Altitude

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In aviation the height of an aircraft is named the altitude. It is expressed in feet. The international foot is defined as exactly 0.3048 metres. In aircraft the altitude is measured with the altimeter. The basis of the altimeter is the same as a 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 very sensitive and the air pressure caused by the weather has great influence on it. Ground radar is perfect for measuring the altitude above ground level and is used for ground warning systems.

With ATC communication two types of altitude can be used.

  1. Altitude = #True altitude
  2. #Flight level


Indicated altitude

The altimeter can be set on three possible references

  1. The "barometric pressure adjusted to sea level" (#QNH) resulting in indicated altitude above Mean Sea Level for a certain region during current weather conditions. While on the airfield the altimeter would read the altitude of the airfield, the #True altitude. If QNH is not available but the height of the departing airfield is known, the alitimeter can be set on the height of the airfield. It is also possible to use QNH from a nearby airfield.
  2. Height above the airfield. While on the runway the indicated altitude would read zero. Near the airfield the indicated altitude would be above ground level, the #Absolute altitude
  3. Standard pressure (29.92" Hg or 1013.25 millibars) resulting in the #Pressure altitude. While on the airfield the altimeter will not display the altitude. It will display the barometric pressure expressed in feet. It provides a standard altitude reading above a certain height.

When to set what depends on the local flight rules.

Absolute altitude

  • Altitude in terms of the distance above the ground (AGL) directly below it.

Ground radar and ground warning systems display absolute altitude. The altimeter can be set to display zero while being on the airfield, the #Indicated altitude will be the absolute altitude while being near the airfield.

In FlightGear there is a "system" altitude, that altitude is AMSL. Some will refer to this altitude as Absolute altitude.

True altitude

  • Altitude in terms of elevation above sea level (AMSL).

After setting the altimeter on #QNH the #Indicated altitude is the true altitude.

Height

  • Altitude in terms of the distance above a certain point.

Pressure altitude

  • Altitude in terms of the air pressure.

The altimeter will display the barometric pressure expressed in feet. (i.e. your expensive piece of equipment has been reduced to a barometer)

  • Pressure altitude gives the same result for every aircraft independent from the departing airfield of the aircraft.
  • Pressure altitude will change with the weather but this is true for all aircraft sharing the same area.
  • When wanting to use Pressure altitude, the altimeter must be set to standard pressure = 29.92" Hg = 1013.25 mbar.
  • Never use Pressure altitude during approach since the altimeter does not display the altitude.

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). 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 important with warm weather, at higher altitudes, with heavy loaded aircraft and with helicopters. Low air density causes less drag (friction), less lift, less engine performance (the blades have less effect and the engine receives less oxygen).

In FlightGear the density of the air is simulated making a helicopter stick to the ground on a warm day on a high elevation helipad or making it impossible for the heavy loaded Antonov 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).

QNH

  • The code QNH is how a pilot would request the "atmospheric pressure at sea-level". In reply ATC will repeat QNH followed with the pressure. These days QNH is also transmitted automatically on ATIS.

To use a barometer as a tool to measure the altitude, the altimeter, means it must be corrected for weather influences. For that the airfield has an accurate barometer and they know the height of the airfield above Mean Sea Level. From those two they will calculate the barometric pressure at Mean Sea Level, the QNH. It will transmit the QNH and with that the altimeter in the aircraft can be corrected for weather influences and will display the #True height. This way the #Indicated altitude will be accurate for a certain area around the airfield as long as the weather remains stable.

QNH is a vital tool to use a height map of an area, hence it is vital during approach of an airfield.

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.

External articles