Aviation uses a wide range of units, some historical, some regional, and some aviation specific.
A nautical mile is a unit of measurement used in both air and marine navigation, and for the definition of territorial waters. Historically, it was defined as one minute (1/60 of a degree) of latitude along any line of longitude. Today the international nautical mile is defined as exactly 1852 metres (about 1.15 miles). The derived unit of speed is the knot, one nautical mile per hour.
The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 1.15078 mph or 0.514 m/s). The ISO standard symbol for the knot is kn, kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.
English Language, the international language of flight: In 1951 the International Civil Aviation Organization (ICAO) instated English as the official language of aviation. ICAO advised all airports and routes to operate in their native language, but to have English available for international flights. English was chosen as the international language because the nations that manufactured and operated the majority of aircraft at the time were English speaking.
Altitude: The primary unit of measurement of altitude and elevation or height is the metre. However, the most widely used unit of measurement in aviation is the foot. Metric altitudes and flight levels are used in certain countries, such as Russia and China.
True Altitude: To display true altitude above sea level, a pilot calibrates the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions.
Flight levels are described by a number, which is the nominal altitude, or pressure altitude, in hundreds of feet, while being a multiple of 500 ft, therefore always ending in 0 or 5. Therefore, a pressure altitude of 32,000 ft (9,800 m) is referred to as “flight level 320”. In the United States and Canada, the transition altitude is 18,000 ft (5,500 m). In Europe, the transition altitude varies and can be as low as 3,000 ft (910 m). There are discussions to standardize the transition altitude within the Eurocontrol area.
