Scatter Factor 2

[TDI]

I have a requirement document that states "no major fatigue damage thru a 25500 flight hour / 17000 flight cycles for economical life, minimum scatter factor of 4"
Does this mean that I need to test to 4 times the life, ie 4*25500 hrs or 4*17000 cycles? And do I also need to factor my fatigue calculations to show a factor of 4 on hrs/cycles?

 

[rb1957]

yes, no ...
fatigue calcs (you know, looking up an s/n curve at your stress) determine the "unfactored" life (so "no" factoring).
but your requirement is for a safe (or factored) life of 25,500 flights, so with a SF = 4 , your calc will need to have an unfactored life of >102,000 flights (so "yes")

 

[EVdave13]

FAA report AFS-120-73-2, which developed a procedure for calculating lives for GA aircraft, has a decent discussion on scatter factor (if you can find it). I'm attaching an excerpt of the relevant pages (last part of pg. 7 and 1st part of pg. 8).

 

[prost]

To find and download this document AFS-120-73-2, I went to the FAA library

http://155.178.136.203/eosweb/opac/index.asp

and put the following words in the e-search box:

Fatigue Evaluation of Wing and Associated Structure on Small Airplanes

Its direct address is

http://www.tc.faa.gov/its/worldpac/techrpt/afs120-73-2.pdf

 

[crackman]

TDI

First of all, the requirements will depend on whether you are certifying to Part 23 or Part 25. If its Part 23, the new document to use is AC 23-13A. The AFS120 is no longer current. Also, if Part 23, are you certifying to 571(a),(b) or (c)? This will affect the factor used. AC 23-13A has a detailed explanation of the various methods and it is what the FAA currently uses for all new programs. If Part 25, normally all safe life certified items use a minimum of a factor of 4 but AC25-571 and AC91-56 will provide some guidance as well.