History of the Flight
On 15 September 2002, a Jabiru UL-450 microlight aircraft (registration G-TYKE) departed Sandtoft for a flight to Rufforth. The pilot, the sole occupant, reported that visibility was 15 to 20 km with an overcast cloud layer at 3,000 to 4,000 feet. After takeoff, the pilot applied carburettor heat for five to seven minutes before switching it off, noting no indication of icing. While passing Drax power station at 1,500 feet, the engine began to misfire. The pilot reapplied carburettor heat and turned on the fuel pump. Increasing the throttle briefly raised RPM, but it then dropped. Approximately one to two minutes later, the engine lost all power. The pilot declared a MAYDAY on the Breighton frequency but received no response. He prepared for a forced landing by turning off switches and the fuel tap, tightening his harness, and placing the fire extinguisher in a side pocket. He approached at 55 kt to a recently seeded field with loose soil. Touching down on the main gear, he held the nose wheel off until about 20 kt. When the nose wheel contacted the ground, the nose wheel leg folded backward, causing the nose to dig in and the aircraft to somersault inverted. The pilot vacated via the passenger door and retrieved the fire extinguisher. The fuel cap was displaced, allowing leakage; he reseated it.
Aircraft Examination
The aircraft was recovered and examined. Defueling produced 5 litres via the electric pump and another 2.5 litres from the fuel drain. No fuel contamination was found in the filter or carburettor bowl. The engine was not shock-loaded and ran normally during a test run.
Weather and Carburettor Icing Conditions
The Meteorological Office estimated temperature and dewpoint at Selby at 13°C and 8°C respectively at 1,000 feet, with a relative humidity of 75%. The carburettor icing probability chart (CAA AIC 145/1997) indicated a serious risk of icing at any power for a typical light aircraft engine without carburettor heat applied. The wind at 1,500 feet was from 010° at 10 kt.
Drax Power Station Effect
The engine began to misfire while the aircraft was about a quarter mile west of Drax power station. The station's eight cooling towers exhaust around 3,000 tonnes of water per hour, significantly raising ambient humidity downwind, possibly for miles. The cooling tower tops were at approximately 400 feet amsl.
Pilot Experience with Carburettor Icing
The pilot reported more carburettor icing incidents with the Jabiru engine than with any other piston engine. He typically applied carburettor heat every five minutes. Previously he cruised at 2,200 RPM (75 kt, 7 L/hr fuel burn). After the accident, he operated another Jabiru at 2,600 RPM (90 kt, 12 L/hr) and found less icing issues.
Discussion
The engine ran normally post-accident, and no fuel contamination was found. Atmospheric conditions were conducive to carburettor icing, and the power loss symptoms were consistent with such icing. The high humidity from Drax power station may have contributed. The low power setting (2,200 RPM) likely also contributed, as carburettor icing forms more readily at lower power due to greater temperature drop and less effective carburettor heat. The pilot suggests operating at 2,600 RPM or above to reduce icing risk.
The AAIB is investigating multiple accidents suspected of carburettor icing and plans a safety recommendation.