What happened
On 27 May 1999, a Cessna 152, registration G-BNKP, was conducting a private training flight from Clacton Airfield in Essex. Following a standard pre-flight briefing and checks, the aircraft departed normally with a student pilot handling the controls. While performing a climbing left turn at approximately 850 feet, the engine experienced a sudden malfunction. The instructor immediately assumed control of the aircraft as the propeller ceased rotation within two seconds. After performing the necessary engine failure procedures and attempting an unsuccessful restart, the instructor declared an engine failure to Clacton tower. The instructor then maneuvered the aircraft into the wind and selected a suitable wheatfield for an emergency touchdown. The landing was completed successfully using flaps, and both occupants escaped the no injuries.
The investigation
Investigators examined the engine at the operator's engineering facility, where they discovered significant damage to the No 4 cylinder and piston. The damage was traced to the No 4 inlet valve falling into the cylinder. A metallurgical analysis determined that the failure began with fatigue cracks in the inlet valve spring cap. These cracks, which originated at the lower inboard edge, propagated through a tensile fatigue mechanism until the cap failed. This failure allowed the valve keys to disengage from the valve stem, causing the valve to drop into the combustion chamber.
While investigators considered whether a maladjusted tappet could have caused the spring to become coil-bound, an overhaul organization noted that such an error would have been clearly detectable during maintenance. Furthermore, an engine overhaul specialist noted that cracked spring caps have been observed in engines previously subjected to overspeeding events. The engine in question had been overhauled to zero hours in May 1998 and had flown 447 hours since that time.