What happened
On March 14, 2025, a Cessna 525 CJ1, registered F-HJAV, operated by Valljet, was en route from Limoges-Bellegrade to Paris - Le Bourget. During the climb, the crew detected inconsistent altitude readings between the primary flight displays (PFDs) and the standby altimeter. To troubleshoot the discrepancy, the crew decided to continue the climb and address the failure once established in cruise.
While at flight level 180, the crew identified an altitude inconsistency and subsequently selected Air Data Computer (ADC) 1 as the primary source for both PFDs. However, the information provided by ADC 1 was erroneous. While the cockpit instruments indicated the aircraft was maintaining a stable altitude, the GNSS altitude was progressively decreasing.
As the aircraft descended through various flight levels, the crew encountered radio communication difficulties with Paris-Charles de Gaulle approach. At approximately 18:42, while the instruments indicated level flight at FL 90, the aircraft was actually at a GNSS altitude of roughly 1,200 feet. This triggered a TAWS TERRAIN PULL UP warning. The captain immediately disconnected the autopilot and transitioned to visual references to maintain separation from the ground. The aircraft eventually landed safely at Le Bourget.
The investigation
The BEA examined the aircraft's air data systems, finding that while ADC 2 and the standby altimeter functioned within tolerances, the information sent by ADC 1 was erroneous. The investigation also analyzed radar and GNSS data, which revealed a massive discrepancy: the altitude transmitted via the transponder (based on ADC 1) showed the aircraft at much higher levels than its actual GNSS altitude.
The investigation also reviewed the crew's response and the air traffic controllers' actions. It was noted that the crew was unaware of a specific QRH procedure for unreliable altitude indications, which had been implemented by the operator following a similar incident in 2022. Furthermore, the controllers did not immediately recognize the crew's report of an altimeter problem as a high-risk situation requiring specific separation protocols.
Findings
- The primary cause of the altitude deviation was a failure of ADC 1, which provided erroneous barometric information.
- The crew's decision to select ADC 1 as the sole source for both PFDs inadvertently caused the flight instruments to follow the faulty data.
- The crew was not trained on the specific memory items for the "UNRELIABLE ALT INDICATION" procedure.
- Air traffic controllers did not perceive the pilot's request regarding the altimeter problem as a signal of an altitude emergency, leading to a lack of necessary lateral separation.
- The aircraft's transponder was transmitting incorrect altitude data, rendering the ground-based MSAW and STCA safety systems ineffective.
- The aircraft's ACAS system was also compromised because it relied on the erroneous altitude data transmitted by the transponder.