Altitude: Difference between revisions

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Altitude (view source)

Revision as of 00:19, 28 August 2020

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Reorganize section headings to be a bit more readable, move QNH to its own definition page
(Touch up introduction paragraph)
(Reorganize section headings to be a bit more readable, move QNH to its own definition page)
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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 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.


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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.
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==
=== Indicated altitude ===
*The [[altimeter]] reading.  
*The [[altimeter]] reading.  
The altimeter can be set to '''three''' possible references
The altimeter can be set to '''three''' possible references
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What you have to use depends on the local [[flight rules]].
What you have to use depends on the local [[flight rules]].


===QNH===
=== Pressure 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 code QNH is used in:
*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.
 
If QNH is not available but the [[#Elevation|elevation]] of the departing airfield is known, set the [[altimeter]] to it instead. The resulting QNH is the correct one. It is also possible to use QNH from a nearby airfield.
 
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''. While on the airfield the altimeter would show the altitude of the airfield.
 
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.
 
 
==Pressure altitude==
*Pressure altitude is the '''barometric pressure expressed in feet''' (this means your expensive piece of equipment has been reduced to a barometer).
*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.
*The altimeter shows pressure altitude, if it is set to the '''standard pressure''': 29.92" Hg = 1013.25 hPa.
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As all planes in the same area experience the same effect they will still fly at the same altitude relative to each other.
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===
=== Flight level ===
*[[#Pressure altitude|Pressure altitude]] divided by 100 is referred to as the flight level.
*[[#Pressure altitude|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 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.
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.
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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.


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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])
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])


 
=== True altitude ===
==General Definitions==
===Height===
The height of something is the '''vertical distance to a certain point'''.
 
===True altitude===
The true altitude is the '''[[#Height|height]] above the mean sea level''' ([[AMSL]]).
The true altitude is the '''[[#Height|height]] above the mean sea level''' ([[AMSL]]).


After setting the [[altimeter]] to [[#QNH|QNH]] the [[#Indicated altitude|indicated altitude]] is close to the true altitude.
After setting the [[altimeter]] to [[#QNH|QNH]] the [[#Indicated altitude|indicated altitude]] is close to the true altitude.


===Absolute altitude===
=== Absolute altitude ===
The absolute altitude is the [[#Height|height]] '''relative to the ground directly below''' ([[AGL]]: above ground level).
The absolute altitude is the [[#Height|height]] '''relative to the ground directly below''' ([[AGL]]: above ground level).


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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.
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|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.


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*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).
*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.
It is very unfortunate that [[Kelpie]] doesn't provide this information.


==Additional information==
==Additional information==
Ranguli
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