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What are the different structural weight limitations that have to be considered when loading large aircraft? Is there a difference in complexity as to whether it's a passenger aircraft or a freighter being loaded?

I think I know the answers, but it's precisely because I have to say "I think" rather than just "I know" that I'm asking the question. I'm hoping those here more technically oriented than I will be able to add to what I know.

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    $\begingroup$ Perhaps we should have a Q&A here regarding those pesky "tall rigid cargo" requirements as well? $\endgroup$ – UnrecognizedFallingObject Aug 9 '15 at 21:55
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    $\begingroup$ Hi Terry, I guess you wait for my input. Sorry, but I don't have much experience here. All I know is that the most severe design requirement for passenger floors is the stiletto heel, and the carpet becomes a structural member to avoid puncturing the composite sandwich floor panel. Freighters should be more complex once freight is not containerized, and here the items must be palletized to avoid exceeding the local floor pressure. The loadmaster must ensure that the load is properly distributed and tied down - after all, in contrast to the flight engineer, the load master is still required. $\endgroup$ – Peter Kämpf Aug 14 '15 at 5:33
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    $\begingroup$ @PeterKämpf It's a tediously complex issue, at least in my opinion, and though I perhaps know more about it than most pilots, there are a lot of holes in my knowledge. I have tended to learn just enough to write software to do large cargo aircraft weight and balance, but there are significant gaps in my knowledge. I was hoping someone here might be knowledgeable in the area, but I'm not surprised that there have been no answers. In the next day or so I'll put up an answer giving what I know. $\endgroup$ – Terry Aug 14 '15 at 7:07
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    $\begingroup$ Great! Once I see your answer, maybe I can add what I know. $\endgroup$ – Peter Kämpf Aug 14 '15 at 9:23
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Is there a difference in complexity as to whether it's a passenger aircraft or a freighter being loaded?

Yes, very much so. The resulting load of a passenger in every seat is nowhere near close to the structural limits (note that I said structural limits, not performance limits). Generally speaking, having to check for the load being within structural limits is a concern for freighters, not passenger aircraft.

There are a lot of structural considerations for the semi-monocoque design used for all modern large aircraft. Using one variant of a 747-400F as an example, its Weight and Balance Control and Loading Manual runs to 584 pages. Items that need to be checked for each load include:

Linear and Floor Loading

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The manual snippet above covers the main deck; there are like tables for each cargo area. The linear loading is the lb/in measured along the longitudinal axis of the airplane. I don't really understand why it's necessary to check both the linear loading and the floor loading, given above as lb/sq ft.

Not only does the linear loading for each cargo compartment need to be checked, but the combined linear loading of all decks together along the longitudinal axis has separate linear limits as tabled below:

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Cumulative Loading

There are cumulative load limits for both the forward body and the aft body. Using the forward body for example, you add up individual items (more often than not ULDs) starting at the front and proceeding to a point over the wing box. I'm not sure exactly how that point is chosen insofar as structural and aerodynamic considerations are concerned. The aft body cumulative load starts from the tail and proceeds to the same point.

Below is a forward body cumulative limits graph. Note that there are two lines: one for use when the ZFW is 610,000 lbs or below, one from that weight up to 635,000 lbs. For this particular airplane, the aft body also has two lines, but the differentiation is based on the takeoff c.g.. Some 747s have only one line for each, some one line for one but two lines for the other. Obviously things can get tediously complex, and loadmasters and the software they're using has to accommodate all the possibilities.

Unsymmetrical Main Deck Limits

Widebody aircraft that can accommodate two rows of ULDs on the main deck have the option of carrying more weight in one container if the container laterally next it is given a lower limit. I know of one operator that didn't want to contend with the limit of, say, the position on the left being higher than the position on the right, and chose to forego the flexibility of unsymmetrical limits. An unsymmetrical limit graphs is below:

So, does anyone want to do a weight and balance for a large cargo aircraft? Fortunately we have software to do that for the great majority of loads that are containerized. However, loadmasters might still have to contend with unusual loads manually.

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    $\begingroup$ The staggered linear load limits are for limiting the fuselage bending moment. This can be critical both in max-g cases and in pitch motions, and the mass should be as close to the center of gravity as possible. The values are staggered to make compliance easier. This is also the reason for the unsymmetrical lateral limits: If one container is lighter than allowed, the one next to it may be heavier, so the total allowable moment around the center of gravity can be fully used. The floor loading is for local strength limits: Heavier loadings will overstress the floor panels and floor beams. $\endgroup$ – Peter Kämpf Aug 17 '15 at 21:27
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    $\begingroup$ The increasing steepness of the limit lines in the Allowable Cumulative Load chart shows that the last lbs of payload added should proportionally contribute less lever arm. I think the X-axis actually shows moment, not length. Again, the lines are in linear steps to make calculation easier. Ideally, the limits would be smooth curves. $\endgroup$ – Peter Kämpf Aug 17 '15 at 21:39
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    $\begingroup$ The staggering may also coincide with changes in the skin and stringer gauges. Normally, the fuselage weighs more per foot/meter around the middle than at the ends of the cylindrical section, because the bending loads are highest in the middle, and to keep stresses constant, thicker cross sections are required. Normally, gauge changes are staggered to make manufacturing easier. $\endgroup$ – Peter Kämpf Aug 17 '15 at 21:42
  • $\begingroup$ It doesn't look so hard. Generally, it comes down to treating the aircraft gently and to distribute loads such that the mass centers around the center of gravity. If the mass of all items is known, planning the loading should take half an hour, and less with more experience. $\endgroup$ – Peter Kämpf Aug 17 '15 at 21:47

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