The flight crew of an A320 was preparing for an RNP approach with LNAV/VNAV minima toward its destination airport, before initiating descent from their cruise Flight Level. The ATIS provided them with an airport QNH of 1001 hPa.

During the descent, ATC cleared the flight crew to descend to 6000 ft QNH 1011 hPa, followed 2 minutes later by a clearance down to 5000 ft QNH 1011 hPa. The flight crew acknowledged both clearances repeating the erroneous 1011 hPa QNH, which was 10 hPa above the current QNH of the airport.

① The aircraft leveled off at 5000 ft QNH 1011 hPa. This placed it approximately 280 ft below the intended altitude of 5000 ft with a correct QNH of 1001 hPa. (fig.1). With autopilot and autothrust ON, the A320 reached its Final Descent Point and ② commenced its final descent using FINAL APP guidance mode. The aircraft was flying with no visual reference and light turbulence through a rain shower.

③ At 1392 ft indicated altitude (1000 ft above the airfield altitude), the aircraft was stabilized in CONF FULL at Vapp and the ND and PFD indicated that it was on its expected horizontal and vertical flight path (fig.2).

④ ATC received a Minimum Safety Altitude Warning (MSAW) when the aircraft was 1.53 NM from the runway threshold and had an indicated altitude of 891 ft.

⑤ The aircraft passed an 802 ft indicated altitude, corresponding to the Decision Altitude (DA) of the published approach plus 50 ft as per the airline policy. ATC transmitted a warning to the flight crew stating that they had an MSAW and asked the flight crew to confirm they had the runway in sight. The PF initiated a go-around 6 seconds after crossing the DA, at 735 ft indicated altitude.

⑥ The aircraft radio altitude indicated a descent to 6ft during the go-around maneuver. ⑦ The flight crew announced the go-around seconds later and were vectored for a second approach.

During the final approach, the flight crew did not detect the erroneous vertical position because:

• The vertical deviation symbol was centered
• Altitude vs. distance checks were correct
• There was no Terrain Avoidance Warning System (TAWS) alert.

Several RA auto-callouts should have been triggered according to the aircraft configuration. However, the cockpit voice recorder data was deleted during subsequent flights, and was therefore not available to confirm if the auto-callouts were triggered or not.

The runway approach lights were not turned ON for their first approach attempt in poor weather conditions, which made it extremely difficult for the flight crew to visually detect the runway. The lights were switched to ON before the second approach, and the flight crew was able to see the runway and correct their trajectory.

An erroneous QNH/QFE value can seriously affect the safety of the flight as presented in the close call event described above.

From the altimetry basics, a 1 hPa difference in the QNH/QFE value creates a 28 ft shift of the barometric altitude displayed on the PFD.

All final approach guidance modes that use the barometric reference are affected by an erroneous entry on the QNH selector.

Managed guidance

The FMS uses the aircraft barometric altitude to compute the deviation of the aircraft trajectory with the computed final descent path. If an erroneous barometric altitude is used, the aircraft will follow a flight path that is parallel to the published path but is shifted either above or below it (fig.3). The vertical deviation symbol, or the FLS symbol, will indicate that the aircraft is on the correct flight path even if it is not the case .

Selected guidance

An erroneous barometric setting will also cause the FDP height above ground to be incorrect when using selected guidance. The flight crew is likely to commence final descent from an incorrect height above ground and therefore fly an approach path that is too high or too low.

The flight crew will not detect an incorrect flight path with altitude-vs-distance checks if the barometric setting is erroneous. These checks use the displayed barometric altitude, which is based on the erroneous barometric setting. The effect is the flight crew will observe that they are at the expected altitude for each distance value, even if the aircraft is flying above or below the published flight path.

Honeywell EGPWS

The relative proximity of the actual flight path to the published path prevents the TOO LOW TERRAIN EGPWS alert from triggering, because the path remains outside of the Terrain Clearance Floor (TCF) alert envelope (fig.4).

ACSS T2CAS and T3CAS

The Premature Descent Alert (PDA) of the T2CAS and T3CAS may also not be triggered depending on the situation.

The final approach path of approaches using ILS, GLS, or SLS guidance are not affected, because the _G/S_ guidance mode uses the ILS signal or a beam computed with an augmented GPS altitude. The final approach path will remain aligned with the correct ILS/GLS/SLS beam even if the intermediate approach segment shifts due to the erroneous barometric setting (fig.5).

Flight crews have two opportunities to detect a barometric reference setting discrepancy. The first is during descent and the second is during final approach.

During descent, when cleared to an altitude, the flight crew should pay attention to a barometric reference that significantly differs from the ATIS barometric reference used for the approach preparation. Such a difference could be a symptom of barometric reference error. In this case, the flight crew should confirm that they have the correct barometric reference from all available sources.

An abnormally decreasing RA audio callout while the barometric altitude is still high above airfield elevation is a clue that the aircraft may be too low on its final approach path. This can be due to a barometric reference discrepancy. However, RA callouts depend on the terrain profile and therefore may not be present if low terrain is located before the runway.

The ALTSM function, currently available on some Honeywell EGPWS standards, compares the barometric altitude on the captain side with the GPS altitude. If the difference exceeds a threshold, the EGPWS emits an “ALTIMETER SETTING” alert, and it is repeated if an incorrect barometric setting is still detected after some time.

Availability of the ALTSM function

A first step of the ALTSM function is already proposed on A320 and A330 aircraft equipped with Honeywell EGPWS standards P/N 965-1676-006, 69000942-151, and 69000942-251. It can be activated on the compatible computer standards via a Service Bulletin (SB). This first step prepares the introduction of the second step of the function that will add a flashing QNH/QFE value on the PFD in addition to the audio alert. It will also protect the QFE setting. This second step will be included in the Landing Surveillance package, to be incrementally certified from 2023 to 2024. It will be available on production aircraft and proposed for retrofit, for both EGPWS and T3CAS computers. More information on ALTSM and on the Landing Surveillance package is available at: https://www.navblue.aero/product/landing-surveillance.

A similar function will be available in the next A350 surveillance computer standard that is planned to be available in 2027 on newly produced aircraft.

The following table provides an overview of ALTSM availability on Airbus aircraft: