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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.
{{WIP}}


With [[ATC]] communication two types of altitude can be used.
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.
#Altitude = [[#True altitude]]
#[[#Flight level]]


==Indicated altitude==
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|true altitude]], or simply ''"altitude"'', is used. In upper airspace, the term [[#Flight level|flight level]] (often abbreviated to FL) is preferred.
 
== Types of altitude ==
 
=== Height ===
The height of something is the '''vertical distance to a certain point'''.
 
=== Indicated altitude ===
*The [[altimeter]] reading.  
*The [[altimeter]] reading.  
The altimeter can be set on '''three''' possible references
The altimeter can be set to '''three''' possible references
#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.
#to [[#QNH|QNH]], resulting in the indicated altitude "matching" the [[#True altitude|true altitude]] (see below).
#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]]
#to the Standard pressure (29.92" Hg or 1013.25 hPa) making the altimeter show the [[#Pressure altitude|pressure altitude]]. This configuration should only be used in high altitudes.
#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.
#to the [[#Height|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|absolute altitude]] ([[above ground level]]).
When to set what depends on the local [[flight rules]].


==Absolute altitude==
What you have to use depends on the local [[flight rules]].
*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.
=== 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.


==True altitude==
This has the big advantage, that ''all aircrafts are using the same altimeter settings'', so the ''same [[#Indicated altitude|indicated altitude]]'' in ''different planes'' in the ''same area'' is the ''same [[#True 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==
The problem with pressure altitude is that changing weather makes the air pressure vary. So you never know exactly which [[#True altitude|true altitude]] you are flying on. This makes pressure altitude very dangerous to use at low altitudes!
*Altitude in terms of the distance above a certain point.
 
==Pressure altitude==
As all planes in the same area experience the same effect they will still fly at the same altitude relative to each other.
*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)
=== Flight level ===
*Pressure altitude gives the same result for every aircraft independent from the departing airfield of the aircraft.
*[[#Pressure altitude|Pressure altitude]] divided by 100 is referred to as the flight level.
*Pressure altitude will change with the weather but this is true for all aircraft sharing the same area.
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.
*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 is referred to as the 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.
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==
=== Density altitude ===
*Altitude in terms of the density of the air.  
*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).  
 
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 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 [http://en.wikipedia.org/wiki/Density_altitude 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]]).
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|true altitude]]). ([http://en.wikipedia.org/wiki/Density_altitude wikipedia])


==QNH==
=== True altitude ===
[http://en.wikipedia.org/wiki/QNH QNH] is a brevity code for "atmospheric pressure at sea-level". It is needed when starting or landing an aircraft, when it is important to have the [[#Indicated altitude|indicated altitude]] as accurate as possible.
The true altitude is the '''[[#Height|height]] above the mean sea level''' ([[AMSL]]).


The code QNH is used in:
After setting the [[altimeter]] to [[#QNH|QNH]] the [[#Indicated altitude|indicated altitude]] is close to the true altitude.
*the pilots request for the "atmospheric pressure at sea-level". In their reply [[ATC]] will repeat QNH followed with the pressure.
*automatic weather reports via [[ATIS]].


If there is neither [[ATIS]] nor [[ATC]] available you can find the QNH value in Environment=>Global Weather.
=== Absolute altitude ===
The absolute altitude is the [[#Height|height]] '''relative to the ground directly below''' ([[AGL]]: above ground level).


To use a barometer (i.e. your [[altimeter]]) as a tool to measure the altitude, it must be corrected for weather influences. For this task, every airfield is equipped with an accurate barometer. By combining this value with the [[#True altitude|true altitude]] of the airfield, they can calculate the barometric pressure at [[Mean Sea Level]], the QNH. It will transmit the QNH via [[ATC]] and [[ATIS]] and with that piece of information the altimeter in the aircraft can be corrected for weather influences and will display an altitude close to the [[#True altitude|true altitude]] of the aircraft. This way the [[#Indicated altitude|indicated altitude]] will be quite accurate for a ''certain area around the airfield'' as long as the ''weather remains stable''.
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|indicated altitude]] will be the absolute altitude as long as the [[#Elevation|elevation]] of the terrain below does not change.


It is vital to set the altimeter to QNH when using a height map, so that the [[#Indicated altitude|indicated altitude]] can be compared to the [[#Elevation|elevations]] on the map, which is of special importance during the approach to an airfield. There have been dramatic accidents where pilots were flying using [[#Pressure altitude|pressure altitude]] during descend instead of the [[#True altitude|true altitude]], making the height map of the area almost useless. In bad visibility mountains have not been noticed and the ground was higher as expected.
In FlightGear there is a "system" altitude, that altitude is [[AMSL]]. Some will refer to this altitude as [[#Absolute altitude|absolute altitude]], which is theoretically incorrect.


==Elevation==
=== Elevation ===
The [[#True altitude]] of positions on the ground is referred to as the elevation. In aviation these positions normally are runways or mountain peaks.
The [[#True altitude|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:
It is vital to know the elevation of a runway before descending towards it. The best sources for elevation data are:
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*Descend: go lower.
*Descend: go lower.


==External articles==
 
==External links==
*http://en.wikipedia.org/wiki/Altitude
*http://en.wikipedia.org/wiki/Altitude
*http://en.wikipedia.org/wiki/Pressure_altitude
*http://en.wikipedia.org/wiki/Pressure_altitude
*http://en.wikipedia.org/wiki/Flight_level
*http://en.wikipedia.org/wiki/Flight_level
*http://en.wikipedia.org/wiki/QNH
*http://en.wikipedia.org/wiki/QNH
*http://www.altitude.nu/
*http://elevationmap.net
 
 
{{Understanding}}


[[Category:Aviation]]
[[Category:Aviation]]
{{Understanding}}
[[ca:Altitud]]
[[de:Flughöhe]]
[[de:Flughöhe]]
[[es:Altitud]]
[[fr:Altitude]]
[[nl:Vlieghoogte]]
[[ru:Высота_полёта]]

Latest revision as of 00:19, 28 August 2020

WIP.png Work in progress
This article or section will be worked on in the upcoming hours or days.
See history for the latest developments.

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

Height

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.

Elevation

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:

  • 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