What happened
On 4 March 2016, an ATR 72-212 A, registration G-COBO, was prepared for a commercial passenger flight from Manchester Airport to Guernsey. While the aircraft had arrived from Guernsey earlier that day, it had been sitting on the ground during periods of falling snow and temperatures near 0°C. Following an external inspection, the crew determined that de-icing and anti-icing treatments were unnecessary, as the commander believed the snow was not adhering to the airframe.
During the takeoff roll, the pilot encountered unexpected control behavior. Upon reaching rotation speed, the aircraft required significantly less aft pressure on the control column than anticipated. As the aircraft climbed, the pilot had to apply continuous forward pressure to maintain the desired pitch attitude. The aircraft's pitch trim moved to its maximum nose-down limit, and the autopilot disengaged multiple times due to the high load on the pitch servomotor.
Due to the excessive physical effort required to maintain the climb attitude, the crew elected to divert to East Midlands Airport. As the aircraft descended into warmer air, the control difficulties subsided. A subsequent inspection revealed no mechanical faults, and the crew concluded that ice contamination had been the cause.
The investigation
The AAIB investigation focused on the aerodynamic behavior of the aircraft and the decision-making process regarding de-icing. Analysis of flight data from the FDR and CVR confirmed that the elevator trim reached its maximum nose-down stop. The manufacturer performed a numerical aerodynamic simulation, which concluded that ice contamination on the upper surface of the horizontal tailplane had disrupted the airflow, creating an aerodynamic imbalance that the trim system could not fully compensate for.
Investigators also examined the crew's experience and training. The commander, while highly experienced, had primarily operated in the mild climate of Guernsey, where freezing precipitation is rare. The co-pilot also lacked experience with ground operations in snowy conditions. The investigation found that the crew's assessment was likely influenced by confirmation bias, as they sought evidence to support their initial belief that de-icing was unnecessary.
Findings
- The aircraft's horizontal tailplane and elevator were affected by ice that formed while the aircraft was parked in freezing conditions.
- The decision not to perform de-icing or anti-icing procedures left the aerodynamic surfaces vulnerable to contamination.
- The crew's decision-making was influenced by an optimistic belief that any existing snow would be removed by the airflow during takeoff.
- A lack of specific experience with winter operations and a reliance on self-study for winter awareness training contributed to the oversight.
Safety action
- The aircraft manufacturer's training organization is updating its conversion course to include simulator profiles of tailplane icing.
- The operator has implemented a computer-based training module for winter awareness, replacing the previous reliance on self-study.
- The operator is enhancing its conversion training by including a dedicated ground training day to discuss technical issues like de-icing.