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 = #Pressure altitude divided by 100

Indicated altitude

The altimeter reading. 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.
  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.

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

#Pressure altitude divided by 100 feet is referred to as the flight level, and 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); so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180" or FL180.

Density altitude

Altitude in terms of the density of the air. This is important at higher altitudes, with heavy loaded aircraft and with helicopters.

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 the 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 and 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.

External articles