What happened
On 25 May 2020, a Sportstar Max, registration G-TMAX, was performing a private flight departing from White Ox Mead Airstrip near Bath. During the takeoff roll, the pilot noted a high-frequency vibration. Because the aircraft had already passed the point of a safe stop, the pilot continued the takeoff. As the aircraft climbed through 250 ft agl, a loud bang was heard and a propeller blade passed over the canopy.
The loss of the blade triggered intense vibrations that caused the forward-hinged canopy to be pulled into the airflow and led to the engine stopping. The engine failure was caused by the carburettors detaching from the inlet manifolds. The pilot managed to perform a forced landing in an upsloping crop field at approximately 200 ft agl. There were no injuries to the two occupants, and no further damage was sustained by the aircraft during the landing. The detached blade was not recovered from the area.
The investigation
The AAIB examined the three-bladed Woodcomp Klassic 170/3/R propeller. While the attachment bolts to the engine were intact, an inspection of the hub screws revealed that three of the twelve screws were below the required torque of 10 Nm.
Examination of the remaining blades showed cracks on the edges of Blades 1 and 3, likely resulting from the extreme vibration following the failure. The retained root section of the failed blade (Blade 2) showed signs of failure due to overload from centrifugal and thrust loads. The manufacturer noted that the failure pattern was similar to previous incidents involving propeller strikes with ground or foreign objects. No evidence of progressive cracking or polymer matrix whitening was found on the blade surface, suggesting the failure was not a gradual process.
Findings
- The propeller blade detached due to a combination of centrifugal and thrust loads during the initial climb.
- The propeller was likely weakened prior to the failure, potentially by a previous strike with a foreign object or the ground.
- The vibration noted during the takeoff roll was consistent with a change in the mass or stiffness of a blade during the run.
- The engine ceased operation because the carburettors were pulled from the inlet manifolds by the resulting vibration.