Synopsis
On 5 November 2010, a Piper PA-28-161 Cherokee Warrior II, registration G-BSPM, was conducting a training flight near Dorking, Surrey. The aircraft suffered a rapid loss of engine power, leading to a forced landing in a ploughed field during which it became inverted. Both pilots, the only occupants, sustained minor injuries. The aircraft was damaged beyond economic repair.
History of the Flight
The flight was planned as a training exercise to simulate deteriorating en route weather conditions. The instructor and student completed the pre-flight inspection together and found no problems. After starting the engine and performing the pre-flight FADEC (Full Authority Digital Engine Control) and propeller adjustment function test, the aircraft entered the runway for a pre-takeoff power check. No abnormal engine indications were observed.
Approximately 12 minutes into the flight, the Compact Engine Display (CED) caution light illuminated and remained lit. The instructor noted that the propeller reduction gearbox oil temperature had increased into the amber range, with no other parameter changes. A PAN was declared, and the instructor decided to divert to nearby Redhill Airfield. About 20 seconds later, an audible power fluctuation was heard, and the engine power gauge fluctuated between 5% and 8%. Both FADEC warning lights flashed rapidly, and the propeller rpm gauge indicated 2,400 rpm. The instructor declared a mayday and executed a forced landing in a ploughed field. After touchdown, the aircraft pitched forward and came to rest inverted. The instructor secured the aircraft and remained with the student until emergency services arrived.
Aircraft and Engine Description
The aircraft was a Piper PA-28-161 originally manufactured in 1981, re-engined with a Thielert TAE 125-02-99 liquid-cooled four-cylinder direct injection diesel engine. The engine drove a variable-pitch propeller through a reduction gearbox, controlled by a single thrust lever providing input to a dual-channel FADEC unit. The cockpit included a multi-function instrument displaying propeller rpm, engine load, reduction-gearbox oil temperature and pressure, and cylinder temperatures. A lightpanel contained two FADEC warning lights, a FADEC test knob, and a CED caution light. The CED caution light was designed to illuminate if propeller speed remained between 2,301 and 2,400 rpm for 20 seconds, or exceeded 2,401 rpm for two seconds.
Investigation
Examination of the aircraft revealed that the engine gearbox had failed due to loss of gearbox oil from a cracked union on the gearbox oil cooler. The oil cooler mounting bracket was found broken, likely allowing movement that caused the cracking. The timing of the bracket failure could not be determined. The instructor confirmed that the gearbox oil level appeared normal during pre-flight and no oil leak was evident at the parking place.
Analysis of data from the engine's FADEC unit showed that during the pre-takeoff FADEC test, the propeller did not respond correctly to commanded pitch changes. The pre-takeoff power check, conducted about four and a half minutes before takeoff, lasted four seconds and saw propeller rpm reach 2,305 rpm for one second—an overspeed insufficient to trigger the CED caution light. Approximately two seconds after starting the takeoff run, propeller speed exceeded the 2,300 rpm limitation and remained in overspeed until gearbox failure 12 minutes after takeoff. The CED caution light remained off until about 20 seconds before power loss; the reason for this was not determined. The only other overspeed indication would have been a red LED on the rpm strip and the digital gauge.
Conclusions
The loss of gearbox oil from the cracked oil cooler union resulted in a loss of oil pressure and subsequent failure of the propeller control system and reduction gearbox, leading to a loss of engine power. It is probable that the failure of the oil cooler mounting bracket allowed sufficient movement of the oil cooler to cause the union to crack. The timing and reason for the bracket failure could not be determined. The lack of evidence of an oil leak during pre-flight and the propeller's failure to change pitch correctly during the pre-takeoff test suggest that the union cracked at some point between engine start and completion of the FADEC test. The failure of the propeller pitch change mechanism allowed the engine and propeller to overspeed during takeoff and in flight until power loss. The pilots were unaware of the problem until the CED caution light illuminated continuously approximately 12 minutes into the flight.