What happened
On 7 February 2018, a Transavia France Boeing 737-800, registered F-GZHO, was performing a ferry flight from Norwich, United Kingdom, to Paris-Orly. During the takeoff roll, the crew observed an IAS DISAGREE alert, followed by AOA DISAGREE and ALT DISAGREE alerts during the initial climb. The crew identified that the right Primary Flight Display (PFD) was providing erroneous speed and altitude data, while the left PFD and standby instruments remained consistent. The pilots decided to continue the flight to Paris-Orly, opting not to perform the full Airspeed Unreliable checklist or notify Air Traffic Control (ATC) due to high workload and the belief that the left-side indications were reliable.
On 8 February 2018, the same aircraft, F-GZHO, was operating a training flight from Paris-Orly to Marrakesh. During the takeoff run, the crew again encountered airspeed discrepancies and the same sequence of alerts. The instructor pilot took control to manage the situation, eventually deciding to return to Orly. The aircraft performed an overweight landing at Orly after 40 minutes of flight time.
The investigation
The BEA examined the aircraft's flight data and the physical condition of the Angle of Attack (AOA) sensors. Maintenance records showed the aircraft had recently undergone a C-type inspection in Norwich. Investigators found that a maintenance technician had inspected the sensors between the two flights but did not follow the Fault Isolation Manual (FIM) procedures, which would have required testing the Stall Warning Yaw Damper Computer (SMYDC). Consequently, the underlying defect remained undetected.
Detailed analysis of the faulty right AOA sensor revealed damage to the internal gears and a deformed shaft. Laboratory testing of the resolver identified a viscous, tacky substance—identified as a contaminant similar to a solvent (THFA) used in the manufacturing process—between the stator and rotor. This contamination caused the resolver to become resistant to rotation, leading to the erroneous data.
Findings
- The primary cause of the instrument discrepancies was the contamination of the right AOA sensor's resolver by a substance that inhibited the relative movement of the internal components.
- The failure of the sensor led to erroneous airspeed and altitude indications and the activation of multiple cockpit alerts.
- The maintenance technician's failure to strictly follow the FIM troubleshooting procedures allowed the faulty sensor to remain in service.
- During both incidents, the crews did not immediately execute the Airspeed Unreliable memory items (such as disconnecting the autopilot and auto-throttle) because they prioritized identifying the erroneous display and believed the flight could be safely continued using the functional side of the instruments.