History of Flight
On 7 August 2002 at 1515 hrs, a Piper PA-28-140 Cherokee (registration G-AYWE) was conducting a PPL skill test on approach to Denham Airfield, Middlesex. The flight had been airborne for about two hours. During the flight, regular carburettor ice checks were carried out with no evidence of ice. The student completed a normal touch-and-go on Runway 24, applying carburettor heat on downwind and final with normal engine response. Cold air was selected after the climb to circuit height (750 ft agl).
For the next circuit, the student was briefed for a glide approach. On downwind, pre-landing checks included selecting full carburettor heat for several seconds. On base leg, full carburettor heat was again selected, then the throttle was closed and descent began from 750 ft agl. Engine speed indicated 1,100–1,200 RPM during descent. Initially, the descent profile was good with an aiming point about one-third along the runway. After selecting 25° flap, a slight undershoot developed.
At about 200 ft agl, approaching the normal APAPI descent profile of 4.5°, the instructor called for power application. The student reacted immediately but there was no engine response. The instructor called "Go-around" and the student applied full throttle. With no response, carburettor heat was selected to 'cold' with no effect. The aircraft struck trees in the undershoot area and impacted the ground, coming to rest on a heading of 040°, about 100 metres short of the runway threshold. Both occupants evacuated unaided. The Airfield Fire Service arrived promptly; there was no fire.
Weather Conditions
The instructor noted that weather conditions were conducive to carburettor icing. Actual conditions: surface wind 270°/7 kt, visibility >10 km, isolated showers, temperature 25°C, dew point 13°C. General Aviation Safety Sense Leaflet 14A indicates serious icing could occur with descent power in these conditions. The Meteorological Office reported frequent heavy showers around the accident time, increasing humidity and the possibility of carburettor icing.
Engineering Investigation
The AAIB examined the aircraft and engine at Denham. The aircraft was severely damaged: left wing severed at root, all three landing gears torn off, nose section damaged, fuselage buckled behind cabin, but cabin intact. Significant fuel release but no fire. Propeller blade damage indicated the engine was running at impact but not developing high power. Impact damage precluded functional testing.
Induction paths were clear. Throttle and mixture controls operated satisfactorily; carburettor heat control was jammed due to impact-crushed housing. When the control cable was cut at the heat flap lever, it moved freely. There was no evidence of pre-impact damage to the heat flap mechanism, which was judged open (cold air) at impact.
Spark plugs were in good condition except the lower plug on the front right cylinder, which was heavily wetted with oil (likely post-impact). Crankshaft, pistons, valve gear and accessory drives continuity was confirmed by hand turning. The carburettor was disassembled; inlet filter clear, main jet unblocked, accelerator pump operated correctly. The electric fuel pump operated satisfactorily; fuel sample was correct colour for AVGAS with no visible water.
No mechanical defect was found to explain the engine failure to respond to throttle application.
Discussion
In the existing weather conditions and with no obvious mechanical defect, the most probable reason for the engine failing to respond would be carburettor icing. Both the instructor and student considered they had fully complied with advice to minimise icing chances. The AAIB are studying recent and historical carburettor icing occurrences for a collective submission to the CAA.