What happened
On 26 September 1999, a Piper PA-28-161, registration G-BSVF, was performing a private flight at Wycombe Air Park. Following a successful touch-and-go landing on Runway 25, the pilot initiated a climb. At an altitude of approximately 200 feet, an audible bang occurred, followed by severe engine roughness.
While monitoring engine gauges, which appeared normal, the pilot declared an emergency and decided to return to the airfield. The pilot initially intended to land on the reciprocal Runway 07 but transitioned to a grass runway approach to avoid a departing aircraft. During the approach, the engine continued to run at idle. Despite applying full flaps, the pilot struggled to reduce airspeed and landed midway down the grass runway. The aircraft'seb skidded during braking and eventually came to a stop after striking the airfield's boundary hedge. The pilot sustained no injuries, though the aircraft suffered extensive damage beyond economic repair.
The investigation
Investigators examined the engine and discovered that the lower spark plug of the No 1 cylinder, including its helicoil insert, had been ejected from the cylinder head. Metallurgical analysis revealed that an oversized helicoil thread insert had been installed in the spark plug hole. The investigation found that the new threads had been cut off-centre, meaning the threads were only partially formed on one side of the hole. This prevented the helicoil from being held securely.
Furthermore, the sealing washer for the spark plug was only in contact with the cylinder head over a 120-degree arc. Evidence of undressed excess threads inside the cylinder head suggested that the installation of the oversized insert occurred while the cylinder was already fitted to the engine, rather than during a formal overhaul. Records for the aircraft did not show any recent replacement of the cylinder or any maintenance related to retapping the spark plug hole.
Findings
- The engine failure was caused by the ejection of the lower spark plug and helicoil insert from the No 1 cylinder.
- The spark plug hole had been improperly modified with an oversized, off-centre helicoil thread insert.
- The improper installation resulted in an insecure fit and inadequate sealing of the spark plug.
- The maintenance work to install the oversized insert appears to have been performed outside of an approved maintenance organization's overhaul process.
- A similar incident involving the same operator and the same aircraft type had occurred only 22 days prior to this accident.