Courtesy of Grandfather, 737 is only significant airliner still in service that has not been subject to a proper full scale fatigue test. Fail-safe fatigue integrity was originally certificated on basis of similarity with 707 and 727. There was also a fatigue test of rear fuselage to prove integrity of rear pressure bulkhead, which was misapplied to prove damage tolerance of fuselage lap splices. All of which ended in tears (pun!!). See
LESSONS FROM ALOHA or
http://www.iasa.com.au/folders/Safet...fromaloha.html
Aloha story mentions Boeing’s untimely letter immediately before accident - saying nothing could go wrong! I’ll share it with anyone interested; just PM me. No, I didn’t steal from Australian Government files. Got mine from Boeing’s file copy tabled at NTSB inquiry. Letter is dated two weeks before accident, so in days of snail mail its arrival was even closer.
Apparently 737 NG had a better fatigue test of fuselage. See
https://www.newsweek.com/boeings-737...problems-63629 including:
“The Aviation Safety Institute's Pat Duggins told me that 38 changes were made to the fuselage before the NG went into production. And then, to make sure that no critical weaknesses remained, Boeing took a standard 737NG fuselage off the 737 assembly line at Wichita, Kans., and tested it to the breaking point. The airframe was pushed through the equivalent of 225,000 cycles (three times the assumed safe life of 75,000 cycles for the NG series) on short duration flights—exactly the way Southwest, for example, uses the 737.The problem, Duggins says, was that the test fuselage did not represent the realities of everyday flight. It lacked a wing box, the core load-bearing part of the wings where they meet the fuselage, and also the landing gear, which transmits particularly forceful stresses to the fuselage on every landing. In addition, he does not believe that the design changes would have given the NG series fuselage a significantly longer life. He raised doubts, when I talked to him, that the tests met Boeing's design requirements. (Boeing disagrees. It asserts that the testing "provides a realistic simulation of complete flights," and adds that "all these loads were represented"; Boeing did not specifically answer my questions about whether the wing box and landing gear were part of the airframe that was tested.)”
I doubt efficacy of this test to replicate fatigue stresses in pickle forks. Furthermore Boeing probably thought fatigue of forks unlikely because they are primarily in compression.
Main job of the forks is to transfer weight of fuselage to wing, putting forks in compression. Secondary tension stresses come from fuselage pressurisation ie fuselage lifting away from wing. So, tension in forks arises from pressurisation together with negative G, and is probably not well replicated in fatigue test spectrum.
With forks we may have dodged a bullet, much as we did with Section 41 cracking on 747. See
https://www.flightglobal.com/FlightP...20-%200317.PDF Just by luck widespread cracks were first found in wreckage of JAL 123 in 1985, though nothing to do with crash. Likewise we almost dodged a bullet with 737 lap joints; except for poor Clarabelle Lansing
All of this I had in mind when penning
https://www.smh.com.au/national/self...03-p536wk.html Plus I was irked by CASA and Qantas disingenuous platitudes downplaying significance of pickle fork cracking.