What happened
On 18 March 2000, a BN2B-26 Islander, registration G-BLDV, was performing a public transport flight from Glasgow to Campbeltown. While cruising at 3,000 feet over Largs, Ayrshire, the pilot heard a noise resembling hail striking the windscreen. Almost immediately, the right engine lost power. The pilot successfully feathered the right propeller and declared a 'Mayday' to Prestwick Approach. Choosing to return to Glasgow due to better weather conditions, the pilot managed a gradual descent using only the left engine. The aircraft landed safely at Glasgow without further incident, though the Airfield Fire Service attended the scene.
The investigation
Investigators examined the right engine and discovered a hole in the lower forward face of the crankcase. The inspection revealed that the crankshaft had failed near the No 1 journal bearing. Significant internal damage was present, including a bent push rod tube, a holed rocker cover, and the presence of a connecting rod end cap and debris within the cowling.
Metallurgical analysis determined that the crankshaft had suffered a long-term high cycle tension fatigue failure. The fracture had progressed through approximately 75% of the crank pin cross-section before the final overload caused the separation. Evidence of oil starvation, including wear and overheating on the rocker shafts, was also noted. Furthermore, the investigation found that the crankshaft journals had been ground 0.006 inches undersize, and microhardness tests indicated that the surface had not been re-nitrided following this grinding process.
Findings
- The primary cause of the engine failure was a catastrophic fatigue failure of the crankshaft.
- The crankshaft journals had been ground undersize, and the lack of a re-nitriding process likely left the surface vulnerable to stress concentrations.
- The failure of the crankshaft led to an overspeed condition, which subsequently caused the failure of the valve spring seats.
- Maintenance records for the component were incomplete, as the crankshaft serial and part numbers were not recorded during previous overhauls.
- The engine had accumulated 8,564 hours since new and 1,453 hours since its last overhaul.