QANTAS lost a plane way back, and almost lost a second one at, I think Esondon. The balance weights fatigued and fell of giving a life time supply of forward stick. The `almost' incedent supplied the answer to why they had a nasty habit of falling out of the sky.
I think the details can be found in one of the 3 vols of air crashes. Damm, can't think of the authors name at the moment.