First Incident (7 February 2018)
On 7 February 2018, a Boeing 737-800 registered F-GZHO, operated by Transavia France, departed from Norwich Airport at 18:40 UTC on a ferry flight to Paris-Orly. The captain was the pilot flying (PF) and the first officer was the pilot monitoring (PM); one passenger was on board. Night visual meteorological conditions prevailed.
During the take-off roll on runway 27, the captain called “check” just before the first officer called “80 knots.” At rotation, the IAS DISAGREE alert appeared on both primary flight displays (PFDs). Shortly after, the AOA DISAGREE and ALT DISAGREE alerts also displayed. The crew observed that the right PFD showed erroneous speed and altitude indications, while the left PFD and the standby airspeed indicator agreed. The captain decided to continue the flight, delaying checklist execution due to high workload in the London terminal control area. At cruising altitude FL200, the difference between PFDs reached about 2,000 feet and 30 to 40 knots, with the right side lower.
The crew subsequently performed the AOA DISAGREE, ALT DISAGREE, and IAS DISAGREE checklists, which referred them to the Airspeed Unreliable procedure. They reviewed but did not apply this procedure, as they had identified the left PFD as correct and confirmed its consistency with the Flight with Unreliable Airspeed table. The autopilot, flight directors, and auto-throttle remained engaged. A TACBDE assessment led to a decision to continue to Paris-Orly. The crew did not send a PAN PAN or notify air traffic control. During descent, they contacted Transavia maintenance. For the approach, they added 10 knots to the reference speed to ensure adequate indicated airspeed on both PFDs. The landing was uneventful. The captain recorded the alerts in the aircraft flight log. A technician inspected the aircraft overnight but found no anomalies.
Second Incident (8 February 2018)
The following day, a different crew operated F-GZHO on a scheduled passenger flight from Paris-Orly to Marrakesh. The pilot in training was PF, while the captain, occupying the right seat, acted as PM for instruction purposes. Take-off from runway 26 occurred at 08:25 UTC in visual meteorological conditions.
During the take-off roll, the PF noticed the speed on his PFD exceeded 80 knots without the instructor’s callout. The instructor then called “80 knots” when the left PFD indicated 90 knots, noting a 10-knot difference between the two PFDs. The IAS DISAGREE alert appeared between 90 and 95 knots. The instructor assumed control, terminated the training situation, and continued the take-off. After rotation, AOA DISAGREE and ALT DISAGREE alerts appeared. The instructor observed that his PFD values were 20 knots and 400 feet below the left PFD and standby instruments. He told the PF to continue flying. The PF re-engaged the autopilot.
At FL90, the crew completed the memory items for Airspeed Unreliable: they disconnected the auto-throttle, autopilot, and flight directors, and set 4° pitch and 75% thrust. The PM issued a PAN PAN call and requested vectors back to Orly. After a TACBDE assessment, the left autopilot and flight directors were re-engaged. The aircraft performed an overweight landing at Orly after 40 minutes of flight.
Post-flight maintenance revealed that the right angle-of-attack (AOA) sensor was abnormally resistant to rotation and produced unusual clicking noises. A test of the Stall Management Yaw Damper Computer (SMYDC) showed error messages indicating an AOA sensor anomaly, leading to its replacement.
Aircraft Information
F-GZHO, a Boeing 737-800, had been delivered new to Transavia France on 9 February 2015. As of 7 February 2018, it had accumulated 8,927 flight hours and 4,397 cycles. It had just undergone a C-type maintenance check performed by KLM UK Engineering in Norwich, during which both AOA sensors were manually rotated 30°; no issues were noted. Transavia France stated that the right AOA sensor had not been removed from the aircraft since delivery.
Maintenance Between Flights
After the first incident, a technician from a subcontractor examined the aircraft overnight based on the logbook entries. He followed the Aircraft Maintenance Manual (AMM) procedures for inspecting AOA sensors, pitot probes, static ports, and the stall warning system, finding nothing abnormal. He reported that the AOA sensor moved freely. He did not consult the Fault Isolation Manual (FIM) and did not perform the SMYDC test required for the AOA DISAGREE alert. Believing that an AOA sensor failure would typically trigger an Autoslat Fail alert (which had not occurred), he speculated that icing might have caused the problem. The aircraft was returned to service.