Accident Details
On 25 June 2008 at 1748 UTC, a Piper PA-28R-201T Turbo Cherokee Arrow III, registration G-BFLI, came to rest in a lake near Bradford Leigh, Bradford-on-Avon, Wiltshire, following a forced landing. The aircraft was substantially damaged and subsequently written off. The sole occupant, a 59-year-old private pilot with 1,273 hours total flying experience (1,098 on type), sustained minor injuries and exited the aircraft unaided.
Background
The aircraft was on its first flight after having two new cylinders (No 2 and No 4) fitted to its Teledyne Continental TSIO-360-FB engine, which had been factory-overhauled in June 2007 and accumulated 71 hours since. Two ground runs were completed satisfactorily before departure.
Flight and Engine Problem
Approximately 90 minutes into the flight at 4,000 ft, the engine began producing a fluttering noise and lost power. The pilot noted normal oil temperature and pressure and briefly adjusted the throttle, but the exhaust gas temperature (EGT) for cylinders No 2, 4, and 6 dropped. Fearing a serious engine failure or fire, the pilot shut down the engine, broadcast a mayday call, and commenced a forced landing.
During the descent, the pilot encountered a tailwind of 30 to 40 kt. While turning final for a selected field, he determined that he would not reach it due to a headwind and would likely hit trees. He opted not to extend the landing gear, applied full flap, reduced airspeed, and made a late turn onto a downwind heading. The aircraft struck a hedge, causing the left wing to detach at the root, then passed through the hedge and came to rest mostly submerged in a lake.
Engineering Investigation
The JPI engine data management system and engine were recovered. Data download showed that EGT for the No 4 cylinder dropped markedly, followed by cylinders No 2 and 6, then the remaining cylinders after engine shutdown. The engine was strip-examined, revealing that the two stud bolts attaching the inlet valve rocker assembly to the No 4 cylinder head had failed, leaving the inlet valve closed. The valve opened satisfactorily when tested manually. Witness marks inside the rocker cover were consistent with the tie-down nuts from the failed bolts.
Metallurgical examination of the fractured bolts found fatigue-initiated failure in both, but no material defects that could have initiated the fatigue. The hardness of one bolt was 33 HRC and the other 35 HRC, slightly above the specified 28-32 HRC range; this was not considered significant. Torque checks on the remaining rocker tie-down nuts (including those on the replaced cylinders and undisturbed ones) indicated correct torque.
Discussion
The engine was likely capable of running and producing sufficient power for continued flight, though with unusual sound and vibration. The pilot's decision to shut down resulted in a power-off forced landing. The pilot had been active most of the day and felt that tiredness may have influenced his decision making. No material defects were found in the failed bolts, and while incorrect torque during reinstallation of the No 4 cylinder rocker could not be ruled out, it could not be confirmed.