AF1632 - Incident: Hop! CRJX at Nantes on Oct 20th 2021

A Hop! Canadair CRJ-1000 on behalf of Air France, registration F-HMLD performing flight AF-1632 from Lyon to Nantes (France), was on approach to Nantes' runway 21 when ATC cleared the aircraft to descend to 3000 feet at QNH was 1002 hPa, the crew however read back 1021hPa, descended to 3000 feet on that QNH and continued the approach. Subsequently a Minimum Safe Altitude Warning (MSAW) activated. The crew corrected and continued for a safe landing on runway 21.

The French BEA rated the occurrence a serious incident and opened an investigation.

On Mar 31st 2023 the BEA released their final report concluding the probable causes of the serious incident were:

The following factors may have contributed to the altimeter setting error:

- a high workload and the F-HMLD crew being excessively focused on the weather conditions associated with the ongoing storm, to the detriment of the application of the standard procedure, which included checking the consistency of the QNH value provided by the ATC against another source of information;
- the APP controller being distracted due to an imminent conflict in another sector of his area of responsibility for which he was preparing the resolution. As a consequence, the controller did not identify that the crew incorrectly read back the altimeter setting.

The following factors may have contributed to the non-detection of the erroneous final path:

- the inherent limitations of the Baro-VNAV function in the event of an altimeter setting error;
- the absence of appropriate safety measures in such situations - in particular, the “consistency check” procedure based on the radio altimeter value appears to be ineffective given the operational context at this point of the flight;
- the controller’s radar display using the standard altimeter reference to indicate the aircraft altitudes.

The following factors may have contributed to the reaction time to the MSAW:

- the QNH not being mentioned when the controller conveyed the warning;
- the word “immediately” not being used in the controller’s warning message;
- the surprise effect experienced by the crew.

The BEA summarized the scenario:

When the crew of F-HMLD were cleared to descend to the first altitude below the transition level and to conduct the approach to runway 21, the PM incorrectly read back the QNH, indicating an altimeter setting of 1021 instead of 1002. This error was not detected by the controller or the PF. 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 because of the turbulence experienced during this phase of flight, which was making it difficult for the crew to read the information written on the flight plan.

Due to this QNH error, the aircraft’s path during the approach was approximately 530 ft lower than the published path. However, the procedures and information on the aircraft instruments did not allow the crew to directly identify the path error in a simple way.

The only instrument information that could have allowed the crew to detect this QNH error was the radio altimeter value, which was abnormally low compared to the procedure altitudes, taking into account the altitude of the area overflown. This check was not clearly defined by the standard operating procedures.

The altitude displayed on the radar display, which is a standard altitude, did not allow the controller to easily detect that the aeroplane was not flying the intermediate approach at the published altitude. Moreover, it was not the responsibility of the controller to carry out this check.

The final descent path checks carried out during a RNP Baro-VNAV approach do not enable the crew to detect a path error due to an altimeter setting error. This limitation inherent to RNP Baro-VNAV approaches is a well-known threat, which pilots are regularly reminded of by the operator during their recurrent training or through internal flight safety documents.

As the aircraft was approaching the missed approach point, a MSAW triggered at the LOC controller’s position. The controller informed the crew of this warning, without initially reminding them of the QNH and without using the word “immediately” in the message. Exchanges between the crew and the controller lasted almost 30 s before the crew realised their mistake and corrected the vertical path.

The crew did not perform a go-around as required by the MSAW procedure described in the operations manual. As they had sight of the runway and were aware of bad weather conditions on the missed approach path, they considered it more judicious to continue the approach.

According to the stabilisation criteria, the approach was not stabilised. During the final approach, due to the inherent limitations of the Baro-VNAV function in the event of an altimeter setting error, the crew could not gain awareness of this destabilisation. The crew were only able to perceive this destabilisation on the triggering of the MSAW.

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