What happened
On 23 August 2012, a Grob G115E Tutor, registration G-BYUB, was performing touch-and-go landings at RAF Cranwell, Lincolnshire, following an aerobatic session. During the initial climb, a loud bang was heard, followed by intense vibrations. The pilot managed to reduce power and perform a successful level forced landing in a nearby field. The aircraft sustained damage to the propeller blade and spinner, and the counterweight assembly was missing.
Approximately four months later, on 9 January 2013, a second Grob G115E Tutor, registration G-CGKC, experienced a nearly identical failure during a flight. The aircraft also landed safely in a field after the propeller counterweight assembly detached, causing severe damage to the blade and spinner. No injuries were reported in either incident.
The investigation
Investigators examined the damage to both aircraft, noting that the destruction of the propeller blades was caused by the counterweight assembly moving radially outward along the pitch change axis. In the second incident involving G-CGSB, investigators recovered a fractured bolt and a portion of a clamp half.
Laboratory analysis of the recovered bolt revealed a fatigue fracture with multiple origins at the base of the thread. The investigation focused on the assembly of the Hoffmann HO-V343K-V/183GY three-bladed propeller. It was noted that the design of the counterweight assembly makes it difficult to ensure the two clamp halves are perfectly parallel when tightening the bolts to the required torque. This difficulty can lead to an eccentric stress distribution, creating a concentrated load on one side of the bolt.
Findings
- The primary cause of the failures was fatigue cracking in the counterweight assembly bolts.
- The fatigue was likely initiated by concentrated static tensile loads combined with operational centrifugal forces.
- The method of installation likely contributed to the failure, as the difficulty in aligning the clamp faces parallel may have caused uneven stress distribution.
- In the case of G-CGKC, the fatigue crack originated from multiple points at the base of the bolt threads.
Safety action
Following the first event, the G115E fleet's operation was temporarily suspended, and all counterweight assemblies were inspected and refitted with new hardware. After the second event, the fleet was again temporarily withdrawn from service. The operator commissioned an independent review, and findings were shared with the propeller manufacturer to develop a strategy for returning the aircraft to service.