What happened
On 4 August 2008, a Grob G115, registration G-BOPT, was conducting a flight at Manchester (Barton) Airport. The flight involved a flight instructor performing a demonstration, followed by a precautionary circuit and a full stop landing. During the landing, the student pilot executed a stable approach and touchdown on Runway 27L. As the aircraft began to decelerate after landing, it started to veer toward the right. The instructor, suspecting a steering issue, applied left rudder to correct the heading, but the aircraft continued to deviate. It was subsequently discovered that the right main wheel assembly had completely detached from the aircraft.
Upon inspection of the aircraft after the event, it was found that the right main landing gear stub axle had broken, causing the entire wheel assembly to separate from the airframe. There were no injuries to the two crew members on board.
The investigation
The AAIB conducted a metallurgical examination of the fractured stub axle. The investigation focused on the point of failure, which was located near a change in cross-section on the axle that formed a shoulder for the inner wheel bearing.
Microscopic analysis revealed two distinct areas of fatigue cracking on opposite sides of the axle. These cracks originated from multiple points on the outer surface. The investigation determined that the axle's symmetrical design allowed it to be installed on the strut in either orientation. Evidence suggested that the first fatigue crack had developed and grown during a previous period of service when the axle was installed in the opposite orientation (rotated 180°). The second region of fatigue cracking developed later, eventually weakening the remaining material to the point where the axle suffered an overload rupture.
Findings
- The right main wheel separated from the Grob G115 due to the failure of the right main gear stub axle.
- The fracture was caused by fatigue cracking originating from the outer surface of the axle.
- The failure occurred in two stages: an initial period of fatigue cracking occurred when the axle was installed in one orientation, and a second period of fatigue occurred after the axle had been reinstalled in the opposite orientation.
- The remaining material was insufficient to withstand the loads during landing, leading to the final rupture.