NTSB Looking into Runaway Trim for Lake Michigan Crash Cause

Five minutes after takeoff and just before radar contact was lost, the pilot of N550BP, a Cessna Citation 550, reported a runaway trim problem. The NTSB now consider this to be their best lead as the aircraft was only carrying a voice recorder and the impact was explosive. All six aboard were killed when the aircraft, carrying two pilots and four members of the University of Michigan's transplant team, crashed into the water 6mls NE of Milwaukee. Runaway elevator trim can be caused by misrigging but is more often associated with an electrical malfunction in the pilot's yoke-mounted trim switches. The control surfaces themselves normally have both electrical limits and, a little further on in their deflection, mechanical stops. It's debatable whether an electrical limit would operate correctly in the event of a switch being miswired and producing trim in the opposite sense. This has happened in a number of incidents (see example at tinyurl.com/2w5xhq). In an EMB110 incident the pilots experienced a sticking trim-switch and reported that "they were unable to pull the electric trim circuit breaker, in accordance with the manufacturer’s procedure, because they were unable to release their control columns." The investigation found that the left yoke-mounted trim switch did not return to the neutral position, when operated and released, due to a sticky substance binding the levers. It also found that the elevator electric trim servo mechanical clutch did not release at the specified setting. (tinyurl.com/2rm2ef). A similar scenario for a Cessna Citation runaway trim accident can be seen at this link