True Airspeed Calculator
The 2% rule is a linearisation, and it drifts.
Work out True Airspeed. The 2% rule is a linearisation, and it drifts. Names the misconception directly.
True airspeed
113 kt
100 kt calibrated · 93 kt over the ground · Mach 0.175
True airspeed exceeds indicated because the air is thinner: the pitot system measures dynamic pressure, and the same pressure at altitude means a higher actual speed. The old 2% per 1,000 ft rule is a linearisation of that and drifts above about 10,000 ft. Temperature matters as much as altitude. On a hot day the same indicated airspeed is a faster true airspeed, which is exactly the same physics as density altitude seen from the airspeed indicator instead of the runway. Ground speed is true airspeed minus the headwind component, and it is what fuel planning uses. A 20 kt headwind on a 100 kt aircraft costs 20% of range and 25% more time — the two are not the same number.
How the True Airspeed Calculator works
True airspeed from indicated, altitude and temperature, with ground speed, Mach number and the density ratio — and the 2% per 1,000 ft rule shown alongside so its error is visible.
Also known as: indicated to true airspeed · tas from cas and altitude · 2 percent per thousand feet rule · ground speed from tas and wind
Frequently asked questions
Why is true airspeed higher than indicated?
Because the air is thinner. The pitot system measures dynamic pressure, so the same reading at altitude corresponds to a higher actual speed through the air.
Is the 2% per 1,000 ft rule accurate?
Good to about 10,000 ft and drifting after that. It is a linearisation of the density ratio, and it ignores temperature entirely, which matters as much as altitude.
What is the difference between CAS and TAS?
Calibrated airspeed is indicated corrected for instrument and position error. True airspeed is calibrated corrected for air density. Below about 200 kt, equivalent and calibrated are effectively the same.
How does temperature change true airspeed?
Hot air is less dense, so the same indicated airspeed is a faster true airspeed — the same physics as density altitude, seen from the airspeed indicator instead of the runway.
What is ground speed?
True airspeed minus the headwind component. It is what fuel planning uses, and a 20 kt headwind on a 100 kt aircraft costs 20% of range and 25% more time — different numbers.
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