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Comment by WalterBright | Hacker News Reader

62,000 flight hours

Flight hours or flight cycles?

I suspect that pressure hull and wing spars experience the greatest loading and fatigue, and may be differentially influenced by overall flight time and takeoff/landing cycles.

Hours. I remember the incident with an Aloha Airlines 737 where fatigue damage caused the failure of part of the structure. Those airplanes were only used for very short hops, hence a high ratio of cycles/hours. While they were within the 62,000 hour limit, the cycles were very high and were not anticipated in the fatigue calculations.

The criteria might have changed since that incident.

Maintenance schedules are based on hours. Pilot experience is also logged as hours.

It's actually both, specifically because of that incident. (Pretty much any time a commercial jetliner crashes, a rule changes, and it's been that way for decades.) There's an hour number and a cycle number for many maintenances, and whichever comes first triggers the maintenance. Hulls, however are rated for a max number of cycles due to fatigue caused by the pressurization/depressurization and that's the part of the plane that cannot be replaced. So the plane usually gets junked or shipped to a developing world airline due to cycles.

NB: Aloha 243 was what I'd had in mind as well, and was when I'd learnt of the distinction between hours and cycles.

For those unfamiliar, a 1988 airline incident in which a Hawaiian airliner experience explosive decompression, though 94 of 95 occupants survived. The lone fatality was a flight attendant who was sucked out through the hole, and if my memory of the post-incident report stands, actually contributed to much of the fuselage damage by enlarging the initial failure region.

<https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243>

Another of my references are the US B-52 Stratofortress fleet, the youngest of which were built 61 years ago in 1963, for which active status is anticipated to extend into the 2050s. My understanding is that wing loadings/unloadings are the limiting factor, with virtually all other parts of the plane being capable of replacement.

<https://en.wikipedia.org/wiki/Boeing_B-52_Stratofortress>

The overall airframe is rated for 32,500 to 37,500 hours, with aircraft experiencing about 380 flight hours/year. The wing upper service being the lowest limit of body parts indicated here:

<https://www.globalsecurity.org/wmd/systems/b-52-life.htm>

This is my only claim to fame, the pilot is my mom’s cousin. Super nice guy.

Hearing him tell the story first hand was pretty incredible. He saw sky, so knew he had a hole… but had no idea it was so extensive until they landed and he walked back there.

Can you imagine the phone footage we would have had from that it if it happened today?!

The phone footage would be ... portrait mode.

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