What happened
On 6 March 1998, a Boeing 767-336, registration G-BNWT, was conducting a public transport flight from Tel Aviv Airport to London. Roughly 15 minutes after departure, the aircraft's left engine experienced a complete loss of oil. The crew initiated an immediate return to Tel Aviv.
During the subsequent landing, the aircraft was over its maximum weight limit. Because the right engine's thrust reverser was inoperative, the braking system experienced significant heat buildup during the rollout. After taxiing to a gate and allowing the 238 passengers and 11 crew members to disembark without injury, the left landing gear tyres deflated due to the activation of the wheel fusible plugs.
The investigation
Investigators examined the left engine and discovered that the accessory gearbox casing had been punctured, which caused the total oil depletion. A detailed strip examination of the engine's high-speed gearbox was performed. This revealed that the starter driven bevel gear had failed, which subsequently caused the damage to the casing and the left-hand fan cowl door.
Findings
- The primary cause of the oil loss was a failure of the starter driven bevel gear.
- This failure originated from high engine starting torque, which created a crack that grew through low cycle fatigue.
- The crack eventually breached the gear diaphragm, causing several teeth to break away.
- The failed component was an older standard part; a Service Bulletin had been issued by the manufacturer to replace these with a more robust version featuring a thicker diaphragm.
- While the operator was in the process of replacing these gears during overhaul visits, the engine in question still contained the older, more vulnerable gear.