Altimeter Setting Procedures
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- Flight level. Standard pressure setting (1013 hPa) is set when flying by reference to flight levels above the transition altitude;
- Altitude. Regional or airfield pressure setting (QNH) is set when flying by reference to altitude above mean sea level below the transition level;
- 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.
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).
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.
System support - the Transponder always transmits the level information as flight level, regardless of the pressure setting. The ground system then converts this information to altitude/height.
- A pilot fails to ensure that standard pressure is set when passing the transition altitude in the climb, and levels the aircraft at a flight level which differs from the cleared level by an amount dependent on the difference between the QNH and 1013 hPa.
- A pilot fails to set QNH when passing the transition level in the descent and levels the aircraft at an altitude which differs from the cleared altitude by an amount dependent on the difference between QNH and 1013 hPa.
- A pilot un-used to landing with QFE set, does not remember that the altimeter now indicates height above airfield elevation or touch-down zone.
- 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.
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:
- BE20/SF34, vicinity Stornoway UK, 2011 (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.)
- B733, vicinity Kosrae Micronesia, 2015 (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.)
- AT45, vicinity Sienajoki Finland, 2007 (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.)
- A310, vicinity Birmingham UK, 2006 (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.)
- H25B, vicinity Kerry Ireland, 2015 (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.)
- B733, vicinity Helsinki Finland, 2008 (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.)
- LJ35, vicinity Masset BC Canada, 1995 (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.)
- MD11, vicinity East Midlands UK, 2005 (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.)
- 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