What happened
On 12 June 2006, a Diamond HK 36 TC, registration G-OSFA, was conducting a training flight at Enstone Airfield, Oxfordshire. The flight was intended to help the pilot re-familiarise themselves with the aircraft type. Following a successful first circuit and landing, the aircraft began its second circuit.
During the landing phase of the second circuit, a loud metallic scraping noise was heard coming from the front of the aircraft immediately after the main wheels touched down. As the nosewheel was lowered, the nosewheel detached from the nose landing gear leg, causing a loss of directional control. The instructor took control of the aircraft, raised the nose, and shut down the engine. The nosewheel was eventually located approximately 150 m from where the aircraft came to rest. There were no injuries to the two crew members on board.
The investigation
Investigators examined the damaged nose landing gear and found that both wheel fork arms had failed due to an upward bending overload. Metallurgical analysis indicated that the left arm failed first, which subsequently placed a twisting load on the right arm.
Evidence from the failure surfaces suggested that a crack had initiated during a prior event, though the timing and nature of that event could not be determined. The examination also revealed that the fork arms had been sulphuric acid anodised, which may have reduced their resistance to crack initiation and propagation. Furthermore, the nosewheel tyre fitted to the aircraft was a 5.00-4 size, which is larger than the specified 4.00-4 tyre. This larger tyre resulted in only 10 mm of clearance between the tyre sidewall and the inside of the fork arms, compared to the standard 15 mm, leading to heavy contact between the tyre and the arms.
Findings
- The nose landing gear failed because the wheel fork arms failed in an upward bending overload.
- A pre-existing crack had initiated from an earlier event and had progressed due to repeated upward cyclic bending loads.
- The use of an oversized 5.00-4 tyre reduced the clearance between the tyre sidewall and the fork arms, causing heavy contact.
- The sulphuric acid anodising on the fork arms may have adversely affected their resistance to surface cracking.
- The aircraft had previously been involved in a landing accident in November 2005 involving a similar nose gear failure.