HighBypass

MD-11 weight limits for upper and lower decks?

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I'm puzzling over the weight and balance for the MD-11. Now, specifically it's for the FSX:SE Sky Simulations MD-11, but I was wondering if I can get some real world values, hence my posting this in Hangar Chat. If the post is deemed to be in the wrong area, my apologies.

All I can seem to find is the respective volumes of the cargo areas and the number of pallets they can contain, but not actual weight limits. The maximum overall payload weight is almost 200000 pounds. However, what amount of that 200k can the upper deck take? Or the lower decks? Or how should that 200k be split between the areas, assuming C of G is within limits of course! Taken to extremes, could the upper deck take a concentrated 140000 load (Main Battle Tank! lol). Or does the floor have to take a more uniformly distributed load?

The actual cargo areas of the real plane are abstracted into FSX via 5 stations discounting the Crew station: First Class, Tourist Class, Tourist Class, Forward Cargo, Rear Cargo. All have differing amounts in them. Should I split up a cargo load by percentages into the different areas I wonder? Or just zero out the pax areas and split the cargo between the forward and rear stations. They would be at a lower c of g position though.. :wacko:

Cheers.

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Some real world numbers  for the MD-11F: ( different operators may have different numbers.. )

The MD-11 can take 70760 kgs on the maindeck. This weight is limited by the weight the barriernet ( or crashnet as it also is called) can hold. This barrier net is located between the cargo area and the cockpit area and it meant to protect the crew against forward shifting load in case of an abrupt stop. Of course the load needs to be "spread out" over the various positions to not exceed the floor / lateral or cumulative limitations.

The forward belly can hold 25400 kgs. The center belly can hold 15880 kgs and aft belly ( bulk loading area ) 3400 kgs,

The total weight restrictions of the various compartments added up will exceed the total capacity of the aircraft.

The MD-11 is not an easy aircraft to balance in terms of COG. You would want to achieve an aft (zfw)COG as this will save fuel.

 

b rdgs / Dick

=

 

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Hi Dick,

Many thanks for the numbers. I shall abstract those out into the FSX stations and have a play. For example, the main deck 70760kg will be split between the 1st class and the two tourist class stations.

One strange observation on the Sky Sim MD-11 is that the stab trim never seems to drop below 5.4 ANU when cruising. Also when I am trying to load the plane with an aft CG I get a  c of g out of trim warning when it goes beyond about 27%, both on the ground and in flight. However I did not begin on the ground with ZFW..

I don't know what "fudges" Sky Sim have applied in the .air file or the aircraft.cfg file, but their MD-11 at least seems to fly nicely with a heavy metal feel (layman's terms of course! :) )

Thank you again for your insight.

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Hi Mark

 

Not sure if you are flying the freighter version or the pax version of the aircraft. I do not have the Sky Simulations MD-11.

A passenger version does not take 70760 kgs on the maindeck. The aircraft itself will be heavier ( seats / galleys etc ). A passenger in general ( body weight and luggage ) represents about 100 kg. So if you carry (let's say) 250 pax you will have 25.000 kgs of payload. Any cargo will be on top of that.

In general, when making the loadsheet and checking the COG you only check the zerofuel weight envelop. This envelop was made such that if you are inside the (ZFW) envelop you will be also within the Tow en Law envelop at all stages of the flight. Contrary to Boeing where you normally have a ZFW and a TOW and a LAW envelop and are required to verify that you are within all 3 envelops.

So when giving MAC percentages it helps if you indicate if you refer to ZFW cog or otherwise.

In RW MD-11 life you would  also be concerned how your fuel is being loaded. There are 2 main fuel schedules ( sequence in which you load / burn the fuel ) and it is important if there is any fuel loaded in the tail and either fuel schedule comes with different COG limits/envelop. Sky simulations may not have modeled this .

B rgds / Dick

 

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Hi Dick,

I've only been flying the freighter version at the moment. Well Sky Simulations may have visually modelled both pax and freighter versions, but the payload stations within FSX are just the pax related ones (First class, 2 tourist class and fwd & aft cargo). The overhead panel in the cockpit has a pax load indicator, even in the freighter! Amusingly, when I input the numbers you gave me, it said I had 981 souls on board! (MD-380Bus anyone? 😄 😄 )

I zeroed the fuel in all the tanks and then input the maximum upper deck weight, the maximum aft belly weight and used some of the fwd belly to get to the max payload, with an aft COG bias. The ZFW COG was over 28% IIRC and this did not trigger a c of g out of limit warning. The nose leg extended a little once I'd "got back in the cockpit" from the payload menu screen, but she did not sit on her tail. Previously, when I'd had fuel on board and played with the payload I had managed to do that - echoing what has happened to more than one MD-11 in real life :) 

However, once I'd fuelled the aircraft up to it's maximum weight, the out of limit warning was displayed despite the COG percentage not going any further back, but this may tie in with what you mentioned about fuel in the tail - The schedule has different COG limits...

In future I'll begin my MD-11 flights from ZFW then add payload and fuel accordingly.

The plane still hand flew very nicely up to around FL250 where I switched on the autopilot. I monitored the COG percentage and it was gradually decreasing as the fuel burned off (COG moving forward). I did not carry out a full flight.

It is quite interesting playing around with the numbers like this, but I cannot say to what depth Sky Simulations have modelled the behaviour of their MD-11. However, the fuel tank sequencing seems to follow the real world quite closely, from what I appear to have found out.

Cheers!

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