Crash loads

[40818]

Hi,

I'm working on a concept scheme in which a structure in a cargo bay has to react a big 9g fwd crash load, all well and good, but in a particular area it will impart a reaction load onto the aft vertical pax door surround. I have tried my best to show it below.

!....!...........!....!
! ! ! !
!....!______!.....!
! ! ! !
! ! ! !
!....! !....!
! ! ! !
! ! ! !
!....! !....!
! ! ! !
!....!______!.....!
! ! ! !
! ! ! !
!....!-->P.....!....!

Badly drawn, but i hope you get the idea.

Before i go down the road of doing any calcs, i dont think its a wise idea to apply a big crash load onto an area which is used to maintain the rigidity and shape of the evac door surround structure. Any ideas?? And would the FAA be happy with such a design. My gut feeling is that its not the brightest idea i have heard. If anybody knows of any documentation that would govern such situations could they let me know.
Thanks in advance.

 

[rb1957]

try FAR 25.809 ... in my experience people have (at least) two ideas on this (deformation of a door surround) ... one group proved the door could be opened with limit loads being applied (obviously unpressurised), a different group demonstrated the the door could be opened after limit load was applied (ie after the limit loads were released).

if your design applies loads near to a door, yeah you're right possibly not the smartest thing, but with enough Al we can fix just about anything. I would consider your analysis as predictive (rather than being the final determination) ... that would be followed up with a test.

good luck, btw, I had no idea what your drawing showed !

 

[der8110]

Look at 14CFR25.807(j). If it is cargo-only, flight crew only, no supernumeraries (or even a few, with exemption), then you could count the overhead hatch (if it has one) as the emergency exit.

Also look at Advisory Circular 25-18 for cargo conversions; some things have changed over the years but many concepts are similar. AC25.783-1A is more modern and deals specifically with doors, and will lead you to other valuable information.

 

[40818]

Thanks both for the pointers.
I have managed to keep the lemmings from the cliff as it were!

P.S my drawign was tryign to represent a dooor cut-out surround structure with door stop stiffeners and adjacent frames, cant understand why you couldn't see it.........

Seriously though, thanks for the help.

 

[rb1957]

i thought you'd drawn the fuselage in plan-view, with the "-> P" being the pax door.

now that you've described it, i was see what you meant ... sills and aux. sills, frames and aux. frames ... probably as good a construction scheme as you can get. obviously doublers (and maybe triplers), presumably an internal doubler joining the caps of the frames and sills at the corners ... if you want to minimise distorsion of the door frame/opening you need to have stiff corners !

 

[40818]

Well the aircraft is a post production mod, and, wait for it..... were not allowed to put pretty patches etc on the outside. As i said, i seem to have made people see common sense about it, even though you could probably spend lots of time, effort and money showing that it would be ok, i would assume that it would turn out to be a certification nightmare.
And wait for it, weight is critical, so i have to scheme a workeable design made from unobtainium that is lighter thsn air.
Regards

 

[rb1957]

actually there is some good reason not to add external doublers, in that you avoid additional inspection tasks. that being said, it sounds pretty stupid in this context (cutting a large hole in the side of the plane) as you're bound to require additional inspections. for us, the main reason for adding external doublers is to hide the rivet CSK heads, as the fuse. skin usually isn't thick enough.

if your skin is thick enough, and you're preapred to rip up large amounts of internal structure (stringers and frames), you could work with internal doublers ... but that sounds like a lot of work. and if you're weight critical, you won't do better than adding a doubler (it's the most effective way to reinforce the corner of a cut-out; you could do it by longerons and frames, but that won't be as effective.

good luck