What happened
On 5 August 2018, a Skystar Kitfox Mk 7, registration G-FBCY, was conducting a private flight from Old Warden Aerodrome to a farm strip near Coventry. During the descent, while the pilot was reducing power for landing, a loud noise was heard followed by a red warning light on the engine monitor. Simultaneously, the engine speed surged from 5,200 to 7,000 rpm. Although the engine appeared to be running, the aircraft suffered a sudden loss of thrust.
The pilot attempted to troubleshoot the issue, including trying to manually select a coarse propeller pitch, but the controls were ineffective. As the aircraft was flying over a built-up area, the pilot diverted to a nearby ploughed field. During the forced landing, the nosewheel collapsed, causing substantial damage to the propeller, engine, nosewheel strut, and right wing. The pilot and passenger both sustained minor injuries.
The investigation
Investigators examined the aircraft's Arplast PV50 three-bladed propeller and its constant speed controller. While the engine's drive system and gearbox were found to be functioning correctly, a strip examination of the propeller hub revealed that the threads on the lead screw within the pitch-change mechanism had been stripped. This mechanical failure allowed the propeller blades to move into a super-fine pitch setting, which prevented the production of sufficient thrust.
The investigation also looked into maintenance history. The aircraft had recently undergone an engine gearbox overhaul, and the propeller had been refitted shortly before the accident. It was noted that the owner was unaware of a 2008 Airworthiness Information Leaflet (AIL) regarding specific inspection and adjustment requirements for this propeller type, as the information had not been easily accessible via the LAA website at the time.
Findings
- The primary cause of the loss of thrust was the stripped threads on the lead screw within the propeller pitch-change mechanism.
- This failure allowed the blades to move into a super-fine pitch regime, resulting in insufficient thrust for flight.
- The pilot's attempt to diagnose the engine issue while flying over a populated area may have delayed the decision to execute a forced landing, contributing to the hurried nature of the touchdown and subsequent nosewheel collapse.