What happened
On 20 October 2021, a Bombardier CL-600-2E25 (CRJ-1000), registration F-HMLD, was operating a commercial flight from Lyon-Saint-Exupéry to Nantes-Atlantique. During the descent phase, the air traffic controller instructed the crew to descend to 3,000 feet with a QNH of 1002 hPa. The crew incorrectly read back the altimeter setting as 1021 hPa, an error that went undetected by the controller.
As the aircraft continued its RNP 21 approach, it descended through 788 feet. At this altitude, the aircraft entered an area where the minimum obstacle clearance altitude was 770 feet. Because the crew was using the incorrect altimeter setting, their onboard instruments indicated an altitude of approximately 1,300 feet. This discrepancy triggered a Minimum Safe Altitude Warning (MSAW) at the controller's position.
The controller alerted the crew to the terrain alert and instructed them to check their altitude. The pilot monitoring initially believed the aircraft was at a safe altitude of 1,200 feet due to the erroneous setting. Upon the controller providing the correct QNH of 1002 hPa, the crew corrected the altimeter setting, which stabilized their flight path. The aircraft subsequently continued the approach and landed safely in rainy conditions.
The investigation
The BEA investigation examined cockpit voice recordings, radar data, and air traffic control communications. The investigation focused on the sequence of the read-back error and the subsequent activation of the MSAW. Investigators analyzed the altitude discrepancy between the aircraft's actual pressure altitude and the altitude indicated on the cockpit instruments. The investigation also reviewed the controller's communication and the procedures for providing altimeter settings under the Standardised European Rules of the Air (SERA).