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Speed units
Aircraft cruising at lower altitudes normally use knots as the primary speed
unit, while aircraft that are higher (above Mach Crossover Altitude) normally
use Mach number as the primary speed unit, though flight plans often include
the equivalent speed in knots as well (the conversion includes allowance for
temperature and height).
In a flight plan, a Mach number of "Point 82" means that the aircraft is
travelling at 0.820 (82%) of the speed of sound.
The widespread use of global positioning systems (GPS) allows cockpit navigation systems to
provide air speed and ground speed more or less directly.
Another method of obtaining speed and position is the inertial navigation system (INS), which
keeps track of a vehicle's acceleration using gyroscopes and linear accelerometers; this
information can then be integrated in time to obtain speed and position, as long as the INS
was properly calibrated before departure.
INS has been present in civil aviation for a few decades and is mostly used in medium to large
aircraft as the system is fairly complex.
If neither GPS or INS is used, the following steps are required to obtain speed information:
An airspeed indicator is used to measure indicated airspeed (IAS) in knots.
IAS is converted to calibrated airspeed (CAS) using an aircraft-specific correction table.
CAS is converted to equivalent airspeed (EAS) by allowing for compressibility effects.
EAS is converted to true airspeed (TAS) by allowing for density altitude (i.e., height and
temperature).
TAS is converted to ground speed by allowing for any head or tail wind.
A - Z of Flight
July 2017
www.alliance-airways.net
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