What happened
On May 23, 2022, an Airbus A320, registration 9H-EMU, operated by Airhub Airlines, was performing a scheduled flight from Stockholm Arlanda to Paris-Charles de Gaulle. Due to ongoing maintenance on the ILS for runway 27R, the crew executed a satellite approach using barometric vertical guidance (RNP APCH).
During the approach, heavy rain significantly reduced visibility. The crew received an incorrect altimeter setting (QNH) from air traffic services, which was 10 hPa higher than the actual pressure (1011 hPa instead of 1001 hPa). This error caused the aircraft to fly a vertical profile approximately 280 feet below the published path.
As the aircraft descended, a ground proximity warning (MSAW) was triggered in the control tower. The air traffic controller attempted to alert the crew but used inappropriate phraseology, which the crew did not hear. The crew subsequently initiated a go-around after failing to acquire visual references. During this maneuver, the aircraft's radio-altimeter recorded a height of only 6 feet above the ground while approximately 0.9 NM from the runway threshold. A second approach was conducted using the same incorrect setting, but the crew successfully acquired visual contact with the ground at an altitude of over 600 feet and landed safely.
The investigation
The BEA examined the impact of the incorrect QNH on the barometric vertical guidance profile and investigated the effectiveness of both onboard and ground-based safety systems. The investigation focused on why the error was not detected by the crew or the air traffic controller, the adequacy of the controller's response to the MSAW alert, and the limitations of the aircraft's instrumentation regarding barometric errors.
Findings
- The primary cause of the near-collision was the execution of a barometric approach using an incorrect altimeter setting, which resulted in a vertical profile significantly below the published minimums.
- Human error during the communication of the QNH was a contributing factor.
- Existing operating procedures for both flight crews and air traffic controllers were insufficient to prevent or detect the use of an incorrect altimeter setting.
- Neither the aircraft's instruments nor the ground-based tools were designed to detect this specific type of pressure error.
- The crew's lack of awareness of the ground proximity was compounded by the absence of an onboard Terrain Awareness Warning System (TAWS) alert during the event, as the system was operating according to its design limitations.
- The air traffic controller's reaction to the MSAW alert was delayed and inadequate due to insufficient training regarding the specific actions required following such an alert.
- Contributing environmental and technical factors included unlit approach lights and reduced visibility due to rain.
Safety action
The BEA issued twelve safety recommendations, including those focused on:
- Reassessing the risks of Controlled Flight Into Terrain (CFIT) related to incorrect altimeter settings during baro-VNAV operations.
- Enhancing ground systems for detecting incorrect QNH transmissions.
- Improving onboard TAWS capabilities.
- Improving air traffic controller training regarding MSAW alerts.
- Maintaining safety levels during the transition to PBN-based approaches in Europe.