PARIS, FRANCE — French investigators emphasize the need for validating altimeter pressure settings after a HOP, Air France regional subsidiary,  CRJ1000 jet descended below the glide path during its approach to Nantes.

The plane with registration F-HMLD faced turbulent conditions while approaching runway 21 from Lyon on October 20, 2021.

The investigation authority, BEA, states that the crew was instructed to descend to 3,000 feet on a QNH pressure setting of 1002mb, but they misread the instruction as 1021mb, which the air traffic controller did not detect. This caused a false reading on the aircraft's altimeter, indicating the jet was higher than it was.

BEA says the crew failed to check the QNH provided by the controller against another source of information, which resulted from difficulties in reading flight plan information due to turbulence caused by a storm. The inquiry also points out that the aircraft instruments and procedures did not enable the crew to easily identify the path error.

The plane continued descending toward the runway, flying 530 feet below the correct glide path. Control of the flight was handed over to the tower, and it was cleared to land. The controller received a minimum safe altitude warning only after the plane descended below 788 feet. The controller then alerted the crew, who initially questioned whether the alert was meant for them, as their altimeter read 1,200 feet on a QNH of 1021mb.

After the controller provided the correct QNH, the crew adjusted their altimeter settings and continued the approach visually, joining the normal glide path 12 seconds later and landing without incident.

BEA emphasizes the importance of validating QNH values using alternative sources of information, such as flight documents, airport ATIS information service, or ACARS communication link. The approach flown by the CRJ1000 to Nantes' runway 21 involves passing over the city, with a minimum obstacle clearance height of 770 feet determined by the 37-story Tour Bretagne office building.

Simulations of the incident showed that without minimum safe altitude alarms, the aircraft would have continued its approach without any alert until the enhanced ground-proximity warning system activated, likely at a radio altitude of 110 feet and 1.6 nautical miles from the runway.

Although the Honeywell Mark V ground-proximity warning system on the aircraft has an optional altimeter-monitor function, this software is not certified for the CRJ1000. Honeywell analysis suggests that if the function were available, it would have alerted the pilots about 30 seconds after descending through the 5,000 feet transition altitude.