French investigators are highlighting the importance of validating altimeter pressure settings, after a Hop Bombardier CRJ1000 descended below the glidepath on approach to Nantes.
The aircraft (F-HMLD) was conducting the approach to runway 21 in turbulent conditions, during the arrival from Lyon on 20 October 2021.
Investigation authority BEA says the crew was instructed to descend to 3,000ft – their first altitude assignment below the 5,000ft transition level – on a QNH pressure setting of 1002mb.
But the crew read back the instruction incorrectly, it states, citing a setting of 1021mb. This error was not detected by the air traffic controller.
This meant the aircraft’s altimeter gave the pilots a false reading, indicating the jet was higher than its actual altitude.
“When resetting the altimeter, the crew did not apply the procedure fully, omitting to check the consistency of the QNH provided by the controller against another source of information,” says BEA.
It says the omission resulted from the crew’s experiencing difficulty in reading flightplan information, owing to the turbulence generated by a storm.
“Procedures and information on the aircraft instruments did not allow the crew to directly identify the path error in a simple way,” the inquiry adds.
The aircraft proceeded to descend towards the runway but flying 530ft below the correct glidepath.
Control of the flight was transferred to the airport frequency after it passed the final approach fix, and it was cleared to land.
Only after the jet descended below 788ft – with the altimeter incorrectly reading 1,300ft – did the controller receive a minimum safe altitude warning, as the aircraft entered a region with 770ft minimum obstacle clearance.
The controller alerted the crew, telling the pilots to check their altitude. One of the pilots initially queried whether the alert was meant for them, because the altimeter was reading 1,200ft on a QNH of 1021mb.
Although the controller then gave the QNH as 1002mb – saying “one thousand and two” – the pilot responded by stating the setting of 1021mb and saying “it is correct”, prompting the controller to give the QNH again, in individual digits.
Instead of executing a go-around, the crew corrected their altimeter settings and continued the approach visually, joining the normal glidepath 12s later while 1.7nm from the runway threshold, and subsequently landing without further incident. None of the 102 occupants was injured.
“This event illustrates how important it is to validate the QNH value by checking it against a value provided by another source of information,” says BEA, using the examples of flight documents, the airport’s ATIS information service, or the ACARS communications link as methods to verify the pressure setting.
BEA points out that the approach flown by the CRJ1000 to Nantes’ runway 21 involves passing over the city. The required navigation performance approach, designated RNP21, includes intercepting the final approach slope at 3,000ft while 8.7nm from the threshold, and features a 3.1° glidepath which is offset from the runway axis by 12°. The minimum obstacle clearance height of 770ft is determined by the 37-storey Tour Bretagne office building.
Simulation of the incident indicated that, had the airport not been equipped with minimum safe altitude alarms, the aircraft would have continued on the approach, in the absence of visual references, not receiving any alert until its enhanced ground-proximity warning system sounded – most probably at 110ft radio altitude with the aircraft 1.6nm from the runway.
Although the ground-proximity warning system fitted on the aircraft – a Honeywell Mark V – has an optional altimeter-monitor function available, this software is not certified on the CRJ1000.
Analysis by Honeywell shows that, had the function been available on the aircraft, it would have triggered an ‘altimeter setting’ visual and aural warning to the pilots about 30s after it descended through the 5,000ft transition altitude.