What happened
On 3 June 1999, a BN2B-26 Islander, registration G-BLDV, was performing a public transport flight from North Ronaldsay in the Orkney Islands. Shortly after the pilot applied full power for takeoff, the aircraft experienced a rightward yaw and a visible reduction in RPM on the right engine. The pilot immediately aborted the takeoff, and the engine subsequently ceased operation. Upon inspection of the aircraft, it was discovered that the No 3 cylinder on the right-hand engine had become detached. While the piston crown remained inside the cylinder, the piston skirt and gudgeon pin lugs had fragmented.
The investigation
The AAIB conducted a metallurgical examination of the engine components, including the cylinder, connecting rod, piston fragments, and the right-hand half of the crankcase. The investigation focused on the engine, which had undergone an overhaul approximately 220 operating hours prior to the incident.
Investigators found that all eight cylinder attachment studs, including two through-bolts, had failed due to high cycle tension fatigue. The analysis indicated that the lower rear stud likely failed first, followed by the plastic deformation of the lower pair of studs as the cylinder detached. Furthermore, the failure of the piston gudgeon pin lugs was attributed to low cycle fatigue, likely caused by the piston becoming partially jammed in the bore as the cylinder moved away from the crankcase.
Crucially, the examination of the cylinder hold-down nuts revealed that the cadmium plating on the thrust faces was undamaged and the chromate passivation film was intact. This lack of deformation indicated that the nuts had not been properly torque-tightened. Additionally, grey paint applied to the engine after assembly had migrated under at least five of the nuts, confirming they were loose when the paint was applied.
Findings
- The primary cause of the incident was the failure of cylinder attachment studs due to a lack of pre-load caused by improper torque-tightening of the hold-down nuts during overhaul.
- The absence of proper torque meant the studs were subjected to pulsating loads from the engine's power strokes, leading to high cycle fatigue.
- Although the overhaul agency's records indicated that a duplicate inspection of the nuts had been performed, the physical evidence contradicted these records.
- No evidence of corrosion or material defects was found in the failed studs.