Altimeter Setting Procedures
Altimeter Setting Procedures
The aircraft altimeter barometric sub-scale must be set to the appropriate setting for the phase of flight. These are:
- Flight level. Standard pressure setting (1013 hPa) is set when flying by reference to flight levels at or above the transition level;
- Altitude. Regional or airfield pressure setting (QNH) is set when flying by reference to altitude above mean sea level at or below the transition altitude;
- Height. Altimeter pressure setting indicating height above airfield or touchdown (QFE) is set when approaching to land at airfield where this procedure is in use. Note that this setting is not used in other portions of the flight (climb, cruise and initial descent).
Aircraft are not supposed to fly level within the transition layer (between the transition altitude and the transition level). When passing through it, their vertical position is expressed in:
- flight levels during climb (i.e. above the transition altitude)
- altitudes during descent (i.e. below the transition level)
If no transition altitude and transition level are defined (which is common for en-route flights outside the TMAs), the vertical position of aircraft shall be expressed in terms of:
- flight levels at or above the lowest usable flight level
- altitudes below the lowest usable flight level
Failure to set the appropriate barometric sub-scale pressure setting may result in a significant deviation from the cleared altitude or Flight Level
Types of Altimeter Setting Error
- The pilot mishears the transmitted pressure setting and sets an incorrect figure.
- The pilot hears the transmitted pressure setting correctly but fails to set it or mis-sets it.
- The pilot fails to change the pressure setting at the appropriate point in a departure, climb, descent or approach.
- Failure to set the appropriate pressure setting can result in deviation from the cleared altitude or flight level leading to level bust, loss of separation from other traffic, and even collision with other aircraft or with the ground (CFIT).
- Loss of situational awareness due to failure to appreciate the significance of a pressure setting (especially QFE as opposed to QNH). This can result in incorrect appreciation of the closeness of the ground possibly leading to an unstabilised approach or collision with the ground (CFIT).
- The existence of appropriate SOPs for the setting and cross-checking of altimeter sub scales and their strict observance is the only universal primary solution to eliminate incorrect altimeter setting.
- Use of the aircraft radio altimeter to monitor the aircraft proximity with the ground can help to improve situational awareness provided that the flight crew are generally familiar with the terrain over which they are flying;
- GPWS/GPWS/TAWS provide a safety net against CFIT and, in the case of TAWS Class 'A' with its option of a simple terrain mapping display, it can also be used to directly improve routine situational awareness.
- Strict adherence to the verification of pressure-altitude-derived level procedure by ATC. This should be done at least once by each suitably equipped ATC unit. The check is performed by comparing the level received from surveillance sources with a voice report by the pilot.
Accidents and Incidents
Events in which the incorrect altimeter pressure setting was either a cause or contributing factor in a Level Bust or CFIT/near CFIT:
On 10 September 2017, the First Officer of a Gulfstream G550 making an offset non-precision approach to Paris Le Bourget failed to make a correct visual transition and after both crew were initially slow to recognise the error, an unsuccessful attempt at a low-level corrective realignment followed. This had not been completed when the auto throttle set the thrust to idle at 50 feet whilst a turn was being made over the runway ahead of the displaced threshold and one wing was in collision with runway edge lighting. The landing attempt was rejected and the Captain took over the go-around.
On 23 May 2022, an Airbus A320 came extremely close to collision with terrain as the crew commenced a go around they did not obtain any visual reference during a RNP approach at Paris CDG for which they were using baro-VNAV reference to fly to VNAV/LNAV minima. The corresponding ILS was out of service. The Investigation has not yet completely established the context for the event but this has been confirmed to include the use of an incorrect QNH which resulted in the approach being continued significantly below the procedure MDA. Six Interim Safety Recommendations have been issued.
On 6 June 2020, a Boeing 787-10 on approach at Abu Dhabi began a low go around from an RNAV(RNP) approach when it became obvious to the crew that the aircraft was far lower than it should have been but were unaware why this occurred until an ATC query led them to recognise that the wrong QNH had been set with recognition of the excessively low altitude delayed by haze limiting the PAPI range. The Investigation found that advice of MSAW activations which would have enabled the flight crew to recognise their error were not advised to them.
On 31 December 2011 a USAF C12 Beech King Air descended 700 feet below the cleared outbound altitude on a procedural non precision approach to Stornoway in uncontrolled airspace in IMC and also failed to fly the procedure correctly. As a result it came into conflict with a Saab 340 inbound on the same procedure. The Investigation found that the C12 crew had interpreted the QNH given by ATC as 990 hPa as 29.90 inches, the subscale setting units used in the USA. The Saab 340 pilot saw the opposite direction traffic on TCAS and descended early to increase separation.
On 12 June 2015, a Boeing 737-300 crew forgot to set QNH before commencing a night non-precision approach to Kosrae which was then flown using an over-reading altimeter. EGPWS Alerts occurred due to this mis-setting but were initially assessed as false. The third of these occurred when the eventual go-around was initially misflown and descent to within 200 feet of the sea occurred before climbing. The Investigation noted failure to action the approach checklist, the absence of ATC support and the step-down profile promulgated for the NDB/DME procedure flown as well as the potential effect of fatigue on the Captain.
On 16 June 2015, the crew of a US-operated HS125 on a commercial air transport flight failed to continue climbing as cleared to FL200 after take off from Kerry for a transatlantic flight and instead levelled at 2000 feet on track towards higher terrain. Prompt ATC recognition of the situation and intervention to direct an immediate climb resolved the imminent CFIT risk. The Investigation found that the two pilots involved had, despite correct readback, interpreted their clearance to flight level two hundred as being to two thousand feet and then failed to seek clarification from ATC when they became confused.
On 26 March 2008, a Ukraine International Airlines Boeing 737-300 being vectored by ATC to the ILS at destination Helsinki in IMC descended below its cleared altitude and came close to a telecommunications mast. ATC noticed the deviation and instructed a climb. The investigation attributed the non-compliance with the accepted descent clearance to the failure of the flight crew to operate in accordance with SOPs. It was also noted that the way in which ATC safety systems were installed and configured at the time of the occurrence had precluded earlier ATC awareness of the hazard caused by the altitude deviation.
On 1 January 2007, the crew of a ATR 42-500 carried out successive night approaches into Seinajoki Finland including three with EGPWS warnings, one near stall, and one near loss of control, all attributed to poor flight crew performance including use of the wrong barometric sub scale setting.
On 11 January 1995, a Learjet 35 on a medical positioning flight and carrying a medical team crashed into the sea while conducting an NDB approach to Masset, British Columbia, Canada. The most probable cause was considered to be a miss-set altimeter.
On 3 December 2005, the crew of a MD-11 freighter failed to set the (very low) QNH for a night approach, due to distraction, and as a result descended well below the cleared altitude given by ATC for the intercept heading for the ILS at Nottingham East Midlands airport, UK.
On 24 November 2006, an A310 descended significantly below cleared altitude during a radar vectored approach positioning, as a result of the flight crew's failure to set the QNH, which was unusually low.
- Doc 8168 (PANS-OPS), Volume I, Flight Procedures - Part VI - Altimeter Setting Procedures - Chapter 3.
- ICAO Video: Altimetry - Basic Principles, external link;
Flight Safety Foundation ALAR Toolkit
EUROCONTROL Level Bust Toolkit
- UK CAA FODCOM 6/2007: Avoidance of Controlled Flight Into Terrain (CFIT) - Operational and Training Considerations
Airbus Briefing Notes