What happened
On 16 March 1998, a Piper PA-34-200T, registration G-OACG, was operating an air taxi flight from Goodwood to France. While climbing through FL 55, the pilot experienced a sudden loss of power from the right-hand engine. This was accompanied by a significant drop in manifold pressure, falling from approximately 35 ins Hg to 25 ins Hg. A passenger noted an audible bang and observed a fine mist emerging from the right-hand engine cowling louvres.
The pilot successfully executed emergency procedures, shutting down the engine and feathering the propeller. While no fire occurred, oil was observed coating parts of the wing and nacelle. With assistance from London ATC, the pilot diverted the aircraft to Southampton, where it landed safely. There were no injuries to the crew or the four passengers on board.
The investigation
Investigators examined the aircraft' and performed a metallurgical analysis of the failed cylinder head. The inspection of the right-hand engine revealed that the upper portion of the No 3 cylinder head had detached from the cylinder barrel. Crucially, the barrel remained intact, allowing the piston to continue moving normally within the bore, which prevented further mechanical damage to the remaining cylinders.
Detailed analysis of the fracture surfaces indicated that a fatigue crack had developed at multiple sites around the internal threads of the cylinder head wall. This crack had progressed through approximately 120° of the circumference before the remaining 240° failed due to mechanical overload. Evidence suggested the crack had progressed through the cylinder wall for roughly 80 flight cycles prior to the final separation.
Findings
- The primary cause of the power loss was the detachment of the No 3 cylinder head from the cylinder barrel.
- The failure originated from fatigue cracks at the inner threaded wall of the cylinder head.
- The fatigue crack had breached the cylinder wall approximately 80 flights before the actual separation occurred.
- Maintenance records showed the engine had operated for 1,646 hours since its last rebuild, with the most recent compression check occurring at 1,562 hours, suggesting the breach likely happened around the time of that check.