History of the Flight
On 15 March 1998, a Piper PA-28-181 (registration G-BHZE) departed Edinburgh for Glenrothes via Dundee with three occupants: the pilot, the club Chief Flying Instructor (CFI), and another member of the Turnhouse Flying Club. While approaching Dundee, the engine failed but the propeller continued rotating. The CFI took control, declared a MAYDAY to Leuchars ATC, and successfully executed a forced landing on Runway 28 at Dundee Airport. No injuries were reported.
Examination of the Engine
Initial inspection revealed damage to the No 3 cylinder valve rocker area and a missing No 3 exhaust valve. The engine was removed and sent for repair. It was found that the No 3 exhaust valve had broken into two parts, both within the upper cylinder. The piston crown showed severe impact damage from repeated contact with the exhaust valve before the top of the piston detached and jammed in the cylinder, trapping the broken valve parts. The lower piston fragmented into numerous small pieces distributed throughout the engine. Further damage included serious wear to camshaft lobes, broken cam followers, and bending of the No 3 exhaust valve pushrod and tube.
Metallurgical Examination
The metallurgical investigation determined that the camshaft and follower damage was secondary. The No 3 exhaust valve had not jammed in its guide, though the guide had migrated and broken up. The valve stem retaining collets had broken; recovered parts showed evidence of poor seating. The valve springs were released, with the lower spring seat fragmented and the upper 'top hat' spring seat missing its inner flange. Debris from the missing flange was found. Fracture surfaces on the upper seat fragments showed repeated impact damage, some corrosion, and a radial crack that appeared to have existed for some time. Microscopic examination revealed coarse material structure with segregation bands and manganese sulphide stringers, consistent with free-machining steel. The valve seat had been machined from rolled bar and surface hardened, resulting in a non-ideal grain orientation. Considerable wear of the seat was noted compared to other similar parts.
Engine Maintenance
The engine had accumulated 644 hours since its last overhaul in July 1995, when it was zero-lifed and fitted to G-BHZE. At that overhaul, visual and magnetic particle inspection of the valve spring seats was performed, and no cracking was detected. The overhaul agency noted that such parts were frequently rejected for cracking found during inspection. The age of the failed valve spring seat could not be determined. The report highlighted the difficulty of inspecting these small parts and suggested that renewal of valve spring seats during zero-life overhauls would be prudent.
Engine Failure Sequence
The investigation concluded that the engine failure resulted from detachment of the No 3 exhaust valve spring upper seat inner flange. This allowed the exhaust valve to drop into the cylinder, causing destruction of the No 3 piston and breakage of the exhaust valve due to repeated piston contact. Post-fracture damage prevented positive identification of the detachment mechanism, but poor collet seating and material quality of the spring seat were factors. The metallurgical report indicated that the cracking of the seat existed before the overhaul, and corrosion occurred during the engine's period of non-use (April 1993 to July 1995).
Safety Recommendation
Based on the findings, the AAIB issued Safety Recommendation 98-54, urging the FAA to require mandatory renewal of all spring seats during zero-life overhauls of Textron-Lycoming reciprocating engines, through amendment of Service Bulletin 240P.