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AP4ATCO - Altitude and Altimeters

From SKYbrary Wiki




Gain an understanding of:

  • how altitude and height are measured in an aeroplane.
  • temperature error.
  • altimeter pressure settings.
  • the basics of Radio Altimeters, GPWS, and TAWS.

The Barometric Altimeter

A barometric altimeter consists of a barometric capsule linked to a pointer by a suitable mechanical or elctronic system. The pointer moves across the dial in response to changes in barometric pressure. The dial is calibrated in feet, or (less commonly) in metres.

Barometric altimeters are provided with a pressure setting control and sub-scale so that the altimeter may be calibrated according to the appropriate pressure setting to indicate flight level, altitude above mean sea level, or altitude above ground level.

The altimeter provides an output to the transponder system to enable the transmission of the flight level or altitude to the air traffic control.

Types of Barometric Altimeter

Barometric altimeter displays may be of two main types:

  • Conventional analogue display;
  • Electronic display.

The main type of conventional altimeter, the Counter-pointer altimeter, is the only type currently approved for use in commercial aircraft.


In modern aircraft, conventional instruments are used mostly as standby instruments.

Modern aircraft are usually equipped with composite Electronic Flight Instrument System displays which combine the functions of several conventional instruments into one. Presentations vary according to the manufacturer's design philosophy. The illustration below shows a typical EFIS display in which the altitude is depicted on a vertical tape to the right of the attitude indicator. In the illustration, the altitude is 5100 ft. The altimeter pressure setting (29.92 in Hz) is depicted in red below the altitude tape.


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 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

altimeter setting
Altimeter Setting

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.

Temperature Error Correction

Pressure altimeters are calibrated to International Standard Atmosphere conditions. Any deviation from ISA will result in error proportional to ISA deviation and to the height of the aircraft above the aerodrome pressure datum.

According to ICAO PANS OPS (Doc 8168) "The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10°C below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above –15°C. For colder temperatures, a more accurate correction should be obtained according to the guidance provided in section 4.3 "Temperature corrections".

When temperature is LESS than ISA an aircraft will be LOWER than the altimeter reading.

For example, if the OAT is - 40 °C then for a 2000 ft indicated altitude the true altitude is 1520 ft thus resulting in a lower than anticipated terrain separation and a potential obstacle-clearance hazard.

Effect of Outside Air Temperature (OAT) on True Altitude - source: Airbus Approach & Landing Briefing Note (see further reading section)

When To Apply Corrections

When the aerodrome temperature is 0°C32 °F <br />273.15 K <br />491.67 °R <br /> or colder, the temperature error correction must be added to:

According to ICAO PANS OPS Chapter 4 "Altimeter Corrections", the pilot-in-command is responsible for the safety of the operation and the safety of the aeroplane and of all persons on board during flight time (Annex 6, 4.5.1). This includes responsibility for obstacle clearance, except when an IFR flight is being vectored by radar.

When pilots intend to apply corrections to the FAF crossing altitude, procedure turn or missed approach altitude, they must advise ATC of their intention and the correction to be applied.

Pilots may refuse Instrument Flight Rules (IFR) assigned altitudes if altitmeter temperature error will reduce obstacle clearance below acceptable minima. However, once an assigned altitude has been accepted, it must not subsequently be adjusted to compensate for temperature error.

Radio Altimeter

A radio altimeter is an airborne electronic device capable of measuring the height of the aircraft above terrain immediately below the aircraft.

The radio altimeter is an important tool to help minimise the risk of Controlled Flight Into Terrain (CFIT), because it provides an independent and unambiguous warning of proximity to the ground, regardless of any navigational uncertainty or error, e.g. mis-setting of the barometric altimeter sub-scale.

Use of the radio altimeter is integral to both the function of ground proximity warning systems and to the operation of aircraft during Cat 2/3 approaches where it is used to determine the position of the aircraft in relation to the applicable decision height.


A ground proximity warning system (GPWS) is a safety net that automatically provides a timely and distinctive warning to the flight crew when the aeroplane is in potentially hazardous proximity to the earth’s surface.

GPWS was introduced in the 1970s as a means to combat the high incidence of Controlled Flight Into Terrain (CFIT) accidents and near-accidents.

Early GPWS used height above ground (measured by the radio altimeter) and rate of closure to determine when the aircraft was in a potentially hazardous situation. Subsequent improvements incorporated aeroplane configuration (e.g. landing gear status) and Instrument Landing System (ILS) glideslope deviation. This 'basic' GPWS was mandated in many countries and was responsible for a worthwhile reduction in the number of accidents. However, basic GPWS suffers from a serious limitation: because the radio altimeter does not look ahead, it is unable to predict a sudden change in terrain, for example, when meeting steeply rising ground.

In 1991, Honeywell introduced their Enhanced Ground Proximity Warning System (EGPWS) which was developed in order to overcome the above limitation. This system combines accurate positional knowledge (normally determined from Global Positioning System (GPS)) with a precise three dimensional map of the terrain, to look ahead of the aircraft as well as downwards. This generates warnings to the pilot if certain parameters are breached. Subsequently, other manufacturers produced similar systems, which are known collectively as Terrain Awareness and Warning Systems (TAWS). The acronym 'TAWS' is used by Federal Aviation Administration (FAA) and by European Aviation Safety Agency (EASA) to refer to this equipment; however, International Civil Aviation Organisation (ICAO) still uses the generic term 'GPWS' in SARPS. To avoid confusion, the terms 'basic GPWS' and 'TAWS' are used in this article to distinguish between the earlier and later systems.

Basic GPWS is no longer mandated in Europe or North America though it may be encountered in other countries or on aircraft where TAWS is not mandated.

Information provided by TAWS

TAWS equipment is classified as Class A or Class B according to the degree of sophistication of the system. In essence, Class A systems are required for all but the smallest commercial air transport aircraft, while Class B systems are required by larger General Aviation (GA) (GA) aircraft and recommended for smaller commercial or GA aircraft.

Aircraft equipment operated by the pilot comprises:

  • A control and indicator unit. The control and indicator unit contains, as a minimum, two lights: a red light to indicate a hard warning (imminent danger) and an amber light to indicate an alert (soft warning or caution).
  • Class A systems must also include a horizontal situation display (HSI), usually integrated as a mode of operation of the Electronic Flight Instrument System.

TAWS is a safety net in which a (Hard) Warning indicates that the aircraft is in a dangerous situation and immediate action is required and an Alert (or soft warning) indicates an abnormal status in relation to terrain which invites prompt review and a possible change of flight path or aircraft configuration.

Appropriate TAWS response procedures for flight crew are determined after careful study of aircraft type performance capability. They must be clearly defined by operators and, in the case of a Warning, should be followed without hesitation as soon as a triggered. Operators normally define different response procedures based upon memory drills for a Warning (sometimes called a Hard Warning) and an immediate review in the case of an Alert (sometimes called a Soft Warning).

The Aircraft Flight Manual or Company Operations Manual must contain the procedures and instructions required for the avoidance of controlled flight into terrain, including limitations on high rate of descent near the surface, as well as detailing abnormal and emergency procedures.

Quiz Questions:

Q1: When the outside air temperature is lower than ISA, an aircraft's barometric altimeter will

  1. show an altitude higher than the aircraft's actual altitude
  2. show an altitude lower than the aircraft's actual altitude


Q2: A radio altimeter is an airborne electronic device capable of measuring the height of the aircraft above terrain immediately below the aircraft.

  1. the height of the aircraft above the touchdown zone
  2. the height of the aircraft above decision height
  3. the height of the aircraft above terrain immediately below the aircraft



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