History of Flight
On July 7, 2014, a Piper Meridian PA-46-500T, registration N406CD, was holding short of the runway at Denver Centennial Airport, Colorado. The engine, a Pratt & Whitney PT6A-42, failed to accelerate after two power lever commands. After canceling the takeoff clearance, the pilot used the fuel control unit (FCU) emergency manual override (MOR) lever to increase engine power. The sudden excess fuel caused a turbine over-temperature. The pilot reported hearing a bang and seeing a fire at the nose of the airplane. He retarded all levers, shut down the engine, and exited. Several small grass fires were ignited by hot fragments of overheated turbine blades ejected from the exhaust.
Engine Examination
Initial examination of the engine, still installed on the airframe, revealed distress and fracture of the compressor and power turbines, with fragments found in the exhaust plenum. All FCU control pressure lines were intact and leak-free. The FCU control linkage and cabling from the cockpit quadrant to the power lever and MOR lever were connected and operated smoothly with full range of travel. The MOR lever resided properly in the detent and required positive force to lift out of detent and advance.
Data Review
Data from the Garmin onboard device showed that during the last taxi, the engine was at a sub-idle condition with gas generator speed (Ng) about 39% and inter-turbine temperature (ITT) 830°C. When the MOR was activated, Ng increased to approximately 47% and ITT reached nearly 1300°C. The maximum operating turbine temperature limit for takeoff is 800°C, and the maximum allowable transient temperature during starting is 1000°C.
Engine Teardown
Examination at Pratt & Whitney Canada revealed no anomalies in engine accessories. The cause of the exhaust fire was concluded to be the sudden introduction of fuel via the MOR activation, combined with the sub-idle engine speed, resulting in high temperature exposure of the compressor turbine blades and subsequent distress of hot section components.
POH Guidance Review
A review of the Piper Meridian Pilot's Operating Handbook (POH) showed that it allowed use of the MOR on the ground to recover from engine rollback at any Ng below 63%, without stating minimum Ng limitations. This guidance contrasted with that of other airframes: the Pilatus PC-12 prohibits ground use of the MOR and restricts in-flight use above 65% Ng; the Cessna 208 allows MOR use only in flight and above 65% Ng. Pratt & Whitney Canada’s Service Information Letter PT6A-053 originally applied only to PT6A-114 engines, but was revised in September 2014 to cover all PT6A engines, emphasizing that the emergency power lever is intended for emergency use only during flight and not for ground operation.