Sat Apr 28, 2018 2:00 pm
#1607841
I've just got round to reading the second half of this thread, and throughout I had thought about posting something on metal fatigue. Then I see the preliminary report saying it was metal fatigue...
One thing about metal fatigue is that it builds up over time. It isn't black and white, it isn't something which shows over-stressing, it can be repeated stressing within limits.
In my Bulldog, I have a fatigue meter. It clicks different counters over at different levels of plus or minus g up to the limits (+6, -3). Each time I pull 1.75g, the 1.75g counter clicks up, assuming I didn't go up to the level of the next counter. These all add up over time, and the more g pulled (or pushed), the higher counters get clicked and the more fatigue is clocked up. The fatigue calculation also takes into account the number of landings. Even assuming no heavy landings, each time you land the spar goes from the wings supporting the fuselage to the fuselage supporting the wings, and vice versa for a take-off. It all adds up.
You can't just go on number of hours flown, the use to which the aircraft is put will be part of the consideration. Circuits? Steep turns? Spins? Spiral dives? Aerobatics? Straight and level cruise for hours? These will all fatigue the airframe, and critically the spar, in different ways. It isn't the case, at least as far as I understand it, that a spar used within (say) its 4.4g limit will have an infinite life. Some aircraft have "multipliers" which are supposed to be used for anything other than straight and level, but I don't know anyone who bothers with that, especially in the training environment.
With the Bulldog, as well as fatigue monitoring, there are also regular NDT eddy current inspections on the spar to check for any cracking.
Does all this mean a Bulldog's spar is weaker than that of (say) a PA28? Hell no, as the Americans say.
It's a cliche to say that some of these aluminium GA aeroplanes were never expected to be in service, let alone training service with the stresses involved, for 30 or 40 years, but this is where we are.