I've had an uncommanded TQ-increase in a DHC6-300 (PT6A-27) last year, while we were 2 hours out of Colombo over the Indian Ocean. I saw the needle move through the redline and pulled the power back immediately. At FL110, prop full fine and power lever in idle, the engine was producing around 30 PSI TQ.
We were able to equal all engine parameters, but kept a 6" power lever split. We shut the engine down in the approach.
We had another FCU shipped out and continued with that. I was told that the problem is probably related to clogging or leaking of internal lines or bellows.
Erik.
BTW, there's a new STC to install -34s and 4-bladed props on a DHC6-200. The TQ-limit remains 42.5 PSI, but you can still get it at high-altitude.
[ 15 July 2001: Message edited by: erikv ]