What happened
On 25 May 2002, a Piper PA-2-161 Warrior II, registration G-BPAU, was performing a circuit check flight near Denham Airfield. The flight was being conducted as a training session with an instructor and a student pilot on board. Prior to departure, the aircraft had been refuelled to capacity, though the engine had been difficult to start, requiring several attempts before running normally. Following successful pre-flight checks, the aircraft departed from Runway 24.
During the base leg of the first circuit, the pilot adjusted the power settings while using carburettor heat and two stages of flaps. Following an increase in power to adjust the aircraft's altitude, the engine ceased operation. The throttle became unresponsive, leaving the aircraft in a gliding state. The instructor took control of the aircraft and, determining that the remaining altitude was insufficient to reach the airfield, maneuvered to avoid obstacles by turning right. While attempting to land in a small field, the left wingtip struck a tree, causing the aircraft to land abruptly. The two crew members sustained minor injuries, and while fuel leakage occurred, no fire broke out. The aircraft was subsequently destroyed.
The investigation
The AAIB examined the circumstances surrounding the engine loss of power. The instructor noted that the fuel supply had been verified as functional by the airfield owner. While the instructor suspected carburettor icing, he noted the difficulty in explaining why icing occurred so quickly after the ground power checks, especially since carburettor heat was already active.
Meteorological data from the Met Office showed a surface temperature of +14.1°C and a dew point of +6.7°C. According to standard icing probability charts, these specific atmospheric conditions presented a risk of moderate icing at cruise power and serious icing during descent. Additionally, the flying club at Denham had reported multiple instances of carburettor icing on the day of the accident and during the preceding days.