Ooops, I hear my phone ringing, been kinda out of it.
Let's see if some distracting meandering can help us out here, Bill_. I can add some ruffles and flourishes to some points, at least.
Corrosion (more than fine surface stuff) is the greater problem, as has been stated, because pitting makes for crack starters. Beyond that, the service and maintenance history are the real considerations.
. . . having to do with cycles of the big planes having to do with pressure expansion, contraction etc. Can you translate that to a 150? Obviously, pressure expansion is not a factor.
Not really comparable; usually there is one pressurization cycle per flight and it only stresses the pressure vessel. Flight, etc. loads stress the whole airplane, in various and changing ways over many times in one vehicle operation.
I would think cycles of take offs and landings and flexing of the landing gear would be the major area along with flexing of the wing spars in flight. is this correct or are there other places where flexing would be a factor, ie. rudder, stabilizer, motor mount?
All this, and more! The honest truth is: the whole airplane, it all flexes to one degree or another. Different maneuvers/inputs produce their own "hot spots." On the F-2 that I work with, the same part may have multiple critical load cases (some in-flight, some by other events), depending on location and the failure mode. And we have an incredible number (that I can't remember right now) of load cases, being a military combat aircraft. But wait, there's more! Vibro-acoustics can produce fatigue as well, on the 150/2 from the motor vibration and the propeller pulses. (The F-35 STOVL gets beat up pretty good by the ground reflections of the vector thrust)
One of the advantages of a large fleet with substantial time in service is the structure is pretty well proven, the major and most minor hot spots identified (an unsung role of the mechanic is this kind of feedback of what is seen in the field). Thus what you are looking for is really just making sure those items, mostly AD's, are up to date. The structural design has several safety factors to handle the limit load stress levels with reasonable room to spare. This means there is a lot of potential life in an aluminum structure. And with this margin, most of the time in conventional structures a crack weakened part will shed load into other structure and the crack grows slowly. This gives you time to find the crack in normal maintenance. And as the other structure overloads as the crack unloads, it yields and bends and gives you further warning of the crack. (Assumes loads are within limits, although overloading can do the same bend-before-break thing if not excessive - the 152 will show skin wrinkles near the MLG if landed in a golf course sand trap, for instance . . . ) This wider range of elastic-plastic behavior of aluminum is part of what makes it so useful in aviation. Not 100% foolproof (it's a human endeavor), but has worked rather well. Pressure vessels are loaded more uniformly and respond differently, hence the separate tracking of pressurization cycles from flight hours.
I guess this is the long way 'round to saying that fatigue, while important, is not at the top of the worry list here, in the GA fleet. There is some growing concern for the very high time/cycle airframes (principally the military and commercial air transports) just because it is new territory. As development times have lengthened and costs have risen, aircraft fleets have not been upgraded as quickly, which used to put most of the "old-timers" out to pasture before this was a significant issue. In addition, aluminum has a peculiarity in that a low stress over a long period of time can cause material failure, too, so there are definite (but high) ultimate time limits to some (parts of) aluminum structures.
From my perspective, I consider "low" airframe time more like a cosmetic issue, nice to have but does not really affect the function now or future. At annual/pre-purchase inspection, 80Q turned out to be in rather nice shape for a 25 yr, 9400+ hr airframe, not to say it does not show it's age and use. Spent 20 of those years in the arid climes doing club and probably some rental trainer duty and having regular maintenance done by the same person for years at a stretch, before coming down here to slumber for awhile until I came along and woke it up. Two recorded events, neither of which produced any airframe damage we could find in inspection (but did engender two engine swaps, one change of owners, and the switch to the Sensenich prop ). Some might be put off by the total time, but I am still shaking my head at my luck in the whole deal.
(OK, more meandering than usual.
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