This section sets out a few operational safety topics with respect to commercial air transport, principally involving large aeroplanes. These topics are illustrated with the latest reports published by the BEA and certain reports from foreign investigations which the BEA was involved in as accredited representative.
The QNH value entered in the on-board systems depends on a chain of information, which is subject to various sources of error, including human error. An altimeter setting error is a threat which directly exposes the aeroplane to the risk of a CFIT. This threat has been known of for a long time but the widespread use of ILS procedures probably contributed to concealing both this threat and its consequences and at the same time, to reducing the pilots’ awareness of this risk. However, for Baro-VNAV approaches which are more and more frequent, the use of an incorrect QNH results in the aeroplane following a descent slope parallel to the published slope without any obvious indication to make the crew aware of the situation.
1. Altimeter setting (QNH) read-back error, triggering of a MSAW on final approach
The investigation into the serious incident concerning the Bombardier CRJ-1000 registered F-HMLD on 20 October 2021 on the approach to Nantes in degraded weather conditions due to a strong low-pressure system showed that the crew read back a QNH value of 1021 although a value of 1002 had been provided by the controller who authorised them to carry out an RNP approach for runway 21. The controller did not pick up the error and the crew did not compare the value with another information source as specified in the operator’s procedures. The aeroplane descended on an approach slope which was around 530 ft below the published path and activated an MSAW. The air traffic controller immediately informed the crew of this.
2. Transmission of incorrect altimeter setting (QNH) by air traffic controller, near-collision with ground during satellite approach procedure with barometric vertical guidance
The serious incident concerning the Airbus A320 registered 9H-EMU on 23 May 2022 on the approach to Paris-Charles de Gaulle was the result of the controller giving a QNH value with an error of 10 hPa (1011 instead of 1001). The aeroplane descended on an approach slope which was around 280 ft below the published path. Crew and air traffic controller operating procedures did not prevent the use of an incorrect altimeter setting. Furthermore, neither the aeroplane’s instruments nor the air traffic controller’s tools were designed to detect this type of error. As the crew had not acquired the required visual references, they decided to carry out a missed approach when the displayed altitude corresponded to the selected minima. During the manoeuvre, the aeroplane descended to 6 ft RA agl at 0.9 NM from the runway threshold, outside the perimeter of Paris-Charles de Gaulle airport. No EGPWS alert was triggered.
3. Insertion of an incorrect QNH value, descent below published approach slope, activation of EGPWS, missed approach
The crew of the Airbus A320 registered VH-VGT carried out a RNP approach to Gold Coast with an incorrect QNH value (1025 hPa instead of 1018 hPa). The aeroplane descended on a slope which was around 200 ft below the published one. The investigation report indicated that before the start of the descent, the correct QNH value provided by the ATIS had been entered in the FMGC for the approach. During the descent, when passing the transition level, a radio communication prevented the copilot (PF) from calling out the procedure which consisted of setting the altimeters using the FMGC approach page. He tried to visually call the captain’s (PM) attention to initiate the check but in the absence of a response and the approach page being displayed, he used the ATIS information noted by hand to set his altimeter. He possibly mistook the cloud cover (025) or the temperature (25°C) for the QNH. When the PM was once again available, he did not display the approach page and the PF called out a setting of 1025 which was used by the PM. The cross-check was carried out with this value. Around 1,000 ft AMSL, the PF questioned whether the approach slope was correct. The automatic call-out at 500 ft RA reinforced the PF's view that the slope was incorrect. At 159 ft RA, an EGPWS "TERRAIN" alert sounded and the crew carried out a missed approach.
