Airhub Airlines A320 Descends Below Safe Altitude Twice on Approach to Paris Charles de Gaulle

Paris, France – On May 23rd, 2022, an Airhub Airlines Airbus A320-200, operating on behalf of Norwegian as flight D8-4311, experienced two significant altitude deviations during its approach to Paris Charles de Gaulle Airport (CDG). The aircraft, registered as 9H-EMU, was flying from Stockholm, Sweden, and was on a Required Navigation Performance (RNP) approach to runway 27R when it twice descended below the safe altitude, triggering safety alarms and necessitating a go-around.

Incident Overview

The flight crew was conducting an RNP approach to runway 27R at CDG when the aircraft descended below the prescribed glide path, prompting a Minimum Safe Altitude Warning (MSAW) at the controller’s desk. The crew, unaware of their proximity to the ground due to an incorrect altimeter setting, initiated a go-around at a dangerously low altitude. The aircraft then repositioned for a second approach, during which it again descended below the safe glide path before finally correcting and landing safely approximately 16 minutes after the go-around.

Contributing Factors

The French Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA) launched an investigation into the serious incident, which they categorized as a Controlled Flight Into Terrain (CFIT) risk. The BEA’s investigation revealed that the root cause of the incident was an erroneous altimeter setting. The crew had set the altimeter to 1011 hPa instead of the correct local QNH of 1001 hPa, leading to a descent profile that was approximately 280 feet lower than intended.

This error was compounded by a series of lapses in communication and procedure:

1. Incorrect QNH Provided by ATC: The approach controller provided the incorrect QNH of 1011 hPa to the crew, which was read back multiple times without being corrected. This mistake was not noticed by either the controller or the flight crew.
2. Lack of Visual References: The aircraft encountered a rain shower on approach, significantly reducing visibility and leaving the crew without external visual references. The approach lights on runway 27R were not illuminated, further exacerbating the situation.
3. Inadequate Response to MSAW Alerts: During both approaches, MSAW alerts were triggered at the control tower. However, the controllers’ responses were delayed and the phraseology used to inform the crew was inappropriate and unclear. This delayed the crew’s awareness of the situation, contributing to the descent below the safe altitude.
4. Systemic Weaknesses: The BEA’s final report highlighted systemic weaknesses in the procedures used by both flight crews and air traffic controllers to prevent altimeter setting errors. Additionally, the aircraft’s Terrain Awareness and Warning System (TAWS) did not issue any alerts during the incident, as it was operating within its design parameters, which do not account for altimeter setting errors.

Investigation Findings

The BEA’s final report, released on July 11th, 2024, concluded that the incident was caused by a combination of human error, inadequate procedures, and insufficient system safeguards. The report emphasized the following key points:

• The erroneous altimeter setting led the aircraft to descend 280 feet below the published glide path, bringing it as close as 6 feet above ground level at one point during the first approach.
• The crew’s lack of awareness of their altitude and the absence of visual references due to weather conditions significantly increased the risk of a CFIT event.
• The delayed and insufficient response by air traffic control, coupled with the ineffective communication of the MSAW alerts, failed to adequately address the risk during both approaches.

Conclusion

This incident underscores the critical importance of accurate altimeter settings during approaches, particularly in poor visibility conditions where visual references are limited. The BEA’s findings serve as a stark reminder of the potential consequences of procedural lapses and the need for robust systems and training to prevent such errors.

In response to the incident, it is expected that both Airhub Airlines and the relevant aviation authorities will review and enhance their procedures to mitigate the risk of similar occurrences in the future. The incident also highlights the need for continuous vigilance by both flight crews and air traffic controllers to ensure the safety of flight operations, especially during critical phases such as approach and landing.