What happened
On 6 August 2013, a Vans RV-9A, registration G-CDCD, was performing a series of three circuits at Wellesbourne Mountford Airfield in Warwickshire. During the second circuit, while on the downwind leg at approximately 1,000 feet, the Wilksch WAM-120 diesel engine ceased operation. Although the propeller appeared to be windmilling, the pilot noted that fuel pressure had dropped to zero. Despite an attempt to switch the fuel tank selector back to the right-hand tank, the engine failed to restart.
The pilot executed a forced landing in a nearby field. During the landing sequence, the aircraft overran the available space, striking a hedge, a wire fence, and small trees. The impact caused the aircraft to tip onto its nose in a near-vertical position. The pilot sustained one minor injury and was later extricated from the wreckage by emergency responders.
The investigation
The AAIB investigation focused on the engine's mechanical state and fuel quality. Upon examination of the engine at the manufacturer's facility, investigators discovered that the pistons had repeatedly struck the exhaust valves. This indicated a loss of timing between the crankshaft and the camshaft.
Further inspection of the timing gear revealed that three of the four bolts securing the gear to the crankshaft had failed. Metallurgical analysis of these bolts showed evidence of high-cycle fatigue. Additionally, the investigation examined the fuel system, where a significant amount of water and dark, discoloured fuel was found within the filter bowl.
Findings
- The engine failure was likely caused by either contaminated fuel or the failure of the timing gear bolts.
- The fuel filter bowl contained a high percentage of water, which the manufacturer estimated was sufficient to cause the engine to stop.
- Three timing gear bolts had suffered fractures due to high-cycle fatigue; however, it is believed the internal engine damage likely occurred during the impact with the fence rather than during flight.
- Testing of the bolts indicated they were below the required specification for hardness and were deficient in chromium and nickel content.