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Altitude: Difference between revisions
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#[[#Flight level]] | #[[#Flight level]] | ||
==Indicated altitude== | ==Indicated altitude== | ||
The [[altimeter]] reading. The altimeter can be set on '''three''' possible references | *The [[altimeter]] reading. | ||
The altimeter can be set on '''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]]. | #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]]. | ||
#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]] | #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]] | ||
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==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. | *Altitude in terms of elevation above sea level ([[AMSL]]). | ||
After setting the altimeter on [[#QNH]] the indicated altitude is the true altitude. | |||
==Height== | ==Height== | ||
Altitude in terms of the distance above a certain point. | *Altitude in terms of the distance above a certain point. | ||
==Pressure altitude== | ==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) | *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 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. | *Pressure altitude will change with the weather but this is true for all aircraft sharing the same area. | ||
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*Never use Pressure altitude during approach since the altimeter does not display the altitude. | *Never use Pressure altitude during approach since the altimeter does not display the altitude. | ||
===Flight level=== | ===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. | *[[#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== | ==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. | *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. | |||
==QNH== | ==QNH== | ||
The code [http://en.wikipedia.org/wiki/QNH '''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]]. | *The code [http://en.wikipedia.org/wiki/QNH '''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. | 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. | ||