What happened
On 5 November 2005, a Piper PA-28-140 Cherokee, registration G-EEKY, was conducting a local area flight near RAF Marham, Norfolk. While descending from 2,000 ft to join the airfield circuit, the pilot attempted to reduce engine speed by retarding the throttle to 1,500 rpm. During this maneuver, the pilot encountered a sudden restriction in the throttle's range of travel, which prevented the lever from being opened sufficiently to maintain level flight.
In an attempt to clear the issue, the pilot engaged the fuel pump, adjusted the fuel tank selector, and cycled the carburettor heat. None of these actions restored full throttle functionality. Consequently, the pilot performed a forced landing in a field approximately 5 nautical miles northwest of RAF Marham. During the touchdown, the nosewheel dug into the soil, causing the propeller to strike the ground and the engine to undergo a shock load. Both the pilot and the passenger escaped the no injuries incident unhurt.
The investigation
The AAIB investigation examined the aircraft' and the environmental conditions at the time of the accident. The aircraft sustained damage to the propeller, nosewheel, right wing tip, and engine cowl. A Chief Flying Instructor attended the site shortly after the landing and confirmed a throttle restriction, though the movement range differed from that reported by the pilot.
Subsequent inspections by a repair agency found no mechanical interference with the linkage between the throttle quadrant and the carburettor. Furthermore, testing of the throttle linkage and carburettor at a specialized facility failed to reproduce the restriction.
Findings
- The ambient temperature and dew point at the time of the flight were within the range for carburettor icing in the cruise power setting.
- While the engine ran smoothly during the descent, ice may have formed downstream of the accelerator pump near the discharge nozzle, potentially causing a mechanical-like restriction.
- The investigation could not definitively rule out a subtle, temporary mechanical restriction that vanished before the aircraft could be examined by specialists.
- The investigation concluded that the accident was caused by a throttle restriction, the exact origin of which remained unidentified.