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In aviation, altitude is the height at which an aircraft is flying. Expressed in feet, an aircraft's altitude is measured with an altimeter. The basis of the altimeter is the same as that of an aneroid barometer: an airtight drum that will change shape depending on the pressure of the surrounding air. The higher the aircraft, the larger the drum. The altimeter is a very sensitive instrument, and as such the air pressure acting on it influences the reading of the altimeter greatly.

In contrast, ground radar is ideal for measuring altitude above the ground without being influenced by the weather. Ground radar is often used for ground warning systems.

In communication with ATC two terms for altitude are used. In lower airspace, the term true altitude, or simply "altitude", is used. In upper airspace, the term flight level (often abbreviated to FL) is preferred.

Types of altitude


The height of something is the vertical distance to a certain point.

Indicated altitude

The altimeter can be set to three possible references

  1. to QNH, resulting in the indicated altitude "matching" the true altitude (see below).
  2. to the Standard pressure (29.92" Hg or 1013.25 hPa) making the altimeter show the pressure altitude. This configuration should only be used in high altitudes.
  3. to the height above the airfield. While on the runway the indicated altitude would read zero. Near the airfield the indicated altitude would be the absolute altitude (above ground level).

What you have to use depends on the local flight rules.

Pressure altitude

  • Pressure altitude is the barometric pressure expressed in feet (this means your expensive piece of equipment has been reduced to a barometer).
  • The altimeter shows pressure altitude, if it is set to the standard pressure: 29.92" Hg = 1013.25 hPa.

This has the big advantage, that all aircrafts are using the same altimeter settings, so the same indicated altitude in different planes in the same area is the same true altitude.

The problem with pressure altitude is that changing weather makes the air pressure vary. So you never know exactly which true altitude you are flying on. This makes pressure altitude very dangerous to use at low altitudes!

As all planes in the same area experience the same effect they will still fly at the same altitude relative to each other.

Flight level

It is used above the transition altitude (18,000 feet (5,500 m) in the US, but may be as low as 3,000 feet (910 m) in other jurisdictions if they do not have higher mountains). When the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180" or FL180.

To assure vertical separation IFR pilots are required to use the altimeter. It is not allowed to use the height measured by GPS systems.

Density altitude

  • Altitude in terms of the density of the air.

This is the only kind of altitude, that is not used for determining the position of the aircraft. Instead it is an important factor for the power your aircraft/helicopter is able to develop in the current situation.

High temperatures cause the air to be less dense. Low air density causes less drag (friction), less lift, less engine performance (the rotorblades have less effect and the engine receives less oxygen). But less friction also enables you to fly faster and more efficient, as less energy is lost for the friction.

Low temperatures cause the air te be more dense. So you have more drag, lift and engine performance (rotorblades have more effect, engine recieves more oxygen) for the price of being slower and burning more fuel as more energy is needed to overpower the friction.

In FlightGear the density of the air is simulated by making a helicopter stick to the ground on a warm day or by making it impossible for the heavy loaded Antonov to depart from Mexico City. The density altitude is calculated from the barometric pressure and the temperature. The higher the temperature, the lower the density, the higher the density altitude (in reference with the true altitude). (wikipedia)

True altitude

The true altitude is the height above the mean sea level (AMSL).

After setting the altimeter to QNH the indicated altitude is close to the true altitude.

Absolute altitude

The absolute altitude is the height relative to the ground directly below (AGL: above ground level).

Ground radar and ground warning systems display the absolute altitude. If the altimeter is set to display zero while being on the airfield, the indicated altitude will be the absolute altitude as long as the elevation of the terrain below does not change.

In FlightGear there is a "system" altitude, that altitude is AMSL. Some will refer to this altitude as absolute altitude, which is theoretically incorrect.


The true altitude of positions on the ground is referred to as the elevation. In aviation these positions normally are runways or mountain peaks.

It is vital to know the elevation of a runway before descending towards it. The best sources for elevation data are:

  • "Location" -> "Select Airport" menu in the simulatior.
  • Atlas provides an accurate height map. One needs to interpret the colours on the map to know the elevation. For IFR and flight planning Atlas is not suited.
  • MPMap is an on-line tool for FlightGear that provides accurate information about runways including the elevation.
  • Airport Diagram Generator will create a PDF file, an accurate map of the airfield. It includes the elevation.
  • Websites that are used for real flight planning. A few problems, the Real runway is not always 100% the same as the FlightGear runway, not all FlightGear runways can be found on those websites (The FlightGear database is more complete).

It is very unfortunate that Kelpie doesn't provide this information.

Additional information

  • QFE: Atmospheric pressure at the airport, can be used to show #Absolute altitude.
  • QNE: Elevation of the airfield above AMSL.
  • QFF: Atmospheric pressure calculated towards AMSL under Standard conditions, this code is not used.
  • ISA-1: International Standard Atmosphere: Atmospheric model of various layers of the earth atmosphere. Each layer has a temperature, pressure, viscosity and density. It is based on an average model of the earth atmosphere. Publication ISO 2533:1975. There is a US model that is synchronised with this standard but is reaching higher.
  • ISA-2: ICAO Standard Atmosphere: Same as ISA-1 but reaching higher and with slightly different values. In aviation this is the standard to use. Publication Doc 7488-CD.
  • Attitude: Most often the Pitch of an aircraft, the angle of the nose in reference to the ground, but there are other definitions.
  • Ascend = climb = go higher.
  • Descend: go lower.

External links