True Airspeed (approx)
Airspeed corrected for altitude.
Formula
Variables
Example
TAS exceeds IAS at altitude
How it works
This approximation estimates true airspeed from indicated airspeed by correcting for the thinner air found at higher altitude. As altitude increases, the aircraft must move faster through the air to produce the same dynamic pressure that the airspeed indicator senses, so TAS rises above IAS. The factor h/60000 is a rule-of-thumb linear correction that is reasonable for many practical aviation calculations in the lower to middle atmosphere.
When to use it
Use this when you need a quick mental estimate of true airspeed from indicated airspeed and altitude, such as during flight planning, performance checks, or navigation estimates. It is most useful when a fast approximation is acceptable and you do not need a full density-altitude or temperature-corrected calculation.
Worked example, step by step
Given: IAS = 140 kt, altitude h = 18,000 ft Step 1: Compute the altitude correction factor h/60000 = 18000/60000 = 0.30 Step 2: Add 1 to get the multiplier 1 + 0.30 = 1.30 Step 3: Multiply by IAS TAS ≈ 140 × 1.30 = 182 kt Final answer: The approximate true airspeed is 182 knots.
Common mistakes to avoid
- Using feet with the wrong altitude scale, such as plugging in meters instead of feet
- Treating this approximation as exact at all altitudes and temperatures
- Confusing indicated airspeed with calibrated or ground speed
Did You Know?
True airspeed exceeds the cockpit reading at altitude, since thin high air understates the real speed.
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