What happened
On October 31, 2012, an ATR 42-500, registration OK-JFJ, operated by Czech Airlines, was conducting a scheduled test flight at Prague/Ruzyně Airport. The purpose of the flight was to verify various aircraft systems, including the Stick Shaker/Stick Pusher, following a recent repainting of the aircraft's exterior.
During the low-speed testing phase, the crew intentionally decelerated the aircraft to find the activation threshold for the stall warning system. Although the 'ICING AOA' light illuminated, the Stick Shaker and Stick Pusender failed to activate at the expected speeds. As the crew continued to reduce speed, the aircraft experienced a sudden loss of lift on the left wing, resulting in a significant left bank and a stall at approximately 15,820 ft. The crew managed to recover the aircraft at 13,800 ft.
Shortly after, the crew attempted a second low-speed test. During this second deceleration, a second stall occurred at approximately 16,000 ft, characterized by a left bank of 35 degrees. The crew successfully stabilized the aircraft at 15,000 ft. A third attempt was made with the flaps extended, which did not result in a stall.
The investigation
The ÚZPLN investigation examined flight data from the QAR and DFDR, as well as pilot testimony and maintenance records. The investigation focused on why the stall protection systems failed to activate despite the aircraft reaching critical speeds. Investigators also reviewed the maintenance procedures used during the recent aircraft repainting process.
Findings
- The primary cause of the stalls was the failure of the Stick Shaker/Stick Pusher system to activate due to frozen Angle of Attack (AOA) sensors.
- The AOA sensors were likely contaminated with water during the high-pressure washing of the aircraft's exterior during the repainting process, as the sensors were not adequately protected.
- As the aircraft climbed into colder temperatures, the water inside the sensor mechanisms froze, blocking the internal moving parts.
- The crew continued to decelerate the aircraft toward stall speeds even after the expected activation thresholds had passed, in an attempt to find the system's activation limit.
- The operator's maintenance program for test flights lacked specific instructions on the minimum speeds required for different aircraft weights or the procedures to follow if the stall warning system was found to be non-functional.
- The crew did not strictly follow the manufacturer's recommended recovery procedures during the first stall event.
Safety action
- The investigation led to a recommendation that the maintenance manual for aircraft repainting must include a requirement to cover AOA sensors during high-pressure washing.
- The operator was advised to update flight test documentation to include specific activation speeds for the Stick Shaker and Stick Pusher for various aircraft configurations and to define procedures for conducting tests if the warning systems are inoperative.