What happened
On 22 June 2001, a Reims Aviation FA-150 L, registration G-BAOP, was conducting a VFR flight from Weston Airfield to Sligo Airport. Approximately 20 minutes into the journey, near Athboy, Co Meath, the engine suddenly ceased operation following a loud noise. The pilot attempted a forced landing in a nearby field but was unable to clear a tall hedge at the far end of the area. As the aircraft approached the hedge at an altitude of roughly 25 feet, it stalled and struck the ground heavily. The impact caused the nose undercarriage to fail and resulted in extensive damage to the wings, fuselage, and engine. While the pilot sustained minor cuts, the passenger, who was pregnant, was taken to the hospital for observation but remained uninjured.
The investigation
The AAIU investigation focused on the sudden engine stoppage and the mechanical condition of the engine. Metallurgical analysis of the crankshaft revealed that the failure originated at the junction of the central main bearing and the crank. Investigators discovered that the main bearing diameter had been modified using a flame spray process to build up metal, followed by grinding. This repair was found to be unapproved and inappropriate for this component. Furthermore, the investigation established that the 'key' coat used to adhere the metal was insufficient, leading to flaking in a critical stress region. The investigation also looked into the maintenance history, finding that the crankshaft had been provided to repairers by the owners without any accompanying service records or airability documentation, which violated UK CAA policy.
Findings
- The engine failure was directly caused by the fatigue failure of the crankshaft.
- The crankshaft failure was due to the use of unapproved repair procedures, specifically the application of metal via flame spray and improper surface preparation.
- The fatigue resistance of the component was significantly reduced because the crankshaft lacked a necessary nitriding coating.
- The aircraft owners acted inappropriately by providing a used component to repairers without any verifiable service history.
- The repair facility failed to follow regulatory protocols by accepting a part without established airworthiness documentation.
- A cracked exhaust stub on the engine may have masked the developing engine vibrations and noise that could have alerted the pilot to the impending failure.
- The use of 4-point harnesses was a critical factor in preventing serious injury during the heavy impact.
Safety action
Following the investigation, recommendations were made to the UK CAA to review regulations regarding the installation of used parts without documentation in the private category. Additionally, calls were made for increased monitoring of repair facilities to prevent the use of unapproved flame spray techniques on crankshafts and to educate the general aviation industry on the risks of using undocumented components.