History of the Flight
On 1 July 2003, a Cessna F152 (registration G-BIJX) was on a training flight from Lydd to Headcorn, Kent. The student, who had 36 hours of flying experience, was the handling pilot. The intention was to perform a touch-and-go at Headcorn and return to Lydd. Engine power checks at Lydd were normal, and takeoff acceleration was normal. During the cruise at no higher than 2,100 feet, the crew observed showers and applied carburettor heat several times as a precaution.
After a direct join to the downwind leg at Headcorn, the student applied carburettor heat for approximately 10 seconds and then set it back to COLD. On base leg, the student re-applied carburettor heat and reduced power to 1,700 RPM to begin a descent. The aircraft was lined up on finals for Runway 29 (grass) with full flap (30°) at 65 KIAS. At about 200 feet agl, the student set the carburettor heat back to COLD.
The instructor reported a stable final approach, with touchdown estimated 30 feet beyond the runway threshold marker boards. During the landing roll, the instructor reduced flap setting to 10° and visually checked the flap position. She then realized the aircraft was not accelerating normally and called for full power. She placed her hand over the student's on the throttle, confirming full power, and glanced at the RPM gauge, which indicated approximately 2,100 RPM. The aircraft still did not accelerate normally, and the airspeed indicator rose slowly. At about 40 to 45 KIAS, the instructor took control and aborted the takeoff because she did not believe the aircraft would gain sufficient speed to clear the hedge at the end of the runway. She closed the throttle, pulled the yoke aft, and applied wheel brakes. At this point, she estimated the aircraft had used more than two-thirds of the 796-metre runway. Braking action was not very effective; she believed they were probably skidding on the short wet grass. The aircraft ran off the end of the runway and penetrated a hedge approximately 180 metres from the runway threshold, coming to rest on a country road. Both pilots were wearing lap and diagonal harnesses and exited the aircraft unaided and uninjured.
Aircraft Examination
Photographs showed damage primarily to the wing leading edges and tail. The vertical tail and horizontal stabiliser had separated from the fuselage but remained attached by control cables. The cabin environment was virtually intact. The engine was examined by the aircraft's maintenance organisation, and no faults were found that might explain a power loss. The carburettor was removed, attached to another engine, and test run; the engine ran normally.
Weather
The wind at the time was reported by the instructor as light and variable. An aftercast from the Meteorological Office estimated surface conditions near Headcorn: wind 260° at 13 kt, temperature 15.5°C, dew point 14.3°C, humidity 93%. Cloud estimate: few/scattered cumulus base at 2,000 to 2,500 feet, deteriorating in showers and thunderstorms to broken cumulonimbus or large cumulus with a base of 1,500 to 2,000 feet. Some heavy rain had fallen at Headcorn before the aircraft arrived.
Advice on Use of Carburettor Heat
The CAA Aeronautical Information Circular (AIC) 145/1997 (Pink 161) advises: on downwind, include a check of carburettor heat; on base leg and finals, select HOT as power is reduced; on some installations, select COLD at about 200/300 ft on finals. For go-around, if carburettor heat has not been selected to COLD on finals, do so concurrently with go-around power. The Cessna 152 Information Manual specifies: before landing, carburettor heat ON (apply full heat before reducing power); it does not specify turning it off before touchdown. In a go-around, full power should be selected before turning carburettor heat off.
Analysis
According to the Cessna 152 Information Manual, normal static engine RPM is 2,280 to 2,380. The instructor later tested another Cessna 152 and noted 2,400 RPM at about 30 kt. The indicated 2,100 RPM during the touch-and-go showed the engine was not performing normally. A reduction from 2,400 to 2,100 RPM at the same airspeed would result in a 23% reduction in thrust, which could explain sluggish acceleration. Wet grass would also reduce acceleration.
Engine examination did not reveal a fault explaining the low RPM. The engine operated normally during takeoff from Lydd, so carburettor icing at Headcorn was considered. Temperature and high humidity placed the risk of carburettor icing in the highest region on the icing chart. Carburettor hot air was selected for 10 seconds on downwind and then reapplied for about 1 minute 40 seconds during descent on base leg and finals to 200 feet at low power. At 200 feet, carburettor hot air was selected COLD. This procedure was not in accordance with the Cessna 152 Information Manual but was close to CAA advice, which notes the flight manual takes precedence.
Carburettor heat effectiveness is reduced at low power settings. It is possible some carburettor ice formed during the descent, with the condition worsening when heat was turned off at 200 feet. Alternatively, ice may have formed during the final 200 feet and on the ground. The AAIB could not find research evidence to support or reject these theories, and it is not known how quickly carburettor ice can form.
After aborting the takeoff, the aircraft failed to stop in the remaining runway length. The landing performance charts indicate on a dry grass runway, the aircraft should have stopped within 210 metres from 54 kt. The instructor reported the grass was wet and very short. CAA Safety Sense Leaflet states landing distance on wet grass up to 20 cm long should be increased by 30%, and if very short, distances may increase by up to 60%. The short wet grass likely caused poor braking and was probably the direct cause of the runway overrun.
Probable Cause
Not stated; contributing factors were lack of acceleration due to reduced RPM (likely carburettor icing) and poor braking on wet grass.