747 Maximum Seat Allowances for Stress Analysis

[auslimbo]

Dear colleagues,

I am currently working on a project to install heavier seats into a 744. Obviously we would prefer to be able to justify the weight increase by comparison to a heavier configuration. the a/c however has not had a heavier configuration, therefore the next step would be to justify the weight by comparing it to the maximum Boeing allowances. A hurdle has however presented itself in that I can not seem to track down a Boeing document that will show me the maximum seat allowances specific to a 744.

Boeing document D6-36238 (Passenger Seat Structural Design and Interface Criteria) has been a good starting point, but provides insufficient information.It does however list the allowances for the 777. (Frustrating)

What D6-36238 gives me is a maximum seat weight of 30lbs per single seat, 60lbs per double, 90lbs per triple and 120lbs per quad. Minimum seat pitch is 30".

Now this is great, but I don't know what seating configurations I can use to justify this. For instance, can I assume that Boeing allows for as many as possible triple economy seats to be placed on LH & RH of A Zone? Therefore, allowing me a load of 600lbs(90lbs + 3 pax @170lbs) every 30" pitch?

In the end what I'm trying to proof is that the seat I am putting in (at 60" pitch) will replace 2 economy seats (I know it's not really a 2 for 1 replacement, but let's just assume).
So say I have a double seat weighing 300lbs @ 60" pitch with a total weight of 640lbs (300 + 2pax @ 170) And am replacing 2 triples (1200 lbs).

This is what I'd like to do. I now need the Boeing allowables (or allowed seating configs) to be able to proof that I can or can not justify it by going down this path.

Any assistance would be much appreciated (no matter how small the assistance is).

Many thanks in advance.

 

[der8110]

You could chase the loads down into the seat tracks and supporting structure, basically reverse engineer it. Then compare to psf allowables which you probably do have.

 

[rb1957]

and are you using 170 lbs pax or the new 185 lbs ?.

if i could elaborate on der8110's post, chasing the loads into the seat tracks is easy enough, the difficulty probably is (not knowing how a B747 picks up the seat tracks) is chasing these loads into the frames (out of the seat tracks) and determining a local allowable.

for an airplane perspective there isn't a problem as the weight your adding will be removed from the cargo (so that the total weight of the plane remains the same. do you need to think about airplane empty weight ? I figure there's a dam' lot of these seats going in (300+?), but then this is a B747 (OEW > 200,000 lbs) so maybe a short story (to say that you've considered the point).

 

[auslimbo]

Thank you for your replies so far.

der8110:
Basically, as you stated is what I intend to do. I can calculate the loads down into the track, but as stated above do not have a clear Boeing guidance or reference for the maximum allowable loads. I am positive that the aircraft structure will easily make the loads that I am creating by adding heavier seats, but without any maximum OEM allowables, the only way I can justify it is by doing an FEA analysis. Which obviously is too time consuming.

Therefore the missing link is the psf allowables as you stated....



rb1957:
I am using the 170lbs pax. It is an older aircraft and therefore 170lbs would only be applicable for it's Type Certificate. But please correct me if I'm wrong.

The aircraft weight does need to be considered, but not at this stage for the structural analysis. The monocoque analysis will have an outcome that the aircraft loading will have to be revised to accomodate the heavier installations in the main deck. This is all being considered.


In summary, chasing the loads into the seat tracks and even chasing the loads into the frame can all be done by FEA or dedicated spreadsheet analysis. However, the reason for my post is to be able to bypass these time consuming analysis by comparing the loads I induce into the structure to the Boeing allowable loads.

I'd appreciate your comments on this.

 

[40818]

auslimbo.

Be carfull with the loads into the seat track. Boeing has substantial test data which validates them. They will have allowable loads both forwards and upwards. Any analysis you make will likely be nowhere near what the test data came up with. I did an example analysis of a local seat track interface (seat to rail) for the vertical component that cam eup with about 10000lb. The test results failed at around 6000lb. You will need the allowable seat to rail applied loads from Boeing to have any confidence. The underfloor floor beams will have cleats to which the seat tracks attach to to help support them, and the floor beams will also have allowable input loads onto them. As for the floor beam to fuselage frame joint; thats always a problem area and very difficult to assess. Your one line statement about "can all be done by FEA.." leaves me a little worried, as you wont have anything to validate your model against. The loading from the heavier seats needs to be reacted through the structure in a correct way, your FEA react the loads in the way you make it.

 

[auslimbo]

Thank you for the advice and I understand your concern.
Please understand that I am trying to validate the seat installation without having to do the FEA or other significant analysis. We have existing FEA analysis for STC'd installations available to use as validation should we need to go down the path of FEA.

At this stage, all I am after is a document reference that actually states the Boeing allowables in a clear cut matter. As stated above, the D6-36238 sort of tip-toes around it for the 747, but a few chapters later clearly states it for the 777. I would like to find another document that does state these allowables. Or a document that clearly states the allowable configurations.


FYI, the installation which we are replacing has very light seats compared to standard. Now, even though the seats we are installing are heavier than the existing seats, the overall install will still be lighter than many configurations that come directly from the Boeing factory.

This a/c has never had a previous configuration that was heavier than the one we are implementing, therefore we can not validate it by saying "a previous config was heavier than the one we are implementing, there has been no structure change, therefore the structure will support the loads" (yes, very simply put, but for illustration only).

And since I can not validate it to another aircraft (I'd first have to prove that the structure is the same - too time consuming), I will have to validate it to the OEM allowables. Which I don't have.

I appreciate all your comments though, please keep them coming.

(and 40818, don't be worried.... I'm sure the DER or the FAA wouldn't sign off on it if we didn't show an accurate validation)

 

[rb1957]

clearly the key is the allowables, determining the loads shouldn't be that much of a problem ... hand calcs (albeit long ones !) or FEA will get you there.

I assume you haven't got access to Boeings detail allowables for the clipping provisions of the seat tracks to the floor beams. You might be able to get a better picture of the stress state in these clips by FE, but I would worry about crippling issues. Non-linear FE would help in redistributing the load, if needed ... if you model a segment of seat track you may find one clip is more highly loaded than another, and NL may model the redistribution due to plasticity ... remember you are talking about the 9g fwd crash case. If you work this way you should consider a limit case (without significant plasticity). Fasteners (shear, bearing, even a little tension) are easy enough.

Alternatively you could build a test rig ... a segment of test track (say 5 frames worth), loaded to include effects of the full track ...

a thought, if you do FE (or hand calc) of the full seat track, what constraint condition are you going to apply to the floor beams (where they support the seat track) ... rigid constraint sounds overly conservative (??), elastic constraint sounds more real but also more work !

 

[40818]

The whole seat track/floor beam/floor panel structure acts to help dissipate the fwd load into the fuselage skin at the sides. I think you have shear panels fitted along the edges if i remember correctly, if you let me know the series (i.e -300, -400 etc) then i will be able to tell you.
Watch out for the down cases, they can be very big for the 747. Some of the load enter into the seat track and the floor panels transfer this load sidewades as shear till its finally reacted by the skin. Some is kept within the track and is passed into the floor beam by the track to beam cleats. The amount that goes in each direction is hard to decide.
What you need to get is the data direct from Boeing, its the only real way you could use it in the report anyway.

The best way you can validate your global FE model is by building it in the same format as the original FEM, and correlating your results with the boeing results, but you need to do this againt a variety of different loadcases.
Boeing will probably give you some data if you ask them if its an STC with their input. As i said earlier, floor beam to frame joint interfaces though are a constant problem and you need to be very carefull in that area.

 

[compositestress]

Note that the document you reference D6-36238 is not FAA approved so that information cannot be used in analysis.