What happened
On 29 June 2004, a Grob G115E Tutor, registration G-BYXJ, was performing a navigation training exercise near Salisbury, Wiltshire. The flight, operated by the RAF University Air Squadron, was conducting a sequence of aerobatic manoeuvres when the aircraft experienced a sudden, violent event. While the aircraft was level at approximately 120 kt, a loud bang and extreme vibration occurred, accompanied by debris passing the cockpit canopy.
The vibration was so intense that the pilot could not read the cockpit instruments. During the event, the canopy detached from the aircraft. The pilot managed to shut down the engine, which halted the severe vibration, and subsequently identified that one propeller blade was missing and another was badly damaged. The pilot performed a successful forced landing in a field of standing crop. Despite the substantial damage to the engine and airframe, there were no injuries to the two crew members on board.
The investigation
The AAIB investigation focused on the mechanical failure of the propeller assembly. Examination of the propeller hub revealed that the No 1 blade had detached because the blade socket in the aluminium alloy hub had fractured. This fracture allowed the blade retaining nut and a portion of the socket to separate from the hub. The departing blade then struck the remaining blades, causing further damage. The investigation also found that the severe vibration had caused the canopy to detach and had resulted in significant damage to the engine mountings and various engine components.
Investigators also reviewed maintenance records and found that a scheduled inspection of the propeller ferrules had been overlooked due to a clerical error in the technical log and a lack of oversight by both the maintenance organisation and the pilot. However, the investigation determined that this missed inspection was not a contributory factor to the failure itself.
Findings
- The primary cause of the failure was a high-cycle fatigue failure of the blade socket in the aluminium alloy hub.
- The fatigue cracks originated in the threads of the socket, likely driven by cyclic stresses.
- A significant reduction in the preload of the propeller blade retaining nut occurs during the first few hours of operation, which reduces the rigidity of the blade in the hub.
- The Grob G115E is known to experience high levels of vibration, particularly at engine speeds above 2,500 RPM, which may have contributed to the fatigue process.
- Evidence of fretting on the thread flanks indicated that the blade retaining nut had been moving within the hub.