Shackle use
Having been a user of shackles since my ATC gliding days (and subsequent marine use) I awaited the 'book' with keen interest to see how this simple 'connecter' fitted in with a high tech escape system.
The answer was simple there was a requirement to connect a rope type end to a metal fitting.(in gliding usually to a weak link assembly) (in marine use a rope to a chain). In both cases the 'pin' is load bearing and the nut stops the pin working loose and falling out. Frequently the pins have 'eyes' which can be wire locked rather than a nut. In both cases shackle deformity can occur when overtightened (mainly due to thread wear). In a case where a shackle gets used on to a more technical fitting and a constant gap must be retained it is common practice to use a internal pin sleeve or spacer that prevents inadvertent gap closure. Was / is there a reason this is not common practice on the seat application or was the requirement for a new pin and nut deemed a reasonable alternative. On the basis of cost and the low number of times this was required it seems incredible that anything else was even considered for such an important component.