prepared for Service (APS) weight. The difference between EW and APS is lost on Microsoft, but I always set the empty weight variable = APS to correct their error. Accordingly when planning a simulated flight; if ZFW is not declared within the download, assume that the maximum payload on any flight must not exceed;
Max Payload = (MLW * 0.94) - APS
If the flight dynamics are realistic the empty_weight variable within the aircraft.cfg will have been set equal to the APS. Reducing fuel below the maximum does not allow an aircraft to carry more than its maximum payload.
ZFW is a certification limit. APS is an airline variable. Some airlines may offer no catering. Some may have lightweight seats. However a typical APS weight for a DC-6B in all economy class is 64,300lbs.
EXAMPLE - DC6B
So if ZFW is typically 0.94 * MLW then ZFW for a DC-6B may be taken to be 88,000 * 0.94 = 82700 thus permitting us to land a fully loaded DC-6B with up to 5300lbs of fuel remaining.
We have just examined why the holding and diversion fuel is always 3128 pounds so allowing 5300 pounds may seem excessive, but of course we must also load a headwind reserve and on some flights we may use none of it!
*The certification authority has ensured that we can never be pressured by airline management to substitute payload for any part of that 5,300 pound reserve fuel allowance.*
Consequently the max payload for a DC-6B with all economy seating is about;
82,700 (ZFW) - 64,300 (APS) = 18,400 (max payload)
By law (during most of the classic era) an airline had to assume that every unseen passenger to whom they sold a ticket weighed 170lbs and they had to calculate the baggage allowance for the flight accordingly. If a particular airline allowed thirty pounds of baggage per passenger the payload seat divisor was therefore 200 pounds.
18400/200 = 92 passenger seats maximum
However if a different airline offers a DC-6B walk on shuttle service specifying only hand baggage which may not exceed ten pounds then the divisor is 180.
18400/180 = 102 passengers maximum
If another airline offers no food and no drinks and completely removes the galley the APS might go down by 900 pounds and the maximum number of passengers allowed to board a DC-6B walk on, no frills, shuttle service might reach 107, which was I believe the largest number of passenger seats that was ever fitted to a DC-6B in commercial airline service within the U.S.
When we need to know the maximum number of passengers that can be loaded *with maximum fuel aboard* we have look in the weight and balance section of a realistic aircraft.cfg where it is always stated in plain English. The DC-6B aircraft.cfg discloses that the maximum payload falls to exactly 10,000lbs with maximum fuel. Again assuming a 30lb per passenger baggage allowance this equates to a maximum of just 50 passengers.
How many passengers a DC-6B can carry depends on what we intend to use it for, but all the information required to work out any of the permutations (real or simulated) is always stated in the relevant aircraft.cfg. If the flight dynamics are realistic it is all there waiting to be read and understood.
EXAMPLE - DC6
The DC-6 was a different and earlier aircraft delivered in two distinct versions, intercontinental and international with many sub variants of those two types. We must ensure that we use the correct aircraft.cfg. The international.cfg explains that payload with full fuel equates to a maximum of 47 passenger seats. Hardly fewer than the larger DC-6B, but the DC-6 has a much lower MTOW, and therefore much less range. Final DC-6 deliveries had an MLW of 85,000lbs, as stated in the handling notes, but most were limited to 80,000lbs with a ZFW of only 74,000lbs and DC-6 APS in all economy configuration was typically 57,500lbs.
74,000 - 57,500 = 16,500lbs and assuming 30lb bag allowance = 82 passenger seats max for short haul.
In practice at least one airline dumped the galley plus catering and fitted 86 seats, probably the largest number fitted to a DC-6 in commercial airline service within the U.S.
EXAMPLE - DC7C/F
As the relevant aircraft.cfg explains the DC-7C was limited to a maximum of only 44 passengers with full fuel for long range operations. Later it had MLW = 111,000 but ZFW was still only 101,500lbs whilst typical APS in all economy seating was 82,000lbs.
101,500 - 82,000 = 19,500 which divided by 200 = 97 seats max for short haul.
Again in practice one airline ripped out the galley and managed to fit 105 seats. The cabin of the DC-7C is bigger than the cabin of the DC-6B, yet it cannot be fitted with as many passenger seats.
So when first introduced the DC-7C typically had fewer seats than the very much smaller DC-6 despite having a much larger cabin. Later after relegation to shorter hauls the DC-7C had far more seats than the typical remaining DC-6. Despite this the DC-7C was restricted to fewer seats than the smaller, much cheaper to operate DC-6B because the DC-7C ZFW and MLW were more restrictive. That is one reason that the older DC-6B was still flying from every major airfield after all the DC-7Cs were rotting and unloved parked around the perimeter.
Again all the relevant information is in each aircraft.cfg and realistic ones usually also provide the non default data we need to substitute within the default aircraft.cfg to turn e.g. a DC-7C into a DC-7CF so that we can fly and land the cargo conversion with a realistic payload, at realistic weights, to discover the difference in performance and handling.
LONG HAUL v SHORT HAUL
So a DC-6B when new and long hauling typically had just under 50 seats and a DC-6 just under 47, but late in life when only short hauling many of the former had almost 107 whilst the latter had no more than 86. The cabin size has little bearing on how many seats may be fitted. Payload is all about the ZFW, which in turn relates to the MLW, which in turn correlates to the strength of the wing spar.
The average number of seats fitted over the lifetime of each of these aircraft, in all parts of the world, would approximate the average of the values above and so;
DC-6 = 61 passenger seats on average
DC-7C = 74 passenger seats on average
DC-6B = 78 passenger seats on average
However the average number of seats over a long time and the typical seating are not the same thing at all since there was a tendency to flip flop from long range operations with minimum seating plus maximum fuel to short range operations with maximum seating and minimum fuel. A DC-7C tended to have about 44 seats or about 97 seats rather than the average of 74.
During flight planning we must ensure that the payload does not exceed the maximum payload, regardless of how little fuel we plan. However the default payload should be compatible with any fuel load. Provided we fuel plan and payload plan we never risk flying an approach at destination in excess of Maximum Landing Weight.
END EXTRACT>>>>>>>
Within the aircraft.cfg, MSFS *requires* declaration of maximum fuel, with payload compliantly limited to not exceed MTOW by default, but that load is illegal and unsafe for many routes. Consumers of MSFS software are reponsible for both payload limitation and fuel limitation, using the 2008 Propliner Tutorial, (else equivalant or real flight planning document), then using the data in the supplied on screen handling notes, to adjust both values until compliant with real world legislation. This must be done using the payload and fuel menu of FS9; (not in the aircraft.cfg). That is the only way flight simulation enthusiasts can hope to replicate real fight plan profiles and schedules for large commercial aircraft, or hope to operate them from the real runway lengths.
As indicated above the distribution of payload and fuel differed greatly over time and airline by airline, but always bounded by ZFW, MSP and MLW. The full 2008 Propliner Tutorial is available from www.calclassic.com/tutorials. It is optimised for keyword and single topic searching. Some elemants do not apply to turbojet and turbofan propulsion, or swept wing aerodynamics, but compliant payload and fuel planning, or the way we are forced to fuel plan for, (unforecast and unforecastable), headwinds within MSFS does not differ.
FSAviator
________________
Greetings vonernsk,
<<Thank you for a very comprehensive and straightforward explanation>>
You are welcome. This topic deserves a good airing in flight sim forums from time to time.
<<Is it correct to consider that the Max Structural Payload (MSP) for a particular a/c is a calculated safety percentage more than the a/c's designated Max Lndg Wt (MLW)? ie ensuring that you always land at or below MLW will mean that you will never breach MSP? >>
No.
To understand why you must focus on my explanation that ZFW is, (and always has been), a safety related value which is essentially a 'political' decision, and is not a 'scientific' or 'engineering' value. A regulatory authority, not Newtonian physics. imposes the gap between ZFW and MLW. All of that gap can be reserve fuel without limiting payload and the owner / employer cannot force the captain of the aeroplane to substitute payload, even if they can prevent that captain from loading all that mass of fuel as a reserve. Because the employer cannot substitute payload they lose most of their profit incentive to intimidate the captain of the aeroplane if he decides to load reserve fuel equal to MLW minus ZFW on a particular flight.
<<ie ensuring that you always land at or below MLW will mean that you will never breach MSP?>>
No.
Aeroplanes often / usually land with less fuel mass remaining than MLW - ZFW. They land with all that fuel (or more) only if;
a) the captain insisted on loading the maximum reserve fuel that does not cause reduction of lawful payload, or reserve fuel that did cause offloading of payload, *and*
b) none of that reserve fuel (that does not cause reduction of lawful payload) was actually used.
Neither proposition is normal.
So for the L.1649A
MLW = 123,000 pounds
ZFW = 116,000 pounds
The captain can load 7,000 pounds of reserve fuel without bumping any passengers or cargo from the flight (subject to a long enough runway in use and large enough headwind vector down that runway).
Sometimes the L1649A captain will not wish to load that large a fuel reserve. In the absence of actual or forecast fog or low cloud at KSFO he will not insist on 7,000 lbs of reserve fuel on the sector from KLAX to KSFO even if he insists on having much more than 7,000lbs of reserve fuel, causing loss of payload, for the more than 20 hour haul from KSFO to EGLL, which requires a huge headwind reserve.
Landing at KSFO at 122,000lbs, (a thousand pounds less than MLW), in no way ensures that MSP was not breached. The whole point of ZFW is that the employer cannot force the captain to load only 3,000 pounds of reserve fuel and an extra 4,000 pounds of payload. OEW plus *everything that is not fuel* must not exceed ZFW. MLW never has any relevance to calculation of maximum payload and conversly ZFW never has any relevance to calculation of maximum fuel.
Read the provided worked example until you understand why maximum fuel loaded can be constrained by either MTOW or MLW, but maximum payload is constrained by ZFW.
That is why I began my reply by saying;
<<Yes you need to take Zero Fuel Weight (ZFW).... into account........>>
You cannot use MLW to calculate maximum lawful payload. You must use ZFW. *Even if you do not load all of the reserve fuel that is compatible with no reduction of maximum lawful payload*.
That is why I explained that if ZFW is not known for a particular aeroplane consumers should calculate ZFW as MLW * 0.94. That 6% of MLW cannot be payload under any circumstance. It can only be fuel we did not use, fuel we used, or fuel we did not load.
In the included worked L1649A example earlier in this thread we decided to load only 5,300lbs of fuel as a reserve for the KLAX - KSFO sector. MLW is 123,000 and if we use none of that reserve fuel we will be commencing the approach at 123000 - (7000 + 5300) lbs = 121,300lbs. We are not allowed to load payload in lieu of the extra reserve fuel we did not load, and we must not commence the approach at 123,000lbs, or even 122,000lbs, nor even 121,500lbs, having loaded more than the lawful maximum 44,000lbs of payload, just because we restricted our reserve fuel.
In practice our maximum approach weight on a short haul flight, depends on ZFW not MLW. Loading the maximum fuel reserve (that does not require us to bump passengers) is not 'normal' on short haul flights.
This may sound like incredibly detailed stuff of no consequence to most MSFS consumers, but they must grasp failing to use ZFW when calculating maximum lawful approach weight causes many / most MSFS consumers to make overweight approaches, after entire overweight flights, during which they could not reach realistic cruising levels, and therefore also could not achieve realistic TAS. Using MLW in our calculation of maximum lawful payload may cause us to operate up to 6% heavier than is lawful, and that is easily enough to constrain available cruising level to a deficient level with deficient TAS, because both our mass and our drag will be up to 6% higher than is lawful.
There is no point consumers demanding flight dynamics within which the errors are less than 6% if they take no care to avoid such consumer / pilot / captaincy errors themselves after downloading. Many consumers take no care at all, and operate aeroplanes at random (over) weights, with random (excess) drag and yet they wonder why they seem to under perform. To experience realistic performance, realistic cruising levels, realistic TAS, and to take off and land successfully on the real runways, consumers must take the time to plan approach weight to be no more than the lawful maximum in real life and MLW is not the only constraint. Constraining approach weight, constrains prior cruise weight and prior take off weight. Realism throughout the prior flight depends on consumer imposition of lawful approach weight.
>
Flight simulation is the demonstration of the operation of a particular real aeroplane, in compliance with all real world legislation and procedures, but in a virtual environment. That is a complex process in many ways. Not least because what constituted full compliance has differed greatly during the history of aviation. This is an FS9 forum where we discuss simulation of a 'century of flight'.
In relation to international law as it stands today the situation is as cited by Volker. Maximum Structural Payload (MSP) is simply ZFW minus OEW and not as cited in my first post relating to the classic phase of aviation history. However it is my understanding that MSP was originally defined differently in international law and could change depending on whether a particular airframe which originally had a 'passenger floor' was re-manufactured with a 'freight floor'.
If in that re-manufacturing process no modifications were made to the wing spar or landing gear, neither MLW nor ZFW would alter. It is my understanding that earlier in the history of aviation the operator concerned might then be able to load a larger payload after fitting a freight floor, but might not be able to load what was then defined as the 'Maximum Structural Payload' (with a freight floor able to bear more payload weight per square foot), because in the past OEW + MSP (as then defined) might exceed ZFW.
It therefore seems to me on reflection that the basic rule during flight simulation (only) is to ignore MSP and to make all calculations using ZFW and MLW, and where we lack knowledge of the real value of ZFW, to set ZFW at MLW * 0.94 in our calculations, even though the gap is not always 6% in real life for several reasons.
Note however that 'light' aircraft usually have ZFW = MLW = MTOW. Of course we can't actually go anywhere with no fuel. 'Fighters' fall into that category, but many bombers and maritime patrol aircraft have MLW far below MTOW and 'may' have a defined ZFW around 6% below MLW. There is no way to calculate or estimate MLW from MTOW, since the former depends on structural fragility, and the latter on power available. Lawful payload is constrained by ZFW. Approach weight is constrained by MLW in theory but sometimes by ZFW in practice (as above). Lawful flight plan range (or combat radius) is instead constrained by MTOW.
If we fail to plan our approach weight to be lawful we cannot expect our flight simulation experience to be realistic during or before that approach. For most consumers the issue has never been how realistic the FD are. The problem for most FS consumers is non compliant operation of the virtual aircraft they procure, and the lack of will to learn compliant operation, in order to experience realism.
>
<<To clarify about zero fuel weight, the idea is to distribute the weight of the plane along the wings as weight (fuel) to reduce the bending moments on the wing structure. In the 747 for example, past a certain point fuel is placed in the center tank, as this is weight located in the fuselage, it reduces the available zero fuel weight. >>
No.
You can lawfully land your real B747 with no payload, but with a huge quantity of fuel remaining and still land below MLW, while above ZFW. Consequently the certification needs to impose additional restrictions on the maximum and minimum mass of *fuel* that can be present in particular tanks, to control spanwise distribution of fuel mass, to prevent fracturing of the main spar and may, or may not, also impose a maximum total fuel remaining value for landing.
As you will realise on reflection, ZERO FUEL weight does not relate to *fuel* present or absent in any way at all, let alone its distribution. The (Maximum) ZERO FUEL weight specified in the certification is exactly the opposite. It exists explicitly to limit the mass (only) of everything present that is *not fuel*. Hence its name.
This does however help to illustrate how easy it is to confuse what limits what during calculation of lawful approach weight.
>
Regardless of all that detail, the bottom line is that both the performance and handling of real and simulated aeroplanes depends significantly on their current weight. Most MSFS consumers fail to constrain aircraft operating weight to realistic (i.e. lawful) values, and then suffer mathematically consequential deterioration of both performance and handling, that the real pilot does not experience, or need to cope with.
This cannot be remedied by Microsoft, or third party producers. It can only be remedied by consumers increasing their knowledge base and flight planning skills. The more realistic the payload and fuel planning undertaken by the consumer, the more closely that consumer's flight simulation experience approximates reality. The larger the aeroplane whose operation is being simulated, the larger the realism gain from realistic approach weight planning.
Simply dumping the fuel before the approach to comply with MLW does not address the lack of realism that was imposed by the consumer throughout the prior flight. Unnecessary % weight imposes *equal* unnecessary % drag, and the consumer then suffers two enduring penalties throughout the flight from a single earlier captaincy (consumer) error.
Flight simulation realism is not something anyone can download. It is a painstaking acquired skill that can only be obtained by reading and understanding real payware training manuals, else freeware on line tutorials aimed explicitly at (FS9) consumers, and which do not assume prior aviation qualifications. Most of the required knowledge base, (even for compliant use of FS9), is nevertheless too complicated, extensive and 'painstaking' to be suitable content for 'forums'. However some available tutorials may explicitly address specific phases of aviation history within which operating and / or flight planning compliance differed from the modern era, and also differed much more, nation by nation.
FSAviator.
this is a post from a thread originally posted at the SOH. The orignal link is here:
www.sim-outhouse.com/sohforums/showthread.php?60721-WEIGHT-WAIT-WEIGHT!-IN-FS2004
__________________________________
Greetings Vonernsk,
Yes you need to take Zero Fuel Weight (ZFW) and Maximum Structural Payload (MSP) into account in FS9 if you hope to have a realistic experience on short haul flights. Depending on the type of aircraft a flight of 3000 miles and eleven hours can be a short haul flight. the flight must be trated as short haul if Maximum Landing Weight (MLW), or ZFW restrict payload.
The weights you ask about are exactly what they say they are.
Once the aeroplane reaches ZFW you must load only fuel, you cannot load payload, or crew, or catering, or booze, or anything else. However some applicable aircraft have an exemption allowing water methanol coolant to be loaded as part of the 'Fuel Weight', but in others it must be included in the Zero Fuel Weight.
In some cases ZFW is what limits the maximum payload you can carry on a 'short haul' flight, but you must never load more than the MSP regardless. Different airlines equip the same airliner differently and they may have very different empty equipped weights (EEW). With many seats in all economy class, toilets and galleys the difference between EEW and ZFW may be modest and much less than the MSP, in which case ZFW will be payload limiting. But if a decade later the same aeroplane hauls cargo EEW + MPS may be less than ZFW. The airline still must not load more payload than MPS, even if more payload does not infringe ZFW.
This arises because there is a complex safety interaction between ZFW and Maximum Landing Weight (MLW). Essentially the regulatory authority sets ZFW so that employers cannot coerce their aircrew employees into squeezing down diversion and holding fuel in order to load extra payload when total payload is below MSP. The owner / employer has no discretion. The captain must legally insist that all weight above ZFW is fuel, and that is how passengers are protected from employers, or aircraft owners, who might put profit before safety.
If you ignore ZFW and MSP you will end up flying 'short hauls' in FS9 at weights much higher than are allowed in real life and what you experience will be unrealistic. Here is a worked example from my L-1649A 'read before flight' text.
BEGIN EXTRACT >>>>>>>>>
**************************
L-1649A Read Before Flight
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The Lockheed L-1649A Starliner was one of the most complicated propliners ever created. In was the last of the great dinosaurs weighing in a full 15,000lbs heavier than the Boeing 377 or Douglas DC-7C. To allow it to fly at all it needed a wing of exceptional span and aspect ratio. The spar of that extended wing was weak. It could depart with a great deal of fuel aboard, enough for San Francisco direct London which is around 4700 miles, but it could not land with significant fuel remaining. The bending moment on the main spar at the moment of touchdown was too great.
Whenever we fly the L-1649A fuel planning is essential. It has the potential to depart at 160,000lbs, yet it must land at no more than 123,000lbs. Of course if we fly KSFO - EGLL we will use over 55,000lbs of fuel and we will be down to around 105,000lbs before we land, but shorter distances require careful planning. The TWA London service began in Los Angeles. We cannot depart Los Angeles at 160,000lbs since we must be down to 123,000lbs when we begin the approach to San Francisco.
KLAX - KSFO is only 300 miles. We must fuel plan accordingly. Fuel planning is explained in detail within Part 6 of the Propliner Tutorial available from www.calclassic.com/tutorials. Our holding reserve will be 45 minutes, our diversion reserve will be 45 minutes, and our headwind reserve will be 15%. To discover the route fuel required we consult the L-1649A handling notes;
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Normal Cruise:
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MAP = 37 inches
RPM = 2200
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PLAN 3200 PPH
Note: - Yields 290 KTAS at FL220
When WEIGHT <= 123,000lbs
Begin ECON CRUISE
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On any short haul flight we intend to land at 123,000lbs so we will never call for econ cruise on a short haul flight. Normal cruise must be planned at 3200 PPH and 290 KTAS nil wind. The Propliner Tutorial explains why we will never reach 290 KTAS and why that is irrelevant.
The route fuel is 300 miles @ 290 KTAS @ 3200 PPH = 3300lbs
Headwind reserve = 3300 * 15% = 500lbs
Holding and diversion reserve (0.75 + 0.75) * 3200 = 4800lbs
So we will load only 8600lbs of AVGAS for KLAX - KSFO regardless of our payload.
How much payload can we carry?
Although we load fuel reserves for every flight we hope we will not use them. We plan to land with them still aboard. When we begin the approach at 123,000lbs we hope to have all 5,300lbs of reserve fuel still in the tanks so that we can use it as reserve fuel over and over again.
We consult the aircraft.cfg to discover our empty weight.
empty_weight=94700 ;APS inc galley, toilets, catering etc
Now we add the fuel we hope to land with. 94700 + 5300 = 100,000lbs.
Since maximum take off weight (MTOW) is 160,000lbs it appears we could add 60,000lbs of payload, but this is false. To protect the main spar payload must never exceed MTOW minus zero fuel weight (ZFW). We consult the aircraft.cfg again.
ZFW = 116000
So maximum payload (including any crew and bags) is 160000 - 116,000 = 44,000lbs
We must never depart KLAX for SFO with more than 44,000lbs of payload or more than 8600lbs of fuel or we will fracture the main spar when we land at KSFO. Our departure weight from KLAX cannot exceed 94700 + 44000 + 8600 = 147,300lbs and anyway our fuel (loaded in the wings and presenting the maximum threat to the main spar) must never exceed 8600lbs on a 300 mile trip.
Trips this short in a propliner always require very careful fuel planning or we will end the flight flying the approach at a dangerous weight. What makes the L-1649A special is the fact that it was designed to fly 4700 miles non stop and *any trip under 3000 miles is a short haul* which prevents us from loading full fuel! We would be dangerously heavy on the approach and would fracture the main spar on landing. Since we will usually intend to fly less than 3000 miles (less than 11 hours) in FS9 we must always conduct careful fuel planning before we fly the L1649A in FS9.
It should come as no surprise that it makes a huge difference to performance whether we depart with 8,600lbs of fuel for KSFO or 59,000lbs of fuel for EGLL. We must not climb to an altitude that we cannot sustain in normal cruise power. We must use the technique described in the handling notes to ensure that we do not climb above our operational ceiling, however light or heavy we depart.
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Climb Power:
Plan 3600 PPH
COWL FLAPS = CLIMB
MAP = 40 inches
RPM = 2500
VSI = 500
IAS will increase then decay
WHEN IAS DECAYS THROUGH 175 KIAS
Begin NORMAL CRUISE and step climb
see www.calclassic.com/tutorials
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*POLAR FLIGHTS ONLY*
Reaching FL150/160 (eastbound/westbound)
Begin ECON CRUISE and step climbs
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END EXCTRACT>>>>>>>>>>>
It is operational ceiling which determine velocity (TAS) whether we employ a constant power cruise technique, or a constant drag (IAS) cruise technique, (and jetliners may insted use a constant Mach technique). If we fail to limit payload, or we load unnecessary fuel, our induced drag rises in parallel and our TAS is limited both for that reason and because our operational ceiling is limited by being overweight. It becomes impossible to replicate real world flight plan schedules, or realistic flight plan cruising levels, if we operate over weight because we become trapped at low altitude, crawling along in thick air.
In the real example above if you ignore ZFW you may load 16,000lbs (80 passengers plus bags) of illegal payload !
Then, having restricted payload versus ZFW, if we fail to fuel plan and load excessive fuel versus MLW we will be massively overweight during the approach, which will require a very high IAS approach, which at a different destination would render the landing runway too short, because when we are massively overweight we must land too fast to stop safely since momentum = mass * velocity squared. The runway length reuired to land is an exponential function of our mass on approach and so we must plan our approach weight carefully, taking into account both MLW and ZFW when hauling virtual passengers, or perhaps instead MLW amd MSP when hauling virtual cargo.
Many FS9 users fail to plan approach weight and operate many / most flights in an illegally overweight condition, trapped at low altitude and therefore at low velocity (TAS).
Here is a further extract, this time from the 2008 Calclassic Propliner Tutorial, which explains how to evaluate ZFW if it is not known for a particular aeroplane. Some handling notes have been revised at Calclassic.com since publication of the 2008 PT, but the principle is illustrated. This extract refers back to earlier sections on holding and diversion fuel planning which are not replicated below.
BEGIN EXTRACT >>>>>>>>>>>>>>>>>
PAYLOAD PLANNING - ZERO FUEL WEIGHT
The parameter that limits payload in real life is zero fuel weight (ZFW). This is the maximum weight to which the aircraft may be loaded before fuel is introduced to the fuel tanks. It is set by the certification authority mostly by reference to the strength of the wing spar. For obvious reasons ZFW is always less than the Maximum Landing Weight (MLW) since even when carrying maximum payload an airliner always lands with some fuel remaining.
The difference between ZFW and MLW correlates to the maximum fuel that the captain may specify as the holding and diversion fuel. When carrying maximum payload, if the aircraft neither holds nor diverts, it may land with all that fuel still aboard, but no more. Although other factors intervene the certification authority will usually set ZFW around 94% of MLW allowing the captain to carry around 6% of the maximum landing weight as holding and diversion fuel, even when airline management require the aircraft to carry its maximum payload.
For the purposes of flight simulation we normally gloss over ZFW, but pay careful attention to MLW. I normally state MLW in the aircraft.cfg and if the relationship between MLW and ZFW was abnormal I usually state ZFW too. I always state MLW in the handling notes or the aircraft.cfg. For instance the relevant section of the DC-6B handling notes says;
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REDUCE < 120 KIAS
FLAP - STAGE 5
Cross airfield boundary 105 KIAS (@ 88,000lbs)
FLARE and LAND
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It is the 88,000lb MLW that determines how many passengers can be aboard a DC-6B. The Maximum Take Off Weight (MTOW) only determines how much fuel may be loaded in addition. Consequently MTOW is the variable that controls maximum range. However odd it may seem MTOW has no relevance to maximum payload at all.
Cabin volume is irrelevant as well since it does not predict the strength of the wing main spar which is what limits MLW. The shock of (a heavy) landing is what matters. Taking off imposes little load on the wing spar. MTOW (fuel load and range) are limited by horsepower to lift the fuel whilst MLW (payload) is limited only by structural weakness.
AIRCRAFT PREPARED FOR SERVICE WEIGHT
Now at this point we must remember that Empty Weight (EW) does NOT include the crew, their bags, hundreds of pounds of oil in the oil tanks, the galley, catering, and so on. Nor does it even include the seats, the number of which varies over time. The weight that includes all these extras is the Aircraft Prepared for Service (APS) weight. The difference between EW and APS is lost on Microsoft, but I always set the empty weight variable = APS to correct their error. Accordingly when planning a simulated flight; if ZFW is not declared within the download, assume that the maximum payload on any flight must not exceed;
Max Payload = (MLW * 0.94) - APS
If the flight dynamics are realistic the empty_weight variable within the aircraft.cfg will have been set equal to the APS. Reducing fuel below the maximum does not allow an aircraft to carry more than its maximum payload.
ZFW is a certification limit. APS is an airline variable. Some airlines may offer no catering. Some may have lightweight seats. However a typical APS weight for a DC-6B in all economy class is 64,300lbs.
EXAMPLE - DC6B
So if ZFW is typically 0.94 * MLW then ZFW for a DC-6B may be taken to be 88,000 * 0.94 = 82700 thus permitting us to land a fully loaded DC-6B with up to 5300lbs of fuel remaining.
We have just examined why the holding and diversion fuel is always 3128 pounds so allowing 5300 pounds may seem excessive, but of course we must also load a headwind reserve and on some flights we may use none of it!
*The certification authority has ensured that we can never be pressured by airline management to substitute payload for any part of that 5,300 pound reserve fuel allowance.*
Consequently the max payload for a DC-6B with all economy seating is about;
82,700 (ZFW) - 64,300 (APS) = 18,400 (max payload)
By law (during most of the classic era) an airline had to assume that every unseen passenger to whom they sold a ticket weighed 170lbs and they had to calculate the baggage allowance for the flight accordingly. If a particular airline allowed thirty pounds of baggage per passenger the payload seat divisor was therefore 200 pounds.
18400/200 = 92 passenger seats maximum
However if a different airline offers a DC-6B walk on shuttle service specifying only hand baggage which may not exceed ten pounds then the divisor is 180.
18400/180 = 102 passengers maximum
If another airline offers no food and no drinks and completely removes the galley the APS might go down by 900 pounds and the maximum number of passengers allowed to board a DC-6B walk on, no frills, shuttle service might reach 107, which was I believe the largest number of passenger seats that was ever fitted to a DC-6B in commercial airline service within the U.S.
When we need to know the maximum number of passengers that can be loaded *with maximum fuel aboard* we have look in the weight and balance section of a realistic aircraft.cfg where it is always stated in plain English. The DC-6B aircraft.cfg discloses that the maximum payload falls to exactly 10,000lbs with maximum fuel. Again assuming a 30lb per passenger baggage allowance this equates to a maximum of just 50 passengers.
How many passengers a DC-6B can carry depends on what we intend to use it for, but all the information required to work out any of the permutations (real or simulated) is always stated in the relevant aircraft.cfg. If the flight dynamics are realistic it is all there waiting to be read and understood.
EXAMPLE - DC6
The DC-6 was a different and earlier aircraft delivered in two distinct versions, intercontinental and international with many sub variants of those two types. We must ensure that we use the correct aircraft.cfg. The international.cfg explains that payload with full fuel equates to a maximum of 47 passenger seats. Hardly fewer than the larger DC-6B, but the DC-6 has a much lower MTOW, and therefore much less range. Final DC-6 deliveries had an MLW of 85,000lbs, as stated in the handling notes, but most were limited to 80,000lbs with a ZFW of only 74,000lbs and DC-6 APS in all economy configuration was typically 57,500lbs.
74,000 - 57,500 = 16,500lbs and assuming 30lb bag allowance = 82 passenger seats max for short haul.
In practice at least one airline dumped the galley plus catering and fitted 86 seats, probably the largest number fitted to a DC-6 in commercial airline service within the U.S.
EXAMPLE - DC7C/F
As the relevant aircraft.cfg explains the DC-7C was limited to a maximum of only 44 passengers with full fuel for long range operations. Later it had MLW = 111,000 but ZFW was still only 101,500lbs whilst typical APS in all economy seating was 82,000lbs.
101,500 - 82,000 = 19,500 which divided by 200 = 97 seats max for short haul.
Again in practice one airline ripped out the galley and managed to fit 105 seats. The cabin of the DC-7C is bigger than the cabin of the DC-6B, yet it cannot be fitted with as many passenger seats.
So when first introduced the DC-7C typically had fewer seats than the very much smaller DC-6 despite having a much larger cabin. Later after relegation to shorter hauls the DC-7C had far more seats than the typical remaining DC-6. Despite this the DC-7C was restricted to fewer seats than the smaller, much cheaper to operate DC-6B because the DC-7C ZFW and MLW were more restrictive. That is one reason that the older DC-6B was still flying from every major airfield after all the DC-7Cs were rotting and unloved parked around the perimeter.
Again all the relevant information is in each aircraft.cfg and realistic ones usually also provide the non default data we need to substitute within the default aircraft.cfg to turn e.g. a DC-7C into a DC-7CF so that we can fly and land the cargo conversion with a realistic payload, at realistic weights, to discover the difference in performance and handling.
LONG HAUL v SHORT HAUL
So a DC-6B when new and long hauling typically had just under 50 seats and a DC-6 just under 47, but late in life when only short hauling many of the former had almost 107 whilst the latter had no more than 86. The cabin size has little bearing on how many seats may be fitted. Payload is all about the ZFW, which in turn relates to the MLW, which in turn correlates to the strength of the wing spar.
The average number of seats fitted over the lifetime of each of these aircraft, in all parts of the world, would approximate the average of the values above and so;
DC-6 = 61 passenger seats on average
DC-7C = 74 passenger seats on average
DC-6B = 78 passenger seats on average
However the average number of seats over a long time and the typical seating are not the same thing at all since there was a tendency to flip flop from long range operations with minimum seating plus maximum fuel to short range operations with maximum seating and minimum fuel. A DC-7C tended to have about 44 seats or about 97 seats rather than the average of 74.
During flight planning we must ensure that the payload does not exceed the maximum payload, regardless of how little fuel we plan. However the default payload should be compatible with any fuel load. Provided we fuel plan and payload plan we never risk flying an approach at destination in excess of Maximum Landing Weight.
END EXTRACT>>>>>>>
Within the aircraft.cfg, MSFS *requires* declaration of maximum fuel, with payload compliantly limited to not exceed MTOW by default, but that load is illegal and unsafe for many routes. Consumers of MSFS software are reponsible for both payload limitation and fuel limitation, using the 2008 Propliner Tutorial, (else equivalant or real flight planning document), then using the data in the supplied on screen handling notes, to adjust both values until compliant with real world legislation. This must be done using the payload and fuel menu of FS9; (not in the aircraft.cfg). That is the only way flight simulation enthusiasts can hope to replicate real fight plan profiles and schedules for large commercial aircraft, or hope to operate them from the real runway lengths.
As indicated above the distribution of payload and fuel differed greatly over time and airline by airline, but always bounded by ZFW, MSP and MLW. The full 2008 Propliner Tutorial is available from www.calclassic.com/tutorials. It is optimised for keyword and single topic searching. Some elemants do not apply to turbojet and turbofan propulsion, or swept wing aerodynamics, but compliant payload and fuel planning, or the way we are forced to fuel plan for, (unforecast and unforecastable), headwinds within MSFS does not differ.
FSAviator
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Greetings vonernsk,
<<Thank you for a very comprehensive and straightforward explanation>>
You are welcome. This topic deserves a good airing in flight sim forums from time to time.
<<Is it correct to consider that the Max Structural Payload (MSP) for a particular a/c is a calculated safety percentage more than the a/c's designated Max Lndg Wt (MLW)? ie ensuring that you always land at or below MLW will mean that you will never breach MSP? >>
No.
To understand why you must focus on my explanation that ZFW is, (and always has been), a safety related value which is essentially a 'political' decision, and is not a 'scientific' or 'engineering' value. A regulatory authority, not Newtonian physics. imposes the gap between ZFW and MLW. All of that gap can be reserve fuel without limiting payload and the owner / employer cannot force the captain of the aeroplane to substitute payload, even if they can prevent that captain from loading all that mass of fuel as a reserve. Because the employer cannot substitute payload they lose most of their profit incentive to intimidate the captain of the aeroplane if he decides to load reserve fuel equal to MLW minus ZFW on a particular flight.
<<ie ensuring that you always land at or below MLW will mean that you will never breach MSP?>>
No.
Aeroplanes often / usually land with less fuel mass remaining than MLW - ZFW. They land with all that fuel (or more) only if;
a) the captain insisted on loading the maximum reserve fuel that does not cause reduction of lawful payload, or reserve fuel that did cause offloading of payload, *and*
b) none of that reserve fuel (that does not cause reduction of lawful payload) was actually used.
Neither proposition is normal.
So for the L.1649A
MLW = 123,000 pounds
ZFW = 116,000 pounds
The captain can load 7,000 pounds of reserve fuel without bumping any passengers or cargo from the flight (subject to a long enough runway in use and large enough headwind vector down that runway).
Sometimes the L1649A captain will not wish to load that large a fuel reserve. In the absence of actual or forecast fog or low cloud at KSFO he will not insist on 7,000 lbs of reserve fuel on the sector from KLAX to KSFO even if he insists on having much more than 7,000lbs of reserve fuel, causing loss of payload, for the more than 20 hour haul from KSFO to EGLL, which requires a huge headwind reserve.
Landing at KSFO at 122,000lbs, (a thousand pounds less than MLW), in no way ensures that MSP was not breached. The whole point of ZFW is that the employer cannot force the captain to load only 3,000 pounds of reserve fuel and an extra 4,000 pounds of payload. OEW plus *everything that is not fuel* must not exceed ZFW. MLW never has any relevance to calculation of maximum payload and conversly ZFW never has any relevance to calculation of maximum fuel.
Read the provided worked example until you understand why maximum fuel loaded can be constrained by either MTOW or MLW, but maximum payload is constrained by ZFW.
That is why I began my reply by saying;
<<Yes you need to take Zero Fuel Weight (ZFW).... into account........>>
You cannot use MLW to calculate maximum lawful payload. You must use ZFW. *Even if you do not load all of the reserve fuel that is compatible with no reduction of maximum lawful payload*.
That is why I explained that if ZFW is not known for a particular aeroplane consumers should calculate ZFW as MLW * 0.94. That 6% of MLW cannot be payload under any circumstance. It can only be fuel we did not use, fuel we used, or fuel we did not load.
In the included worked L1649A example earlier in this thread we decided to load only 5,300lbs of fuel as a reserve for the KLAX - KSFO sector. MLW is 123,000 and if we use none of that reserve fuel we will be commencing the approach at 123000 - (7000 + 5300) lbs = 121,300lbs. We are not allowed to load payload in lieu of the extra reserve fuel we did not load, and we must not commence the approach at 123,000lbs, or even 122,000lbs, nor even 121,500lbs, having loaded more than the lawful maximum 44,000lbs of payload, just because we restricted our reserve fuel.
In practice our maximum approach weight on a short haul flight, depends on ZFW not MLW. Loading the maximum fuel reserve (that does not require us to bump passengers) is not 'normal' on short haul flights.
This may sound like incredibly detailed stuff of no consequence to most MSFS consumers, but they must grasp failing to use ZFW when calculating maximum lawful approach weight causes many / most MSFS consumers to make overweight approaches, after entire overweight flights, during which they could not reach realistic cruising levels, and therefore also could not achieve realistic TAS. Using MLW in our calculation of maximum lawful payload may cause us to operate up to 6% heavier than is lawful, and that is easily enough to constrain available cruising level to a deficient level with deficient TAS, because both our mass and our drag will be up to 6% higher than is lawful.
There is no point consumers demanding flight dynamics within which the errors are less than 6% if they take no care to avoid such consumer / pilot / captaincy errors themselves after downloading. Many consumers take no care at all, and operate aeroplanes at random (over) weights, with random (excess) drag and yet they wonder why they seem to under perform. To experience realistic performance, realistic cruising levels, realistic TAS, and to take off and land successfully on the real runways, consumers must take the time to plan approach weight to be no more than the lawful maximum in real life and MLW is not the only constraint. Constraining approach weight, constrains prior cruise weight and prior take off weight. Realism throughout the prior flight depends on consumer imposition of lawful approach weight.
>
Flight simulation is the demonstration of the operation of a particular real aeroplane, in compliance with all real world legislation and procedures, but in a virtual environment. That is a complex process in many ways. Not least because what constituted full compliance has differed greatly during the history of aviation. This is an FS9 forum where we discuss simulation of a 'century of flight'.
In relation to international law as it stands today the situation is as cited by Volker. Maximum Structural Payload (MSP) is simply ZFW minus OEW and not as cited in my first post relating to the classic phase of aviation history. However it is my understanding that MSP was originally defined differently in international law and could change depending on whether a particular airframe which originally had a 'passenger floor' was re-manufactured with a 'freight floor'.
If in that re-manufacturing process no modifications were made to the wing spar or landing gear, neither MLW nor ZFW would alter. It is my understanding that earlier in the history of aviation the operator concerned might then be able to load a larger payload after fitting a freight floor, but might not be able to load what was then defined as the 'Maximum Structural Payload' (with a freight floor able to bear more payload weight per square foot), because in the past OEW + MSP (as then defined) might exceed ZFW.
It therefore seems to me on reflection that the basic rule during flight simulation (only) is to ignore MSP and to make all calculations using ZFW and MLW, and where we lack knowledge of the real value of ZFW, to set ZFW at MLW * 0.94 in our calculations, even though the gap is not always 6% in real life for several reasons.
Note however that 'light' aircraft usually have ZFW = MLW = MTOW. Of course we can't actually go anywhere with no fuel. 'Fighters' fall into that category, but many bombers and maritime patrol aircraft have MLW far below MTOW and 'may' have a defined ZFW around 6% below MLW. There is no way to calculate or estimate MLW from MTOW, since the former depends on structural fragility, and the latter on power available. Lawful payload is constrained by ZFW. Approach weight is constrained by MLW in theory but sometimes by ZFW in practice (as above). Lawful flight plan range (or combat radius) is instead constrained by MTOW.
If we fail to plan our approach weight to be lawful we cannot expect our flight simulation experience to be realistic during or before that approach. For most consumers the issue has never been how realistic the FD are. The problem for most FS consumers is non compliant operation of the virtual aircraft they procure, and the lack of will to learn compliant operation, in order to experience realism.
>
<<To clarify about zero fuel weight, the idea is to distribute the weight of the plane along the wings as weight (fuel) to reduce the bending moments on the wing structure. In the 747 for example, past a certain point fuel is placed in the center tank, as this is weight located in the fuselage, it reduces the available zero fuel weight. >>
No.
You can lawfully land your real B747 with no payload, but with a huge quantity of fuel remaining and still land below MLW, while above ZFW. Consequently the certification needs to impose additional restrictions on the maximum and minimum mass of *fuel* that can be present in particular tanks, to control spanwise distribution of fuel mass, to prevent fracturing of the main spar and may, or may not, also impose a maximum total fuel remaining value for landing.
As you will realise on reflection, ZERO FUEL weight does not relate to *fuel* present or absent in any way at all, let alone its distribution. The (Maximum) ZERO FUEL weight specified in the certification is exactly the opposite. It exists explicitly to limit the mass (only) of everything present that is *not fuel*. Hence its name.
This does however help to illustrate how easy it is to confuse what limits what during calculation of lawful approach weight.
>
Regardless of all that detail, the bottom line is that both the performance and handling of real and simulated aeroplanes depends significantly on their current weight. Most MSFS consumers fail to constrain aircraft operating weight to realistic (i.e. lawful) values, and then suffer mathematically consequential deterioration of both performance and handling, that the real pilot does not experience, or need to cope with.
This cannot be remedied by Microsoft, or third party producers. It can only be remedied by consumers increasing their knowledge base and flight planning skills. The more realistic the payload and fuel planning undertaken by the consumer, the more closely that consumer's flight simulation experience approximates reality. The larger the aeroplane whose operation is being simulated, the larger the realism gain from realistic approach weight planning.
Simply dumping the fuel before the approach to comply with MLW does not address the lack of realism that was imposed by the consumer throughout the prior flight. Unnecessary % weight imposes *equal* unnecessary % drag, and the consumer then suffers two enduring penalties throughout the flight from a single earlier captaincy (consumer) error.
Flight simulation realism is not something anyone can download. It is a painstaking acquired skill that can only be obtained by reading and understanding real payware training manuals, else freeware on line tutorials aimed explicitly at (FS9) consumers, and which do not assume prior aviation qualifications. Most of the required knowledge base, (even for compliant use of FS9), is nevertheless too complicated, extensive and 'painstaking' to be suitable content for 'forums'. However some available tutorials may explicitly address specific phases of aviation history within which operating and / or flight planning compliance differed from the modern era, and also differed much more, nation by nation.
FSAviator.