Lessons learned 4: near collision with the ground during a barometric approach with an incorrect QNH

Link to the investigation report: https://bea.aero/en/investigation-rep...

Synopsis: The crew of the Airbus A320 registered 9H-EMU were carrying out scheduled flight NSZ4311 between Stockholm Arlanda airport (Sweden) and Paris-Charles de Gaulle airport (France). Work was being carried out on the ILS for runway 27R at CDG, so the crew carried out a satellite approach with barometric vertical guidance (RNP APCH down to LNAV/VNAV minima).

During the approach, in a rain shower which severely impaired visibility, the crew were given an incorrect altimeter setting (QNH) by the air traffic service with a difference of 10 hPa (1011 hPa instead of 1001).

An error in the altimeter setting will mean that the aeroplane's actual altitude does not correspond to the altitude displayed. For approaches with barometric vertical guidance, the vertical profile and the associated vertical guidance are thus affected. Using a QNH value 10 hPa higher than the actual value resulted in the approach being flown on a vertical profile that was parallel to but around 280 ft (85 m) below the published vertical profile.

The design of the IFR procedures is based on normal operations and thus does not take into account an incorrect altimeter setting. The crews' operating procedures and those of the air traffic controllers 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.

At low height, a ground proximity warning (MSAW) was triggered in the control tower. Several seconds later, the air traffic controller informed the crew of the situation using an incorrect and inappropriate phraseology. The crew did not hear this announcement and continued their descent.

After reaching the indicated altitude corresponding to the chosen minima, the crew carried out a go-around because they had not acquired the visual references needed to continue the landing. During the manoeuvre, the minimum recorded and corrected radio-altimeter height was 6 ft, i.e. about 2 m, when the aeroplane was about 0.9 NM from the runway threshold, outside the limits of Paris-Charles de Gaulle airport. As per design, there was no on-board ground proximity alert (TAWS) during the event. In their statements, the crew indicated that they had not been aware of this proximity with the ground.

The second approach was also carried out with the same incorrect QNH. This time, the crew acquired visual contact with the ground, at a height of more than 600 ft. The PF corrected the flight path and landed without further incident.