What happened
On 14 May 2007, a Pitts S-2A, registration G-PTTS, was conducting a type conversion training flight at Leicester Airport. The pilot in command was occupying the rear seat to instruct a student pilot. Following two successful circuits from Runway 28, the aircraft was on its third touch-and-go when it experienced a sudden and complete loss of thrust at an altitude between 50 and 100 feet.
The pilot immediately executed a forced landing on a disused section of the runway. This area was characterized by a rough surface, debris, and several young trees. Despite the terrain, the aircraft remained upright and sustained damage to the rudder, the right lower mainplane, the aileron, the tail wheel, and both brake calipers and wheel spats. There were no injuries to the two crew members on board.
The investigation
Investigators found that the engine continued to run at idle speed after the landing. Testing revealed that the propeller blades were stuck at the fully coarse pitch angle. The investigation focused on the propeller control unit (PCU), which was removed and bench-tested by a licensed engineer. The test confirmed a failure of the internal relief valve spring, which caused a loss of the oil pressure required to control the propeller hub.
It was noted that while many single-piston engine aircraft are designed to move blades to a fine pitch position during an oil supply failure, the system on G-PTTS caused the blades to move to the coarse pitch stop. The investigation also found that the aircraft's Flight Manual, which included UK CAA Supplements, contained no information regarding this specific propeller behavior during a PCU failure.
Findings
- The primary cause of the loss of thrust was a failure of the internal relief valve spring within the propeller control unit, leading to a loss of controlling oil pressure.
- The pilot was unable to diagnose the issue because the specific behavior of the propeller following a PCU failure was not documented in the aircraft's Flight Manual.
- The aircraft's propeller system was designed such that a loss of pressure caused the blades to move to the coarse pitch stop, rather than the fine pitch position typical of other aircraft.
Safety action
- The AAIB issued Safety Recommendation 2007-054, advising the Civil Aviation Authority to ensure that information regarding specific propeller behavior following a PCU failure or similar malfunction is made available to all pilots operating such aircraft on the UK register.