What happened
On 22 March 2022, a DHC-1 Chipmunk 22, registration G-BXHA, was conducting a training flight near Sevenoaks, Kent. During the initial climb following takeoff, the pilot noted subtle vibrations within the airframe. Because the pilot lacked recent experience on this specific aircraft type, they were unable to determine if the vibration was within normal parameters and elected to continue the flight.
While performing manoeuvres, the vibrations increased abruptly to a level so severe that the pilot found it difficult to read the cockpit instruments. The pilot decided to return to the airfield. During the taxiing phase, the intensity of the vibration caused the low voltage warning light to be dislodged from the instrument panel.
The investigation
An investigation into the incident revealed significant damage to the propeller assembly, including a cracked propeller and cracks in both the forward and aft boss blocks. The aircraft was equipped with a Fairey Reed A66753/X1 fixed-pitch aluminium propeller.
Investigators found that the forward boss block had suffered a primary fatigue fracture. This crack originated from a worn area on the inner diameter, which had been caused by fretting and corrosion from the engine hub splines. Additionally, the aft boss block experienced three separate overload failures. The propeller itself sustained two fatigue cracks, one of which extended through a hub bolt hole.
While the surface finish of the bolt hole radii and the propeller face did not meet the manufacturer's required specifications, this was not identified as the primary cause of the failure. The investigation also noted that a previous service bulletin for this propeller type recommended visual inspections but did not specifically require checking the inner diameter of the boss blocks for wear or corrosion.
Findings
- The primary cause of the failure was fatigue cracking in the forward boss block and the propeller, which resulted from freetng and corrosion.
- The cracks in the boss blocks and the propeller developed concurrently, likely due to abnormal loading on the assembly.
- The existing service bulletin for the Fairey Reed propeller did not mandate inspections of the boss blocks' inner diameters for signs of corrosion or wear.