What happened
On 26 July 2003, a Stampe SV4C(D), registration G-OODE, was conducting a private flight near Redhill Aerodrome. The pilot, intending to practice aerobatics, had climbed to 1,000 feet and established a cruise speed of 1,900 RPM. Approximately one mile north of the airfield, the engine began to falter. The pilot observed an object detaching from the propeller hub, followed immediately by the complete separation of the propeller from the engine.
Following the loss of the propeller, the pilot declared a MAYDAY, informed Redhill of the engine failure, and initiated a forced landing. Due to standing crops limiting available landing sites, the pilot identified a green area within a former quarry that had been filled and grassed over. The pilot successfully executed a three-point landing at the site, and no injuries were sustained during the incident.
The investigation
An investigation by the AAIB revealed significant damage to the front of the engine. The failure resulted in fractures to the crankshaft, the No 1 cylinder connecting rod, and the forward portion of the engine crankcase. The separation of these components was what allowed the propeller to detach. While the propeller itself remained intact in flight, it was found broken on the ground following its impact with the earth.
Metallurgical analysis of the engine components identified that the big-end bolts had failed, subjecting the big-end cap to a high twisting load. This led to the subsequent failure of the connecting rod and the crankshaft. The examination of the crankshaft revealed fatigue cracking originating at the radius of the No 1 crank pin. Investigators also noted significant corrosion pitting on the crankshaft, particularly around the crank pins.
Engine records indicated the engine had accumulated approximately 1,192 hours since its last full overhaul. While the engine was being operated within the permitted extensions allowed by CAA Airworthiness Notice No 35, the investigation highlighted that certain internal components, such as the crankshaft, cannot be inspected for fatigue or corrosion without a complete engine disassembly.
Findings
- The primary cause of the propeller separation was a fatigue failure of the crankshaft.
- The fatigue crack likely originated from corrosion pitting on the crankshaft surface.
- The failure of the big-end bolts and the connecting rod was a secondary result of the initial crankshaft fracture.
- There was no evidence of insufficient lubrication or excessive wear in the engine components.
- The engine was being operated beyond its nominal 1,000-hour overhaul period under permitted extensions.