What happened
On July 26, 2013, an ATR-7CO-212A, registration PP-PTU, was operating a commercial flight from Maceió to Salvador. While cruising at Flight Level 160, the aircraft entered atmospheric conditions conducive to ice accretion. Although the aircraft's anti-icing system was active, the crew had not activated the pneumatic de-icing system.
As ice accumulated, the aircraft's performance degraded, leading to a reduction in Indicated Airspeed (IAS). The crew experienced a loud vibration and a noise similar to a compressor stall. In an attempt to manage the situation, the crew reduced engine power to 20% torque, which caused the airspeed to drop to 10 knots below the minimum required speed for icing conditions. This triggered the aircraft's stall protection systems, including the stick shaker and stick pusher. The pilot's attempt to counteract the stick pusher resulted in a loss of control. The aircraft eventually recovered after descending to Flight Level 110, and the flight landed safely at Deputado Luis Eduardo Magalhães International Airport with no injuries to the four crewmembers or 58 passengers.
The investigation
CENIPA's investigation focused on the failure to manage ice accretion and the subsequent aerodynamic stall. The investigation examined the activation of the ice protection systems, the crew's response to the vibration and airspeed decay, and the effectiveness of the stall protection systems. The investigation also reviewed the aircraft's maintenance records and the crew's decision-making process during the emergency.
Findings
- Failure to activate de-icing systems: The crew did not activate the pneumatic de-icing system despite being in icing conditions.
- Inadequate airspeed management: The reduction of engine power led to the airspeed falling below the minimum operating limits for icing conditions.
- Improper stall recovery: The crew's decision to counteract the stick pusher contributed to the loss of aircraft control.
- Ineffective Crew Resource Management: Communication errors and confusion regarding engine shutdown procedures hindered effective task management.
- Inaccurate perception: The crew failed to accurately assess the impact of ice accumulation on the aircraft's performance and energy state.