Background
The aircraft, an ATR 72-212A registered G-COBO, arrived at Manchester Airport from Guernsey and remained on the ground for over an hour in snowing conditions with a temperature of 0°C. The flight crew decided that no de-icing or anti-icing treatment was needed, as they did not believe snow was settling on the aircraft.
Flight History
During takeoff for the return flight to Guernsey, the commander, who was the pilot flying, found that less aft pressure than expected was needed to rotate the aircraft. After liftoff, maximum nose-down pitch trim was required to maintain the appropriate climb attitude. The autopilot was engaged four times but disengaged on each occasion as designed, and the commander had to apply continuous forward pressure on the control column to retain the desired pitch attitude. While climbing, the pitch trim indicator showed full nose-down deflection, and a pitch mistrim message was displayed. The crew actioned the Pitch Mistrim drill from the Quick Reference Handbook, which only required autopilot disengagement. The commander decided to divert to East Midlands Airport (EMA) if control difficulties persisted after level-off. At cruising level FL170, the commander reported having to exert excessive forward pressure and announced the diversion. During descent, the aircraft flew out of icing conditions and the control difficulties dissipated. The crew assessed that ice contamination had caused the problem. A normal landing was made at EMA at 0954 hrs.
Aftermath
Post-flight inspection revealed no ice on the aircraft, but analysis by the manufacturer concluded that from the start of the flight until the latter stages of the descent, the airflow over the horizontal tailplane and elevator was disrupted by ice contamination. No faults were found during an engineer inspection later that day. The operator downloaded flight data from the Flight Data Recorder and Cockpit Voice Recorder before the aircraft was returned to service. The AAIB was notified three days after the incident and classified it as a serious incident.