Total Time Air Frame
Cycle count - take-off/landing (airframe pressurization cycle counts)
A large transport aircraft like 777 may have more hours than a smaller jet like 737, but it may have fewer cycle count because of longer mission profile. Fatigue is a function of frequency and magnitude of stress changes. A frequently pressurized cabin, repeated heated and cooled jet engines are some parameters to use before spending more time on the detail records.
Airlines are quick to discard older airplanes when TAF/Cycle count reaches economic apex, the useful life, maintenance cost and not so efficient fuel burn engines do not justify economic benefit of buying newer aircraft.
On the other hand, for low utilization application, buying an used aircraft may be a good choice when capital investment relative to its utilization operating cost is low.
An airline may schedule a jet over 10 hours flying time a day, that could easily account for 2,500 hours a year. Many private jet don't see that kind of utilization schedule, therefore a commercial jet on its final lag of "final D" check before heading to bone yard is a good suitable candidate for conversion.
Airframe manufacture designs aircraft with a life-cycle limit in mind before it has to be completely checked out. Maintenance Planning Data manual (MPD) is a good place to start.
737-BBJ 1998, TTAF=3500 hours (300 hours /year)
