With 60 kt of headwind, however, the time would be increased to a little more than 23 minutes. For example, a 140 nm distance would take an aircraft flying at 420 kt (no wind) 20 minutes. The moment when an aircraft reaches the Transfer of Control point depends on their ground speed. Groundspeed has two major uses in air traffic control: If the aircraft are flying at different levels, a difference of 2000-3000 ft would result in 6 kt speed difference, the lower aircraft being faster. Therefore, Mach number, being the same part of a now lower value, is reduced.Īs a rule of thumb, if the two aricraft are flying at the same level, a difference in Mach number of 0.01 would result in 6 kt groundspeed difference if the wind is calm. This is because air density (and consequently, the speed of sound) decreases with altitude. If an aircraft maintains Mach number, TAS (and therefore groundspeed) reduces when an aircraft climbs. As a result, in case of tailwinds higher aircraft move significantly faster and in case of head winds it is possible that aircraft at different levels maintaining the same IAS would also fly at similar groundspeeds. an aircraft that is flying 4000ft above would be about 30 kt faster in terms of TAS/groundspeed.Īdditional factor here is that usually (but not always) windspeed also increases with altitude, generally reaching its highest at the tropopause. As a result, if two aircraft are maintaining the same IAS and tracks at different levels and the wind is the same, the higher aircraft will fly faster in terms of groundspeed.Īs a rule of thumb, TAS would change by 7 kt per 1000 ft, e.g. This is because air density decreases with altitude and consequently, higher speed is required to obtain the same dynamic pressure. If an aircraft maintains IAS, TAS (and therefore groundspeed) increases when an aircraft climbs. Groundspeed is a vector sum of True Airspeed (TAS) and wind velocity.Ī graphical representation of TAS, wind velocity and ground speed Source: ICAO Doc 9426 Relations to Other Speeds The speed of an aircraft relative to the surface of the earth.

Crackdown 3