¶ Usual Units
This guide describes the usual units of measurement you will encounter in aviation. As an ATC you will have to manipulates these so it is important for you to understand them.
¶ Speeds
Knots
Knots (kt) is the unit of measure of speed in aeronautical navigation. It is used to reprends the speed of wind or aircraft.
One knot represents one Nautical Mille (NM) per hour.
1kt = 1NM/h = 1.52km/h
Mach
The Mach numer (M) is a dimension-less number expressing an aircraft's speed as a ratio of the local (in the aircraft's environment) speed of sound.
An aircraft flying at Mach 1,0 is moving at a speed equal to the speed of sound. A commercial jet flying at M0,82 moves at 0,82 x the speed of sound. As the vast majority of IFR aircraft are unable to fly at supersonic speeds (M>1,00) the Mach number is expressed as the decimals only.
BAW123, Maintain mach decimal 78 or less.
¶ Altitudes
Feets
Feets are the unit of measure of height, altitude or depth. It can represent an aircraft's altitude or the height of clouds above ground.
A feet is about 0,3 meters.
Flight Levels
Flight levels is the unit of measure under standard barometric setting. A flight level corresponds to 100ft. For example FL100 corresponds to 10.000ft.
Flight levels are primarily used for cruise altitudes.
¶ Pressure
Hectopascals
Hectopascals (hPa) is the unit of measure of atmospheric pressure. It is given in both the METAR and the ATIS to accurately set altimetric references. The standard atmospheric pressure at sea level is 1013,25 hPa.
Inch of Mercury
The inch of mercury (inHg) is the main unit of measure for atmospheric pressure in North America. The standard pressure at sea level is 29.92 inHg.
¶ Distances
Nautical Miles
The nautical mile (NM) is the unit of measure for distance.
One nautical mile is about 1,852 kilometer.
Meter
The meter (m) is the second main unit of measurement for distance. It is used to represent length rather than distance, for example: runway length or horizontal visibility.