What happened
On 8 July 2001, a modified Stampe SV4C, registration G-HJSS, was conducting a private flight from Fenland Airfield in Norfolk to its base at Shoreham. While traversing the airspace over Kent, the crew detected a smell of burning. Initially, the pilot believed the odor might be coming from the ground, but the smell persisted and appeared to originate from the front of the engine.
During the flight, both engine tachometers ceased functioning. While this was a known issue related to vibrations in the Renault engine drive, the situation escalated when the engine lost power that could not be recovered by advancing the throttle. The aircraft descended below 2,000 feet. The crew then heard two rapid bangs and observed an object flying off the front of the aircraft. The pilot subsequently performed a successful forced landing in a nearby field. There were no injuries to the two crew members, and the aircraft sustained no damage during the landing, though the crew discovered upon disembarking that the propeller had detached.
The investigation
Investigators examined the aircraft and found that while the landing itself was undamaged, the front cowling was scorched and the aluminium propeller backplate had nearly melted. An inspection of the six bolts used to mount the propeller revealed significant distress, including evidence of bending, stretching, fracturing, and overheating. The propeller hub and one blade were recovered, both showing signs of heat-related burning. The investigation noted that this specific propeller design, which is wooden with a glassfibre facing, lacks a dowel or key arrangement to provide secondary positioning.
Findings
- The primary cause of the incident was the loss of torque in the propeller mounting bolts, which led to friction-induced heating.
- The overheating and subsequent failure of the bolts caused the propeller to detach from the engine.
- The shrinkage of the wooden propeller due to drying out, likely exacerbated by the particularly wet winter of 2000/2001, contributed to the loosening of the bolts.
- The maintenance organization noted a trend of encountering loose bolts on various wooden propeller aircraft during the same summer season.