Jumpindan
30th Apr 2014, 03:21
Hello, I have a question about un-commanded idle rollback/ FCU failure on the PT6 motor. In the last month, we have experienced two cases of idle rollback in our Twin Otter equipped with -34 motors. It happened once to each engine, and both times the first flight out of the shop after 125hr checks, so I'm thinking it is something the maintainers are doing wrong during routine power plant phase. Both times, engine starts were completely normal and aircraft was taxied without issue. Engines were run up for preflight system checks and pressures and temps were as expected. While waiting for clearance on the ramp, with power at idle (54% NG in our configuration) there was a sudden drop to 40% NG and TQ was 0psi. T5 rose slightly but still in safe limits (640) and the engine was non-responsive to its power lever. There was a slight rise in fuel flow to the effected engine but oil pressure and temp were normal. Engine was shutdown without issue, cowls popped and PY lines checked for leaks to governor, or any obvious signs of failure, and engine was restarted. Problem could not be recreated. Second incident was identical but two weeks later on the opposite engine after the next 125hr phase. I have only seen this once before in a PT6 and it was with a loose PY connection into the governor. I was speculating that an improper compressor wash could be causing a water blockage in the pneumatics creating the fcu failure that then cleared itself once the engine spooled down. I am not a mechanic, but we are flying in some areas were quality work can be hard to come by and I am looking for a starting point for next time they work on the plane. Motors are young, less than 3,000hrs, and plane is flown 10-15 tach hours a day. Any thoughts? Thanks in advance.