I was a Huey crewchief in the 80’s after that Chinook accident. Saw the footage in a safety film. The Chinook had skydivers on board, one of the rotors stopped in flight, one or two were able to jump out before the Chinook nosed down and slammed into the ground. We were told that the transmissions were overhauled, and cleaned with walnut shells. Apparently the shells were used like sand blasting to clean transmissions. After that was found out all Hueys, Cobras, and Chinooks had to have extensive maintenance to ensure oil ports were cleared and the transmissions were flushed with a cleaner and fresh oil three or four times, just to ensure that whatever residue from the walnut shells was completely removed. I dont know exactly what residue from the walnut shells (oil or dust) but the internal transmission oil ports were clogging.
Amazing something that small (blasting dust) can lock up the transmission that is being fed that much power.