What happened
On 10 December 2009, a Cessna 172M Skyhawk, registration G-ECON, was performing a private flight from Dunkeswell back to Bournemouth Airport. During the takeoff run on Runway 05, the pilot experienced a sensation similar to hitting a pothole. After the aircraft became airborne, the pilot and passengers performed a visual check of the landing gear and wheel spats, which initially appeared normal. However, after the aircraft had travelled roughly 200 metres along the runway following touchdown, a loud noise occurred, and the rear passenger reported that the left mainwheel had detached from the aircraft.
The separation caused the outer section of the wheel hub, the tyre, and the inflated inner tube to depart the aircraft. The incident resulted in damage to the left wheel spat, hub, and brake disc. There were no injuries to the pilot or the two passengers on board.
The investigation
Investigators identified debris consisting of fibreglass on the runway at Dunkeswell, which was linked to the aircraft via a distinctive paint scheme. Although the pilot had noted nothing unusual during the pre-flight inspection, they had commented that inspecting the wheels was difficult while the spats were fitted. The wheels had been last visually inspected by the same maintenance organisation approximately 19 flying hours earlier during a 5-hour inspection.
Examination of the recovered components revealed that while the inner hub and bearing assembly remained attached to the axle via the retaining nut, the outer section of the hub had separated. Of the six bolts used to clamp the hub sections together, four were recovered. The investigation found that the first two to three threads in two of the threaded holes in the inner hub section had been stripped. The threads on the bolts themselves remained intact.
Findings
- The separation of the wheel hub was caused by the unwinding of the clamping bolts.
- Four of the six bolts had moved out of their locating holes, and the remaining two bolts had had their threads stripped due to the loss of tension.
- The failure was likely caused by either improper initial installation or the release of torque on all bolts, allowing vibration to unwind them.
- The maintenance organisation used no compound or physical locking devices on these bolts, which are required to be torqued to 190–200 lb-in.