1.5 factor of safety?

[Nigel]

Ok, here is a question. How did the FAA settle on the 1.5 factor of safety? Did it come from the proposed handbook for airplane design Oct 1927? Or does it even have it's origins in aircraft design? Anyone have any idea?

Nigel Waterhouse B Eng (Hon's)
Can-Am Aerospace,LLC, Canadian Aircraft Certification Centre

http://www.canamaero.com

 

[SWComposites]

One rumor that I vaguely remember is that the ratio of Ftu to Fty for one of the early aluminum alloys was ~1.5, therefore the DUL/DLL factor was set at 1.5. Somewhere in my musty files of paper I think I have some more historical info; if I think of where it might be I try to dig it up.

Regards,

Steve

 

[MikeHalloran]

I've been told that one military plane was designed with a FS of 0.95. Which effectively relies on everyone else's factors.

The plane aaw production, and combat, and did not suffer a reputation for structural failures.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Well, you could argue that all these lifetime extension programs are an in-depth re-evaluation of FS.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Kwan]

A gross weight increase would be a re-evaluation of margin of safety or perhaps a reduced flight envelope. Lifetime extension deals with normal flight loads used to evaluate fatigue life.

The factor of safety is 1.5 as required by the FAA. The military can choose their design requirements. I've seen military aircraft that exceed limit load frequently and intentionally.

 

[Nigel]

All these comments are good, but they don't answer the question. I have been through all the early regulations, all the way back to the late 30's, and 1.5 has always been there. I cannot believe they started at 1.5, since most common factors of safety are around 4 to 5.

Nigel Waterhouse B Eng (Hon's)
Can-Am Aerospace,LLC, Canadian Aircraft Certification Centre

http://www.canamaero.com

 

[prost]

You could never design an airplane to SF4 or 5; it would not be economical. So you make up for the reduced SF by inspect and repair.

 

[Sparweb]

The number was not firmly established in 1929, when Alexander Klemin wrote "Airplane Stress Analysis". The book has a section on "Department of Commerce Requirements", which explains a variety of terminology and factors that would be confusing if I didn't quote the entire chapter.
Although he comes at the numbers from empirical sources at the US Army and Navy, the factors he ends up using aren't too different from what we use today (N_man=3.98 instead of 3.8). Some are related to power loading, which is also interesting. Again, different terminology is used, but he does mention the "factor of safety". He used a factor of 2. That is:
"The load factor times the basic load is, then, the 'design load' or twice the maximum probably load that the plane is required to withstand."

Which is as close as he gets to saying that you multiply the limit load by the factor of safety for ultimate load.

Aeronautics Bulletin 7 was published by the Dept. of Commerce in 1934. It states:

"The minimum factor of safety for any aircraft structure or component thereof shall be 1.50 unless
otherwise specified."

And it's been that way ever since.

Why do you ask (If I may ask).


Steven Fahey, CET

 

[RPstress]

I believe there's some background in "The Safety of Structures" by Sir Alfred Pugsley. A good university library may well have a copy (Bristol, UK, used to have one). From memory he mentions that there used to be a value of 1.625 (1.6?). I also have a half a memory of there being a proof factor for civil aircraft in the past. Military factors were 1.125 for proof and 1.5 for ultimate until recently. I believe (but can't say for sure) that modern fly-by-wire military planes have used a proof factor of 1.0 (i.e., just limit, like civil) and an ultimate factor of 1.4 (some of the 1.5 was taken up by allowance for exceeding the flight envelope, which doesn't happen (in theory, anyway) with fly-by-wire). In fact, if my memory of a conversation a decade or more ago can be trusted, the 1.4 could in theory have been 1.38, so someone somewhere had a pretty exact idea of how much of the 1.5 was flight envelope exceedance.

-RP.