What happened
On 6 October 1999, a Kolb Twinstar MK3 (Modified), registration G-MYOG, was conducting a private flight from Redhill to Lydd. The weather conditions were clear with light winds. After approximately one hour of flight, the pilot contacted Lydd radio for circuit information and switched to the alternate fuel tank. Shortly after this action, the engine RPM decreased, the engine ran roughly, and then ceased functioning.
The pilot issued a MAYDAY call and began procedures for a forced landing. While attempting to reach a chosen field, the pilot experienced a steeper descent than expected, likely due to the propeller being stationary. In an attempt to reach the landing site, the pilot raised the aircraft's nose, which led to a loss of airspeed and insufficient pitch authority. The aircraft struck the ground heavily and came to rest in an inverted position. The pilot and passenger sustained one serious and one minor injury respectively.
The investigation
The AAIB examined the engine and the failed crankshaft. The investigation focused on the mechanical cause of the engine stoppage and the circumstances of the descent. The engine had approximately 300 hours of total running time since new. The investigation also looked into the structural impact of the crash and the effectiveness of recent modifications to the aircraft's seating.
Findings
- The engine failure was caused by a crankshaft failure resulting from rotating/bending fatigue cracking.
- The crack originated at the intersection of the shaft and a rotating counterbalance weight, which featured a sharp corner with minimal blend radius.
- A loose fit was identified between the crankshaft and its aft end support bearing, creating radial clearance.
- This loose fit increased bending stresses at the adjacent bearing, leading to the fatigue failure.
- The pilot's decision to spend time communicating the intended landing location to Lydd radio may have diverted attention from aircraft control during the emergency.
- Modifications to the seat pans and cushions (Service Bulletin KSB1) helped absorb impact energy, which mitigated the severity of the injuries.