What happened
On 20 February 2006, a Slingsby T6/B, registration G-BLTU, was conducting a three-hour navigation training exercise from RAF Wyton. While flying at approximately 1,100 ft, the pilot experienced a noticeable reduction in engine power. The engine RPM dropped from a cruise setting of 2,300 rpm to roughly 2,100 rpm, and subsequently fell further to 1,700 rpm.
In an attempt to rectify the issue, the pilot checked the throttle position and activated the electrical fuel pump. Despite these actions, the engine power continued to decay, making it impossible to maintain altitude. The pilot declared a distress call to Marham and, to avoid trees during a potential landing into the wind, elected to perform a forced landing in a recently harrowed field located 13 miles north of RAF Marham.
Upon touchdown, the aircraft's main wheels hit the ground first, followed by the nose wheel. The nose wheel sank into the soft, turned earth, causing the nose leg to break and the aircraft to nose over, coming to rest inverted. The pilot was able to exit the aircraft through the shattered canopy without none injuries.
The investigation
The AAIB examined the engine and fuel system of the G-BLTU. The investigation found no evidence of fuel contamination, and both the mechanical and electrical fuel pumps were fully operational. The induction and ignition systems were intact, and spark plugs suggested the engine had been running slightly rich. Maintenance records showed the engine had completed fewer than 500 hours since its last factory overhaul with no recent reported faults.
Meteorological data from the time of the accident indicated a temperature of 5°C with a dew point of 2°C, alongside reports of sleet and rain showers. Analysis of these conditions suggested a significant risk of carburettor icing.
Findings
- The primary cause of the power loss was likely carburettor icing.
- The weather conditions at the time were highly conducive to icing, with a moderate risk at cruise power and a serious risk at descent power.
- The pilot may have had insufficient time to clear the ice using carburettor heat before committing to the forced landing.
- The aircraft sustained damage to the landing gear, propeller, engine, and structure due to the nose wheel digging into the soft ground.