Issue awareness and continuous improvement are key to safe operations. Following recent serious incidents on the effects of incorrect barometric altimeter settings, particularly, when operating below the transition altitude, EASA has recently issued SIB 2023-03.
The SIB was developed in consultation with several subject matter experts. This article provides the chance to explain the wider context of the SIB and cover some of the wider topics associated with this safety risk.
In aviation, vertical navigation based on barometric altimetry and vertical references on navigation charts traditionally rely on the use of local barometric pressure, i.e., QNH (or QFE); hence, operating with an incorrect altimeter setting could lead to flying closer to terrain or obstacles than expected. It may also lead to a loss of separation with other aircraft. In the worst-case scenario, having an incorrect barometric altimeter setting could lead to a loss of adequate terrain clearance and, in the worse case, a CFIT.
An incorrect barometric altimeter setting is a known vulnerability that, in some cases, has proved to degrade pilots’ situational awareness. An incorrect QNH/QFE below the transition level/altitude could result in minimum safe altitudes being infringed, including minimum vectoring altitudes, published decision altitudes, step-down altitudes, etc.
In particular, an incorrect barometric altimeter setting could affect the safety margins that protect a variety of approach procedures that rely on barometric altimetry for vertical navigation (e.g., RNP APCH down to LNAV/VNAV minima, RNP AR APCH) or that are flown using the CDFA technique that rely on a BARO-VNAV equipment onboard to compute the vertical profile and to provide vertical guidance along the descent (e.g., NDB, VOR, LOC, RNP APCH down to LNAV).
It is particularly worth highlighting that when using barometric altimetry for vertical navigation, altitude/distance cross checks in the standard operating procedures do not detect an incorrect barometric altimetry setting.
On the other hand, vertical guidance provided by ILS, SBAS or GBAS is not vulnerable to an incorrect barometric setting and, contrary to vertical guidance based on barometric altimetry (e.g., supported BARO-VNAV equipment), errors in barometric altimeter settings can be detected through altitude (glide path) checks.
In particular, when vertical navigation relies on barometric altimetry, a precise barometric altimeter setting is paramount; otherwise, an incorrect vertical profile will be flown, i.e., either lower or higher than desired, depending on whether the incorrect QNH (or QFE) is, respectively, greater or lower than the actual QNH (or QFE).
The diagram below helps to highlight what the situation might look like from a practical perspective.
Photo credit: Airbus
On the above figure, a 10 hPa error in altimeter setting translates into 280 ft altitude error. This means that the altitude displayed may differ significantly from the actual altitude.
It is worth noting that the effects of an incorrect barometric altimeter setting are similar to those associated with very low or high temperatures, whose mitigation requires error corrections on barometric altimeters readings (temperature corrections).
Setting the correct barometric values in the cockpit requires a number of different people to do the right things. This is where the possibility for error comes in, these include:
There are a number of barriers in the system that help to prevent a barometric setting error, identify one quickly if it happens and then to mitigate the risk of an accident the longer such a situation continues.
Following voice communication procedures is key to preventing or detecting an incorrect barometric altimeter setting. In particular, it is important to do the following:
Another barrier is the effective use of systems that help ATS units to monitor the progress of approach operations, e.g., the minimum safe altitude warning (MSAW) system or an approach path monitoring (APM) system, which are helpful against the risk of CFIT. When using these systems, there should be local ATS procedures and ATCO training to ensure that controllers understand the functionality in use and can react in different circumstances, including the issuance of clear messages to pilots.
As safety nets are commonly adapted to the ATS unit, this needs to be considered when constructing the procedures and training. Recent incidents have shown that, when safety nets alerts are triggered a prompt and accurate reaction from the controller is essential to enable pilots to take timely corrective action.
Pilots must remain attentive to any ATS related messages and react promptly, including the execution of a go-around, when necessary. For example, a low altitude warning transmitted by and ATS unit includes, as part of the standard phraseology, the QNH, together with an instruction to check altitude, which can definitely help to detect an incorrect QNH setting: Low altitude warning ((aircraft call sign) LOW ALTITUDE WARNING, CHECK YOUR ALTITUDE IMMEDIATELY, QNH IS (number)[(units)]. [THE MINIMUM FLIGHT ALTITUDE IS (altitude)])
Should a barometric pressure error occur, if the pilots have external references they are more likely to identify the situation and take the right action. This is particularly a challenge in poor visibility. This means that the use of aeronautical ground lights can be particularly important to help manage this risk.
The rules in ATS.TR.150 say that all lights should be operated at night and close to sunset or sunrise, or “at any other time when their use, based on meteorological conditions, is considered desirable for the safety of air traffic”. It could help if runways that are in use for landing operations had their runway and approach lights kept on in all meteorological conditions in order to help pilots acquire visual references during the approach.
It is worth reminding that approach lights are meant to be on whenever the runway lights are on.
In addition to the barriers already in place, to prevent the risk of incorrect barometric setting and mitigate its potential consequences, the following practices are recommended:
Through EASA’s work in the European Operator’s FDM (EOFDM) forum, the group have developed 6 possible methods to help monitoring incorrect altimeter settings using your FDM programme.
While the SIB specifically covers operational aspects related to incorrect barometric pressure settings, there are some other areas that deserve consideration, in particular:
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