What happened
On 21 June 1998, a BN2A Mk III-1 Trislander, registration G-AZLJ, was conducting a scheduled cargo flight from Liverpool to the Isle of Man. During the cruise phase at Flight Level 60, approximately 20 minutes after takeoff, the aircraft experienced a sudden vibration and a rightward yaw. The engine speed on the right engine dropped to 2,000 RPM.
The pilot attempted to shut down the malfunctioning engine; however, before the propeller could be feathered, a loud noise occurred and the propeller stopped completely. This event took place roughly 2 to 4 seconds after the initial power loss. Although the pilot noted that oil pressure had dropped to zero, oil and cylinder head temperatures remained within normal limits. Due to the aircraft being near its maximum weight and the unfeathered propeller, the aircraft descended at a rate of 100 to 200 feet per minute despite maximum power being applied to the remaining two engines. The pilot notified Manchester ATC and diverted to Blackpool, where a safe landing was achieved. There were no injuries to the crew.
The investigation
Following the incident, the AAIB conducted a metallurgical examination of the right engine. The inspection of the engine revealed a significant rupture in the top of the crankcase near the No 6 connecting rod big end. A detailed strip examination at an overhaul facility uncovered extensive internal damage. Investigators found fragments of the big end cap, along with parts of two attachment bolts and one nut.
Further examination showed that the No 4 exhaust valve rocker arm had fractured and the No 4 inlet valve push rod had failed due to bending. The investigation established that these valve components failed because metallic debris from the primary engine failure had become lodged in the valve operating mechanism, increasing the stroke of the push rods and causing them to overload.
Findings
- The primary cause of the engine failure was the detachment of the No 6 connecting rod big end cap.
- This detachment was triggered by the loss of a single retaining nut from one of the big end cap attachment bolts.
- The loss of this nut caused the remaining attachment bolt to fail under extreme bending stress.
- The failure of the No 4 valve rocker arm and push rod were secondary effects caused by metallic debris obstructing the valve mechanism.
- There was no evidence to explain why the retaining nut had unscrewed from the bolt.