Originally Posted by
Pilot DAR
I'm thinking about it differently, and, purely speculating, as 737's are outside the scope of my work. As the cracks appear at the last aft fastener holes of a four fastener cluster, I'm not thinking that there is a lot of motion or load between those four fasteners, they're going to act as a unit, even if the spar is bending a little in that area. The presence of a crack indicates a load exceeding the local capacity right there, it's not the whole cluster, just that one last line of two fasteners. So what load separates the area of structure of that cluster of four fasteners (and those above) from those below? Not so much wing bending, but yaw torsion between the wing and the fuselage. The force which would have to be resisted if lots of rudder were being applied. I've always wondered about the applicability of Va in the yaw axis, and this defect continues my wonder about that. As those pickle fork fittings would appear to be the sole structure which resist a wing to fuselage yaw load transfer, perhaps they're just not quite up to it. The presence of winglets could aggravate a yaw reaction a little (winglets would not reduce yaw reaction anyway!).
But, of course, that's all speculation on my part. I'm just thinking about aging aircraft and primary structure cracks, as I'm dealing with them on other aircraft these days. Obviously a load in direction or magnitude, or both, is exceeding what the engineers accounted for, and as we consider repairs, we must consider what load is being carried, which was originally inadequately considered.
FWIW I note some sort of clevis fitting in the pics near the cracked area- which is on the rear spar to wing box area. Could the clevis area be part of the landing gear arrangement and during landing with a heavy load ( short haul/cycle ) and/or hard landings wind up pushing partly sideways thru the landing gear ' side struts' or some sort of connection ???.
BE interesting to find the flight time vs cycles for the carriers that have found the highest incidence of cracks or other high-over stressed indications.