What happened
On 15 March 1998, a Piper PA-28-181, registration G-BHZE, was conducting a pleasure flight from Edinburgh to Glenrothes via Dundee. The aircraft was carrying a crew of two and one passenger. As the aircraft approached Dundee Airport, the engine failed, although the propeller continued to rotate. The Chief Flying Instructor, who was part of the crew, took control of the aircraft and declared a MAYDAY to Leuchars ATC. The pilot successfully performed a forced landing on Runway 28 at Dundee Airport, and all three occupants escaped without injury.
The investigation
Investigators examined the Lycoming O-360-A4M engine and discovered extensive internal destruction. Specifically, the No 3 cylinder exhaust valve had broken into two pieces, and the piston had suffered massive impact damage. The piston crown had been severely damaged by repeated contact with the broken valve parts, eventually causing the top of the piston to detach and jam within the cylinder. This fragmentation also led to secondary damage to the camshaft lobes and cam followers.
Metallurgical analysis of the components revealed that the No 3 exhaust valve spring upper seat had lost its inner flange. The examination of the fractured surfaces showed evidence of corrosion, suggesting a crack had been present for some time. The material used for the valve seat was found to be a high-sulphur steel with a coarse microstructure, which was not ideal for the application. Additionally, the valve stem retaining collets were found to have been poorly seated.
Findings
- The primary cause of the engine failure was the detachment of the inner flange of the No 3 exhaust valve spring upper seat.
- This failure allowed the exhaust valve to drop into the cylinder, leading to the destruction of the piston and the subsequent breakage of the valve.
- Contributing factors included the poor seating of the valve stem collets and the substandard material quality of the spring seat.
- Evidence suggested that a crack had existed in the seat prior to the engine's last overhaul, with corrosion occurring during a period when the engine was not in use.
Safety action
- Safety Recommendation 98-54: To prevent similar failures and the resulting massive engine damage, it is recommended that the FAA require the mandatory replacement of all upper valve spring seats during 'zero-life' overhauls of Textron-Lycoming reciprocating engines by amending Service Bulletin 240P.