Flight levels and Altitude understanding
In
aviation,
a flight level is the nominal altitude
of an aircraft referenced to a standard pressure datum, as opposed
to the real altitude above mean sea level.
To
see why flight levels are used, a discussion of the means of measuring altitude
is necessary.
Historically,
altitude has most easily been measured using an altimeter,
which is essentially a calibrated barometer
- it measures air pressure, which decreases with increasing altitude. To
display altitude above sea level the pilot must recalibrate the altimeter
according to the local air pressure from time to time, to take into account
natural variation of pressure over time and in different regions. If this isn't
done, different aircraft may be flying at different heights even though their
altimeters show the same altitude. More critically, different aircraft may be
flying at the same height even though their altimeters show different heights.
Clearly this is a safety issue.
Flight
levels solve this problem by defining altitudes based on a standard pressure of
1013.2 mb (29.92 inHg used in U.S. and Canada). All aircraft
operating on flight levels calibrate to this same standard setting regardless
of the actual sea level pressure. Flight levels are then assigned a number
which is the apparent altitude ("pressure altitude") to the nearest
thousand feet, divided by one hundred. Therefore an apparent altitude of 12,000
feet is referred to as Flight Level 120 (except in the United States and Canada
-- see note below). Note that aircraft may be at some other actual height than
12,000 feet, but since they all agree on a standard pressure, no collision risk
arises.
Flight
levels are not used close to the ground, for perhaps obvious reasons -
obstacles are fixed to the ground and so their absolute height needs to be
known. A vertical region extending from a defined transition altitude to the
lowest available flight level is known as the transition layer - pilots will
use altitude based on the local pressure below this level, and flight levels
above. The altitude of the lowest flight level varies from country.
An
estimate of the real altitude is based on air pressure at the aircraft and the
reported local air pressure at sea level (if there is no sea, this is a virtual
value by adjusting the value at the ground for its elevation). However, to
avoid collisions between two planes, their real altitudes are not important, but
only the difference between them. This difference solely depends on the air
pressure at both planes, and does not require knowledge of the local air
pressure on the ground.
Therefore
air traffic control assigns a plane a "flight level" (a nominal
altitude), based on an altitude scale with a one-to-one correspondence with air
pressure at the plane. Thus basically the plane is assigned an air pressure.
The flight level corresponds to the real altitude that would be concluded from
the air pressure, if the air pressure at sea level were 1013.2 mbar
(29.92 inHg or 101.32 kPa).