Reference
Aviation numbers, explained.
Every quote we send carries units most people never have to think about. These pages explain the ones that change what you pay — each with a calculator, so you can check our figure rather than take it.
What you are billed on
Chargeable weight
Why a light, bulky box is billed on its size — and why the same box weighs differently on an air waybill than on a courier label.
CalculatorDistance and the nautical mile
Where 1,852 metres comes from, and why our distance is shorter than the line on a road map.
CalculatorHow fast, how high
Knots, ground speed and wind
Why the return leg is rarely the same length in time as the outbound one.
CalculatorFeet and flight levels
Why FL350 is not 35,000 feet, and what QNH and the transition altitude change.
CalculatorMach number
A ratio, not a speed — and a different ground speed at every altitude.
CalculatorReading a flight
The phonetic alphabet
The full ICAO spelling alphabet, with the number words most tables leave out.
ToolRegistration marks
How to read TC-, D-, N- or G- on a tail, and what it tells you about the operator behind a quote.
LookupWhy these units, and not metres and kilograms
It is often said that aviation uses imperial units. That is not quite the position. The International Civil Aviation Organization publishes its standard for units of measurement in Annex 5 to the Chicago Convention, and that standard is the International System of Units — SI.
The nautical mile, the foot and the knot survive alongside it as a temporary dispensation. They were meant to be phased out; no termination date has ever been fixed, because withdrawing units that every chart, instrument and clearance in the world is written in would create more risk than it removes. So the exceptions have outlived the rule, and a quote from us will carry NM, feet and knots for the foreseeable future.
Two of these units are defined exactly rather than approximately, which is worth knowing when you check our arithmetic: one nautical mile is 1,852 metres and one international foot is 0.3048 metres, both by agreement rather than by measurement.
Sources: ICAO Annex 5, Units of Measurement to be Used in Air and Ground Operations; International Extraordinary Hydrographic Conference, Monaco 1929, for the 1,852 m nautical mile; the 1959 international yard and pound agreement for the 0.3048 m foot.