What happened
On 31 October 1998, a Davis BW Denney Kitfox, registration G-FOXC, was conducting a private flight from Perth to East Fortune. While traversing the Firth of Forth, the pilot detected an odor of burning coming from the engine. In response, the pilot executed a 180-degree turn to return to land, reducing engine power from 5,400 to 4,000 RPM and directing the aircraft toward Sorbie airfield.
As the aircraft descended through 3,700 feet, the engine ceased operation abruptly. The pilot managed to identify a suitable field and performed a safe landing into the wind. There were no fatalities and no injuries to the two occupants on board.
The investigation
Post-landing inspections of the engine revealed substantial damage, specifically that the crankshaft had failed and subsequently penetrated the engine casing. Upon dismantling the engine, investigators identified a failure in the crank web located near the main bearing at the rear of the engine. Analysis of the fracture surfaces revealed clear evidence of fatigue crack growth. The cracks originated on the 'big end' side of the web, where the forging had been machined to a flat surface, creating an abrupt change in cross-section.
Investigators also observed significant fretting damage on the main bearing. This damage was attributed to vibrations and loads resulting from the loss of the plastic cages that maintain the position of the balls within the bearing races. The engine had been in service for approximately 96 hours since its installation.
Findings
- The primary cause of the engine failure was a fatigue crack in the crankshaft web.
- The crack initiated at a point where the forged material had been machined, creating a stress concentration.
- The loss of plastic bearing cages contributed to bearing damage via vibration and instability.
- Two potential mechanisms for the cage failure were considered: chemical contamination from combustion byproducts or excessive crankshaft vibration.
- The aircraft was not equipped with a 'hydro damper,' a component recommended by the manufacturer and UK agents to mitigate torsional vibration in engines with electric starters.