What happened
On 9 August 1998, a Cessna 140, registration N76402, was conducting a private flight in the Cranfield area. After approximately 50 minutes of flying at altitudes between 1,500 and 2,000 feet in good visual conditions, the pilot prepared to return to Henlow. Following a descent to 1,000 feet and the selection of carburettor heat, the engine power dropped significantly. Although the propeller continued to windmill, the engine failed to produce sufficient power.
The pilot attempted several troubleshooting steps, including checking the throttle position and switching fuel tanks, but was unable to restore power. Realising the aircraft was beyond gliding range of Henlow, the pilot identified a strip of land north of Meppershall Airfield and declared a MAYDAY. While attempting to establish a pattern at Meppershall, the pilot tried to regain power by reselecting the fuel tank and toggling the carburettor heat, but the engine remained non-functional.
During the final approach to Runway 02 at Meppershall, the pilot found the aircraft was too high. In an attempt to lose altitude, the pilot used full flaps and a side slip. As the aircraft approached the end of the strip, the pilot pitched the nose up and turned left to avoid trees and cables. After clearing the obstacles, the pilot descended and attempted to flare. Upon touchdown, the right main landing gear collapsed, causing the aircraft to veer right and strike the nose heavily against the ground. There were no injuries to the two occupants.
The investigation
Following the accident, investigators examined the recovered aircraft and the stripped carburettor to identify the cause of the engine failure. An inspection of the fuel system revealed that the inlet to the carburettor was blocked.
Detailed examination of the Stromberg type NA-S3A1 carburettor showed that the needle valve seat had become unscrewed from the float chamber housing. This movement caused the needle valve to close, preventing fuel from entering the carburettor. While the seat and housing featured holes for wire locking, no locking wire had been applied.
Technical records showed that the carburettor had undergone significant maintenance in November 1997, which included re-facing the valve seat and lapping the needle and seat together. Although the manufacturer's manual specifies that the seat should be torqued and then lockwired once the float level is set, the investigation established that the seat had loosened during operation.