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From 50 fps turbulence intensity at VC to 90-100 fps turbulence intensity at VC. The amended FAR design criteria was introduced in 1980 or 1981 for commercial airliners. What prompted this change?

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  • $\begingroup$ Assuming you're referring to FAR 25.341? $\endgroup$
    – mins
    Commented Jul 26, 2021 at 19:31
  • $\begingroup$ I believe so. I've seen the 90 fps design criteria be referenced in documents from 1980 or 1981 at the earliest. I'll try see if I can dig those up later. $\endgroup$ Commented Jul 26, 2021 at 21:42

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According to the FAA, "The final PSD continuous turbulence criteria were based on those studies and were codified in Appendix G to part 25 in 1980."

This matches the timeline you suggest, but the Appendix released in 1980 does not specifically mention 90fps. (See Amdt. 25–54, 45 FR 60173, Sept. 11, 1980.)

Bottom line: research through time over the actual nature of turbulence and analytical procedures for modeling it resulted in the rule change, as explained in sufficient detail the first reference.

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The FAA made that change in 2015. Link to historical revisions of FAA regulations, specifically 25.341 gust and turbulance loads. You can see the 90fps was not in the 1996 revision, 1984 revision, or original 1965 rule.

Link to Amendment Number: 25-141, Effective Date: 02/09/2015 https://drs.faa.gov/browse/excelExternalWindow/C86DBFEBB41D816F86257DE80072BA20.0001

If the above link breaks try https://drs.faa.gov/browse/FAR/doctypeDetails browse to 25.341 Amendments - 25-141 (02/09/2015) , 25-86 (03/11/1996) , 25-72 (08/20/1990) , Initial (02/01/1965)

Rule making explanation. Generally just aligning FAA and EASA requirments with little to no practical effect on current production aircraft. https://www.federalregister.gov/documents/2014/12/11/2014-28938/harmonization-of-airworthiness-standards-gust-and-maneuver-load-requirements

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