What happened
On 06 July 2010, a Windlass Aquilla microlight, registration ZU-CGD, was conducting flight training at Jag Airfield in the Cullinan area of Mpumalanga. An instructor and a student pilot were performing a series of touch-and-go landings on the airfield's grass runway. During an attempt to take off for a subsequent landing exercise, the right main landing gear wheel detached from the stub axle. This sudden mechanical failure caused the aircraft to veer sharply to the right, resulting in a ground loop that took the aircraft off the runway. The impact caused substantial damage to the engine, propeller, wing, and undercarriage. The student pilot, seated in the front, sustained serious injuries, including a fractured left wrist and facial injuries, while the instructor sustained minor facial abrasions.
The investigation
SACAA AIID investigators examined the wreckage and the mechanical components of the landing gear. The investigation focused on the right landing gear wheel securing bolt, which connects the wheel to the stub axle. Physical examination of the bolt revealed that a fatigue crack had developed, originating within the screw thread at the bolt head. Additionally, investigators noted roughness when rotating the wheel by hand, indicating that the wheel bearing was worn. The aircraft had accumulated 7 .63.3 total airframe hours at the time of the accident, well below the 2,000-hour replacement interval specified by the manufacturer for the axle and bearing assembly. The investigation also assessed the condition of the airfield, noting that the grass and dirt runway surface was notably rough and uneven.
Findings
- The primary cause of the accident was the failure of the right landing gear wheel securing bolt due to metal fatigue.
- The uneven and rough surface of the grass runway subjected the retaining bolt to variable loading, which facilitated the progression of the fatigue crack.
- The right landing gear wheel bearing was found to be worn, exhibiting detectable roughness during manual rotation.
- The weather conditions at the time of the incident were fine, with calm winds and good visibility, and did not contribute to the event.
Safety action
- It is recommended that RAASA, in coordination with MISASA, evaluate the condition of runway surfaces at unlicensed aerodromes to ensure they are suitable for takeoff and landing operations.
- A review of the necessity for specific scheduled inspection and replacement intervals for landing gear wheel bearings and securing bolts is recommended, in consultation with the manufacturer.