What happened
On 25 June 2008, a Piper PA-28R-201T Turbo Cherokee Arrow III, registration G-BFLI, was conducting a private flight near Bradford Leigh, Wiltshire. The aircraft was on its inaugural flight following the recent installation of two new cylinders. Approximately 90 minutes into the flight, while at 4,000 ft, the pilot noted an unusual fluttering noise accompanied by a loss of engine power. Data from the engine management system confirmed that exhaust gas temperature (EGT) readings for cylinders 2, 4, and 6 dropped significantly.
Fearing a fire or total engine failure, the pilot shut down the engine and declared a mayday. During the descent, a 30-40 kt tailwind made it impossible to reach the intended landing field. To avoid trees, the pilot attempted a low-speed approach with full flaps but struck a hedge. The impact caused the left wing to detach, and the aircraft came to rest almost entirely submerged in a lake. The pilot sustained minor injuries and was able to exit the aircraft without assistance.
The investigation
The AAIB examined the engine and the JPI data management system. Analysis of the EGT data showed the temperature in the No 4 cylinder dropped sharply, followed by a rapid decrease in the EGT of the other cylinders on that side of the engine.
Engine strip-down revealed that two stud bolts attaching the No 4 cylinder inlet valve rocker assembly to the cylinder head had failed. This failure caused the inlet valve to remain closed. Metallurgical testing of the broken bolts showed evidence of fatigue-initiated failure, but no material defects or issues with hardness were identified. The investigation also checked the torque of the remaining rocker tie-down nuts on the newly installed cylinders, all of which were found to be within the correct range.
Findings
- The engine failure was caused by the failure of two stud bolts on the No 4 cylinder rocker assembly, which prevented the inlet valve from opening.
- The failure of these bolts was due to fatigue.
- The engine likely remained capable of producing sufficient power for continued flight, despite the unusual noise and vibration.
- The pilot's decision to shut down the engine was influenced by the possibility of an imminent fire or catastrophic failure, and by the guidance in the Pilot’s Operating Handbook regarding engine roughness.