Altimeter Setting Error Triggers Terrain Alert on Approach to Nantes (F-HMLD)

Incident Overview and Context<br>On October 20, 2021, a Bombardier CL-600-2E25 (CRJ-1000) registered F-HMLD, operated by HOP! for a commercial passenger flight, was on approach to Nantes-Atlantique airport in France. The aircraft was carrying a pilot in command, a pilot flying, two cabin crew members, and 98 passengers. The incident occurred during a period of strong low-pressure weather associated with Storm Aurore over northern France.<br><br>## Sequence of Events<br>During the descent, air traffic control cleared the crew to descend to 3,000 feet with a QNH of 1,002 and for an RNP 21 approach. The crew incorrectly read back the QNH as 1,021, an error that went undetected by the controller. Approximately two minutes later, as the aircraft descended through 788 feet actual altitude, a Minimum Safe Altitude Warning (MSAW) was triggered at the controller's position. The controller alerted the crew to check their altitude. The pilot in command responded that they were passing through 1,200 feet with a QNH of 1,021. The pilot flying simultaneously applied a nose-up input, stabilizing the descent rate. The controller then clarified the correct QNH was 1,002, and the crew corrected their altimeter settings and flight path. The MSAW ceased, and the aircraft joined the normal approach slope 1.7 nautical miles from the runway threshold, landing safely in the rain.<br><br>## Approach and Meteorological Conditions<br>The approach to runway 21 at Nantes-Atlantique requires the aircraft to fly over the city of Nantes, with specific minimum altitudes determined by local obstacles, including the Brittany Tower. The RNP 21 procedure involves intercepting the final approach slope at 3,000 feet. At the time of the incident, the airport was experiencing falling pressure, with the QNH dropping from 1,009 hPa at 13:00 to 1,002 hPa at 17:30. Wind information provided during the approach indicated winds from 200 degrees at 30 knots, gusting up to 40 knots. Precipitation radar showed a rain shower arriving at the airport, and the crew reported landing in the rain. The crew had visual references during the approach, including the Precision Approach Path Indicator (PAPI), when they corrected their path.