What happened
On 28 September 2002, a Cessna 152, registration G-MASS, was conducting a dual training flight near Denham. The flight, commanded by a flying instructor with a student pilot on board, proceeded through a climb to 1,900 feet over Amersham. During the flight, the instructor selected carburettor air to the 'hot' position and reduced power to 1,800 RPM to begin a descent towards Maple Cross. Shortly after this power reduction, the engine experienced a sudden loss of power, with engine speed dropping significantly.
Despite attempts by both the student and the instructor to restore power by adjusting the throttle and carburettor air controls, the engine failed to respond effectively. The instructor took control of the aircraft, maneuvering away from built-up areas and selecting a suitable field for an emergency landing. After executing a forced landing approach with full flaps, the aircraft successfully touched down in a field, clearing a low fence. There were no injuries and no fatalities during the incident.
The investigation
Investigators examined the aircraft's engine and induction system, as well as meteorological data for the area. A maintenance examination identified a deteriorated O-ring in the fuel priming pump; however, investigators determined that this defect was unlikely to cause the observed symptoms, as the volume of air leaking through the small-bore priming pipes would be insufficient to cause such a significant power loss.
Meteorological analysis revealed that the atmospheric conditions at the time of the incident—specifically the temperature and high relative humidity—were highly conducive to the formation of ice within the induction system. The investigation also considered the impact of the instructor's training and the legal constraints regarding practicing forced landings under existing regulations.
Findings
- The primary cause of the engine power loss was likely carburettor icing.
- The aircraft was operating in atmospheric conditions that presented a high risk for induction ice formation.
- The reduction in power to 1,800 RPM, combined with the use of carburettor heat, likely facilitated the ice build-up.
- The successful forced landing was attributed to the instructor's regular practice in simulated engine failure procedures.
Safety action
- Safety Recommendation 2004-11: The Civil Aviation Authority (CAA) was advised to review the safety implications of Rule 5 regarding its impact on realistic engine failure training and to publish guidance on acceptable best practices for flight instructors.