What happened
On 20 October 2005, a DA40D, registration G-CCLB, was being prepared for a training flight at Rochester Airport, Kent. While the pilot was taxiing the aircraft toward a mown area for engine power checks, the aircraft performed a sharp left turn at a very low speed. During this maneuver, the nose landing gear wheel separated from the nose leg. This separation caused the propeller to strike the ground, resulting in damage to the nose gear, propeller, and engine due to shock loading. There were no injuries to the two crew members on board.
The investigation
An investigation by the AAIB examined the failed component and also inspected another aircraft of the same type, G-CCUS, operated by the same flight school, after cracks were discovered in a similar location. Metallurgical analysis conducted at the Materials Centre at QinetiQ revealed that fatigue cracking had occurred at the top of the nose wheel swivel/castoring pivots on both aircraft.
Testing showed that the hardness of the pivots was significantly lower than the manufacturer's specifications. While the steel used was the correct alloy, the hardness levels indicated that the material had not achieved the required strength and toughness. The investigation found that the heat treatment process used by the subcontractor was appropriate for a different type of steel but not for the SAE 4130 steel used in the pivot.
Findings
- The primary cause of the failure was a manufacturing error in the heat treatment process, which left the pivot material with insufficient strength.
- The heat treatment applied to the 4130 steel was incorrect, resulting in a hardness level far below the required 320 HV.
- Post-manufacturing quality checks failed to identify the error because inspections were only being performed on the outer surfaces of the struts, rather than the internal components.
- Fatigue cracks had initiated at multiple points within the radius of the pivot due to stress concentrations.
- The failure was exacerbated by the fact that the incorrect heat treatment made the material more susceptible to fatigue under normal in-service loading.