Introduction
On 16 March 1998, a Piper PA-34-200T, registration G-OACG, was being operated as an air taxi from Goodwood to France. The aircraft, manufactured in 1978, was powered by two Continental TSIO-360-EB piston engines. The flight crew consisted of a single pilot holding a Commercial Pilot's Licence with an Instrument Rating, who had accumulated 8,500 total flying hours, including 2,000 hours on type. There were four passengers on board.
Sequence of Events
While climbing through Flight Level 55, the pilot noted a sudden loss of power from the right-hand engine, accompanied by a rapid fall in manifold pressure from 33-35 inches of mercury to approximately 25 inches of mercury. The front right-seat passenger reported hearing a bang at the time of the power loss, and a fine mist was observed emanating from the louvred grill on top of the right-hand engine cowling. The pilot shut down the affected engine and feathered the propeller in accordance with the emergency checklist. No fire was observed, though a film of oil was present over parts of the nacelle and wing. Both engines had performed normally during start-up, power checks, takeoff, and climb prior to the incident. Although outside controlled airspace, London Air Traffic Control assisted with an immediate diversion to Southampton, where the aircraft landed safely and the passengers disembarked.
Examination Findings
Examination of the right-hand engine revealed that the upper portion of the No 3 cylinder head had detached from the barrel. The barrel portion of the cylinder remained intact, allowing the piston to move normally within the bore. The failure did not mechanically affect the other cylinders, limiting engine damage. The fracture occurred around the upper threads where the cylinder head is screwed onto the cylinder barrel; the lower portion of the cast aluminum cylinder head remained attached to the barrel. Detailed fractographic analysis showed that a fatigue crack had propagated across the cylinder head wall through approximately 120 degrees of the circumference, with the remaining 240 degrees failing through mechanical overload. The fatigue initiated at multiple sites around the inner threaded wall of the cylinder head. Event markers within the fatigue fracture surface indicated that the crack progressed for about 80 flights between breaking through the cylinder wall and final separation.
Maintenance Details
The maintenance organisation reported that the failure occurred at 1,646 hours after the engine rebuild. The previous compression check had been performed at 1,562 hours, suggesting that the breakthrough occurred around the time of that check. The mechanical properties and chemical composition of the cylinder head were not determined, but no mechanical discontinuities that would have reduced the basic fatigue strength were observed. Following the incident, when a replacement cylinder was fitted at the No 3 position, the maintenance organisation noted that one magneto was faulty, with irregular running and internal arcing. However, it was considered unlikely that this magneto issue contributed to the initiation of the fatigue crack, as it would have remained undetected through the intervening hours of operation.