Aviation numbers
Ground speed, and the wind that sets it.
Airspeed is what the aircraft does. Ground speed is what arrives. The gap between them is wind, and it is why the same leg is two different lengths depending on which way you fly it.
- 1 kt = 1 NM/hOne nautical mile per hour — 1,852 metres, by definition.
- 2 h 30 against 1 h 40The same 1,000 NM leg at 500 kt, into and then out of a 100 kt wind.
- Block timeChocks off to chocks on. That, not en-route time, is what a charter is invoiced against.
Three speeds, and only one of them books freight
An aircraft moves through a body of air that is itself moving. Airspeed is its speed through that air; ground speed is its speed over the map. The difference is the wind, and it is the reason two identical aircraft on the same route land at different times.
The unit is the knot — one nautical mile per hour. Because a nautical mile is one minute of latitude, a speed in knots is also a rate of progress across a chart, which is why the unit has outlived every attempt to replace it.
Ground speed ≈ airspeed − headwind component, or + tailwind component.
Block time = en-route time + taxi. It runs chocks off to chocks on, and it is what a charter is contracted and invoiced against.
Wind on the nose, on the tail, and across
Take an aircraft doing 500 kt through the air over a 1,000 NM leg. With 100 kt straight on the nose the ground speed is 400 kt and the leg takes 2 h 30. Turn round and the same 100 kt is behind: 600 kt, and 1 h 40. Same aircraft, same distance, fifty minutes apart.
The case most explanations skip is wind from the side. A pure 100 kt crosswind takes nothing off directly — but the aircraft has to point partly into it to hold the track, and the part of its speed spent doing that is not spent going forward. At 500 kt the drift correction is about 11.5° and the ground speed falls to roughly 490 kt. Small, and worth knowing the sign of.
Why we price a return as two legs
Doubling an outbound time is the most common estimating error on a round trip. In prevailing westerlies the eastbound leg is routinely faster than the westbound one, sometimes by a wide margin at jet-stream altitudes. Where a quotation from us shows a return, the two directions were estimated separately.
What the calculator leaves out
- Climb and descent, where the aircraft is neither at cruise speed nor at cruise altitude.
- Airway routing, which is longer than the great circle and varies with the day's airspace.
- Wind changing along the route and with altitude — the figure here is one steady wind.
- Holding, slot delays and any en-route stop.
Treat it as an honest first estimate. The desk's figure comes from a flight plan.
Ground speed is solved with the standard wind triangle: the wind is resolved into head and cross components against the track, and the drift correction is applied before the forward component is taken. One knot is one nautical mile per hour, and one nautical mile is 1,852 m by the 1929 Monaco definition.