What happened
On 2 May 2014, a Quad City Challenger II, registration G-MYIX, was conducting a check flight near Louth, Lincolnshire. After departing from North Coates Airfield, the pilot reached an altitude of approximately 1,0 and attempted a timed climb to 2,000 ft. During the application of full power, the engine RPM suddenly increased rapidly, indicating a loss of connection between the engine and the propeller.
Realizing the drive to the propeller had failed, the pilot throttled back and transitioned into a glide. The pilot attempted a forced landing in an arable field, approaching from the southwest. While the approach appeared stable, the pilot discovered a height difference in the terrain near a drainage ditch. An attempt to avoid the embankment resulted in a stall and a heavy impact. There were no injuries to the two occupants, though the aircraft sustained significant damage to the nosewheel, left main wheel, nose cone, and the underside of the floor pan.
The investigation
The investigation focused on the mechanical failure of the engine's drive system. The aircraft utilizes a belt drive to connect the engine driveshaft to the propeller shaft. Post-accident analysis revealed that the drive belt had slipped out of the lower pulley groove.
Detailed inspection of the propeller drive components showed that the lower pulley was rotating in an elliptical pattern. This was caused by excessive wear on the internal surface of the pulley and an accumulation of aluminium alloy on the shaft. Although the securing bolt was tight, the components had been slipping periodically. The pilot noted that previous intermittent RPM fluctuations, which he had previously attributed to carburettor icing, were likely symptoms of this mechanical slippage.
Findings
- The engine RPM runaway was caused by the drive belt migrating out of the lower pulley groove.
- The lower pulley was rotating eccentrically due to internal wear and material build-up on the shaft.
- Misalignment between the engine and propeller shafts is considered a likely cause, potentially stemming from maintenance performed 19 flight hours earlier.
- Records indicated the lower pulley had been replaced recently, but the work lacked the required Permit Maintenance Release.
- The pilot's previous observations of engine RPM fluctuations were likely related to the progressive failure of the drive system.