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Altitude: Difference between revisions
small changes to pressure altitude and to general definitions
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==Pressure 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. | |||
* | 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]]''. | ||
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! | |||
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]] 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). 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. | ||
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. | ||
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==General Definitions== | ==General Definitions== | ||
===Height=== | ===Height=== | ||
The height of something is the '''vertical distance to a certain point'''. | |||
===True altitude=== | ===True altitude=== | ||
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). | |||
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. | 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. | ||