Altitude: Difference between revisions

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Altitude (view source)

Revision as of 12:31, 6 November 2011

125 bytes removed ,  6 November 2011
moved QNH category in the "Indicated altitude" paragraph
(some improvements to QNH)
(moved QNH category in the "Indicated altitude" paragraph)
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==Indicated altitude==
==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 hectopascal) 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]].
 
What you have to use depends on the local [[flight rules]].
 
===QNH===
[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 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 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.
 


==Absolute altitude==
==Absolute altitude==
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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 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]]).


==QNH==
[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.
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''.
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.


==Elevation==
==Elevation==
Hamster
172

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