What happened
On 5 November 2010, a Piper PA-28-161 Cherokee Warrior II, registration G-BSPM, was conducting a training flight near Dorking, Surrey. The flight was intended to simulate deteriorating weather conditions for the student pilot. Following a standard pre-flight inspection and engine start procedures, the aircraft departed normally.
Approximately 12 minutes into the flight, the instructor noticed the Compact Engine Display (CED) caution light illuminate and remain lit. Simultaneously, the gearbox oil temperature began rising into the amber range. Shortly thereafter, the engine experienced a sudden power fluctuation, with the power gauge dropping between 5% and 8% and the propeller RPM increasing to 2,400 rpm. The instructor declared a mayday and performed a forced landing in a ploughed field. During the landing, the aircraft pitched forward and came to rest in an inverted position. The two crew members sustained minor injuries.
The investigation
The AAIB investigation involved an examination of the aircraft at the site and a detailed analysis of the engine's Full Authority Digital Engine Control (FADEC) data. The engine, a Thielert TAE 125-02-99, was removed and sent to the manufacturer for assessment.
Data downloads revealed that the propeller had been overspeeding since shortly after the takeoff run began. While a pre-takeoff power check had been performed, the propeller speed had reached 2,305 rpm for one second, which was insufficient to trigger the CED caution light. The investigation also looked into the engine's oil system and the physical condition of the oil cooler mounting hardware.
Findings
- The loss of engine power was caused by the failure of the propeller reduction gearbox due to oil loss.
- This oil loss originated from a cracked union on the gearbox oil cooler.
- The failure was triggered because the mounting bracket for the oil cooler had broken, allowing the cooler to move and subsequently crack the union.
- The engine had been in an overspeed condition from the early stages of flight, but the pilots were unaware of the issue until the oil temperature warning appeared.
- The timing of the bracket failure could not be determined, though it occurred after the pre-flight inspection and the FADEC test were completed.