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For instance the Pilatus PC-12 life is either 27,000 cycles or 30,000 flight hours, whichever comes first.

I understand that the limit life in cycles is mainly due to fatigue in terms of pressurization/depressurization, but what about the maximum flight hours?

Imagine a world where the aircraft would fly only one flight of 30,000 flight hours, then there would be only one pressurization cycle, and one takeoff, so what limits the aircraft to fly more than 30,000 FH?

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    $\begingroup$ Most of the components in an aircraft need maintenance based on the number of flights hours. I am not familiar with the Pilatus, but most probably a lot of components need to be replaced at the 30,000 flight hours mark making it economically speaking infeasible to restore it to flying conditions $\endgroup$
    – Brilsmurfffje
    Commented Nov 19, 2019 at 15:44
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    $\begingroup$ I think some components of the airframe could experience fatigue with flight hours as well. Think wing flex with turbulence, for example. $\endgroup$
    – Fred Larson
    Commented Nov 19, 2019 at 17:25

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Some fatigue related issues that are not directly tied to departures and arrivals, like wing bending from gust loads, but may be randomly continuous throughout the flight, need an hour limit. Taking it to a comical extreme, if the airplane flew for 3 years straight without ever landing because it had a new pixie dust powered engine, it would have close to 26000+ hours of bending fatigue time on the wings and tail, engine mounts etc., but only one flight cycle.

Also, there will be an analysis that establishes the average amount of time per flight cycle, overall across the fleet, and this may be applied in some cases to give an hours life limit that is roughly equivalent to the target number of cycles, when applied in the aggregate. That ratio of hours/cycles suggests that the average flight time per cycle for the PC-12 is somewhere around 1.1 hours. In the Regional Jet world, the average flight cycle is about 1.3 hours. The 30000 hour limit prevents a PC-12 operator who does a lot of long range flying with 2 hour+ trips from running his airframe up to 50000 hours before it gets to 27000 cycles.

30000 does't sound like a lot, but corporate aircraft generally live fairly pampered low-utilization lives and take quite a long time to get those hours, perhaps 30 to 40 years. A regional jet gets to 30k in only 10 years.

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    $\begingroup$ You also get aircraft used in Skydiving whose cycle can be as quick as 15 minutes. The PC-6 used to be about 12 minutes to climb to 12000AGL and let the skydivers out; dive at the ground and level at 6000AGL; line up the runway and dive at the ground, level out and land. I once had to stay in the aircraft and boy, was that an exciting ride! $\endgroup$
    – simon at rcl
    Commented Nov 20, 2019 at 14:31

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