Background
Two Grob G115E Tutor aircraft, operated by the UK Ministry of Defence and maintained on the civil register, suffered similar propeller failures in separate incidents. The fleet consists of 119 aircraft, and the airframe/engine/propeller combination is unique to this fleet. The propeller type is a Hoffmann HO-V343K-V/183GY three-bladed, constant-speed unit with wooden blades sheathed in carbon-composite. Each blade has a counterweight assembly secured by two semi-circular clamps and bolts.
First Event: G-BYUB
On 23 August 2012, G-BYUB completed an aerobatic detail and touch-and-go landings at RAF Cranwell. During the initial climb after a final takeoff, a loud bang was heard followed by severe vibration. The pilot reduced power, lowered the nose, and landed in a level field of crops near the airfield. Post-event examination revealed severe damage to one propeller blade, which was missing its counterweight assembly. The spinner was disrupted. Despite a search with metal detection equipment, the missing assembly was not found.
Second Event: G-CGKC
Approximately four months later, on 9 January 2013, G-CGKC experienced a similar occurrence. During initial climb, a counterweight assembly detached, causing severe vibration and damage to a propeller blade and spinner. The aircraft landed safely in a field. A fractured threaded bolt portion with a nut was found on the ground below the propeller. The aircraft had completed about 60 flight cycles since the counterweight assemblies were re-installed with new nuts and bolts after the first event.
Investigation Findings
For G-BYUB, unusual markings in a ferrule recess were noted, and a lightning strike was suggested as a possible contributor but not confirmed. The counterweight assembly was missing, preventing a definitive cause.
For G-CGKC, detailed examination of the recovered bolt and nut showed that the bolt had suffered a fatigue fracture with multiple origins at the thread root on the side closest to the counterweight. The fracture face exhibited ten distinct beach marks, corresponding to the final ten flight cycles. The clamp half was deformed plastically. The investigation concluded that the fatigue failure likely resulted from elevated local static stress due to non-parallel clamp faces during installation, which concentrated stress on one side of the bolt. This stress, combined with operational centrifugal loads, initiated the fatigue mechanism.
Subsequent Actions
After the first event, the fleet was temporarily suspended. All counterweight assemblies were removed, examined, and refitted with new hardware before return to service. After the second event, the fleet was again withdrawn from service for further investigation.