History of the Flight
On 26 September 1999, at approximately 1310 UTC, a Piper PA-28-161 (registration G-BSVF) was conducting a private flight from Wycombe Air Park, Marlow. After completing a touch-and-go landing on Runway 25, the pilot applied full power and initiated a climb. At around 200 feet above ground level, a loud bang was heard and the engine began running very roughly. The pilot carried out engine-related cockpit checks, observing that all engine gauges indicated normally. Deciding to return immediately, he declared an emergency to air traffic control (ATC). At about 800 feet agl, he turned to land downwind on the reciprocal Runway 07. However, while making his approach, he heard ATC clear another aircraft for takeoff on Runway 25, prompting him to realign for the grass Runway 07. He observed the other aircraft climbing away and continued his approach, with the engine still running at idle. Selecting full flap near the runway threshold, he found difficulty reducing airspeed and touched down about midway along the grass runway. Maximum wheel braking was applied, but the wheels skidded on the grass. The aircraft came to a halt after colliding with the airfield's boundary hedge. The pilot was not injured.
Examination of the Engine
Initial examination revealed that the No 1 cylinder lower spark plug, complete with its helicoil insert, had been blown out of the cylinder head. Metallurgical examination of the cylinder, spark plug, and helicoil insert showed that an oversized helicoil thread insert had been fitted into the spark plug hole. The new threads cut to receive the insert had been cut off-centre, resulting in threads only partially formed on one side, which prevented the helicoil from being held securely. Additionally, the sealing washer between the spark plug and cylinder head had contacted only about 120 degrees of the cylinder head's circumference during operation. Evidence of excess threads inside the cylinder head that had not been dressed out strongly suggested that the oversized helicoil was fitted after the last overhaul by an approved maintenance organisation, with the cylinder still on the engine.
Previous Overhaul
The engine had been overhauled to zero hours in 1994 by a manufacturer-approved overhaul organisation. During that overhaul, the No 1 cylinder was refurbished, including rework of the lower spark plug hole. This rework involved partial filling (re-metalling) of the hole and tapping a new thread using a manufacturer-approved jig to accept a standard-size helicoil insert. Excess thread at the inner end was dressed out per manufacturer instructions. Inspection of aircraft and engine log books, as well as the operator's spares provisioning list, revealed no record of any subsequent replacement of the No 1 cylinder or work to re-tap the spark plug hole and fit an oversized helicoil. The engine had accumulated 1,934 hours since the 1994 overhaul.
Previous Similar Incident
Twenty-two days before this incident, on 4 September 1999, the same operator experienced a similar blowout of the No 1 cylinder lower spark plug on another Piper PA-28-161 aircraft. AAIB examination of that cylinder showed very similar evidence to the accident aircraft, though the spark plug and helicoil from that earlier event were not available for examination. That engine had been overhauled in 1995 by the same manufacturer-approved organisation, with a standard-size helicoil fitted. Again, no record of subsequent cylinder replacement or spark plug hole rework was found. That engine had accumulated 1,515 hours since its 1995 overhaul.