What happened
On 14 August 2020, a Cessna 525A, registration N222NF, departed Paris-Le Bourget for Deauville-Normandie. Shortly after takeoff, the pilot experienced a total failure of the left Primary Flight Display (PFD). While attempting to manage the failure by transferring information to the Multi-Function Display (MFD), the aircraft encountered further complications as the autopilot disengaged and several warnings, including overspeed and pull-up alerts, were activated.
As the flight progressed, the pilot noted significant discrepancies between the altitude and airspeed indicated on different screens. The pilot declared an emergency and requested to return to Le Bourget. During the return, the aircraft's altitude information became highly unreliable. While the pilot's instruments provided conflicting data, the air traffic controller at Paris-Charles de Gaulle received altitude information from the aircraft's transponder that suggested the plane was much higher than its actual position. This discrepancy led the controller to instruct the pilot to descend, at one point bringing the aircraft dangerously low.
After the pilot switched the transponder to a different source, the radar altitude finally aligned with the aircraft's actual position. The pilot eventually regained visual contact with the runway and performed a visual landing at Le Bourget without further incident.
The investigation
The BEA investigation focused on the cause of the instrument discrepancies and the failure of the left PFD. While the cause of the initial PFD screen blackout could not be reproduced, investigators examined the aircraft's air data systems. Maintenance inspections revealed that the contamination of the static port by an insect and sand had caused the Air Data Computer 1 (ADC1) to provide erroneous altitude and speed information. This faulty data was being transmitted via the transponder to air traffic control.
Findings
- The primary cause of the conflicting flight data was contamination of the ADC1 static and pitot lines by an insect and sand.
- The pilot was managing a high-workload environment, flying a high-performance aircraft alone, without autopilot, and navigating through alternating visual and instrument meteorological conditions.
- A confirmation bias occurred because the altitude information provided by the aircraft's transponder matched one of the pilot's erroneous instrument readings, leading both the pilot and the controller to believe the aircraft was at a safe altitude.
- The aircraft's transponder was transmitting incorrect altitude data to the Paris-Charles de Gaulle control center, which triggered a Minimum Safe Altitude Warning (MSAW) that was not visible to the Le Bourget controller.
Safety action
- The BEA recommended that the FAA update Cessna flight manual procedures to provide specific instructions for handling inconsistent air data information, particularly for aircraft configurations with multiple PFDs and ADCs.
- The BEA recommended that EASA ensure manufacturers develop comprehensive procedures for situations involving doubtful or erroneous altitude readings.
- The BEA recommended that the DSNA ensure air traffic controllers are trained to recognize that altitude information on their screens is derived solely from the aircraft's transponder and are prepared for the specific management of flights where the pilot expresses doubt regarding vertical position.