Relevant FAR for Impact Loading?

[Burner2k]

Hi folks,
Which are the relevant FARs for designing structures for impact loading? I am only aware that in interiors, seat tracks need to be designed to withstand 16G of dynamic loading. I think FAA uses a similar terminology called as Dynamic Conditions. I would love to get better clarity...but I would like to start with reading relevant FARs and probably its interpretation from Advisory Circulars.

 

[rb1957]

impact from where ?

from pax ... you're on your way

from the ground ... 25.561

gusts (dynamic) ... somewhere in the flight loads (I could look it up, as easily as you can)

another day in paradise, or is paradise one day closer ?

 

[aerostress82]

Which FAR is this exactly? If it is FAR Part 25, that document is 240 pages. You can scan it pretty easy in 2-3 hours reading most of it.

EDIT:
OK. I went the extra mile to check where your 16.0g came from, and I guess you are looking for FAR Part 27 (rotorcraft). Found this if it helps. Other than this I'm blank in this area:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 43.13-2B.pdf

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[rb1957]

16g is also the new FAR25 standard for seats

another day in paradise, or is paradise one day closer ?

 

[aerostress82]

Thanks rb1957! I see that it is mentioned in chapter 25.562. The gravitational accelerations in FAR documents should really be arranged according to a standard.

In one chapter is mentioned as 9.0g and then at another chapter it is 16g. That's why I missed it in the search from the pdf. They should rearrange them all "with one decimals" in every chapter/section to be consistent.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[Burner2k]

RB1957 & Aerostress82,
Thanks for replying.

I was discussing the concept of DUL with some one and they made a statement which seemed wrong to me but I could refute as I wasn't sure. He said that FAR defines DUL for impact loading (say impact on a wing from a discrete sources i.e. bird or a huge hail stone) as DUL = 2 x DLL. I have never worked with discrete sources impact static design and thus trying to get an understanding of how FAA defines loading condition for the same.

I will look up 25.561 as RB says. I will also search for discrete sources in FAR.

 

[rb1957]

I haven't heard of that ... what he's really saying is that impact loads have an additional factor, of 1.33, to increase the 1.5 (from 25.303 i think) to 2.

This is done for cabin pressure ... 25.365d says the ultimate pressure (pressure alone) is 1.33* the standard ultimate pressure (in combination with other loads). ie, we know what limit cabin pressure is (when we make up a limit loadcase involving cabin pressure, we know what the ultimate version of this limit load is (1.5*), well for pressure alone, there's an extra factor so ultimate cabin pressure is 2*limit.

I have not heard of this being applied to impact loads. To me it seems like your colleague is saying that impact loads are 2*static loads, which is a rule of thumb.

another day in paradise, or is paradise one day closer ?

 

[aerostress82]

So I went ahead and checked all the birds for you in FAR Part 25.
There are fat and skinny birds. Fat bird is 8 lbs, whereas the skinny bird is 4 lbs.
- For damage tolerance, they are considering the skinny bird.
- For bird impact analysis on empennage structure, they are considering the fat bird. (which I guess could be the case for all primary structure of the aircraft. I guess this is what your colleague was implying).
- However, for pilot windshield bird impact analysis, again, the skinny bird is on demand. (But on this one there is a minimum requirement of 15 degree angle between the window pane angle and the longitudinal aircraft axis)

I guess for explicit analysis they are utilizing 2*DLL. But again it is not very clear for all parts. Please let me know if you find a clear definition of these analyses somewhere.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[Burner2k]

RB1957 & Aerostress82,
Thanks for your replies.

RB, I did read about cabin pressure after you posted. I was not aware of that requirement/regulation. Anyways, I guess I will probe this further in to more detail as when required.

Cheers...

 

[stressebookllc]

Food for thought:

http://www.stressebook.com/emergency-landing-dynamic-loads-14-cfr-subpart-c-section-25-562/

Aerospace Stress Analysis and FEA Courses

http://www.stressebook.com

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