What is the difference between indicated airspeed and true airspeed?

Indicated airspeed (IAS) is what your airspeed indicator displays - it measures dynamic pressure. True airspeed (TAS) is your actual speed through the air. At sea level on a standard day, they're equal. As altitude increases and air becomes thinner, TAS becomes higher than IAS for the same indicated reading.

Why does true airspeed increase with altitude?

At higher altitudes, air is less dense. Your airspeed indicator senses less dynamic pressure, so it reads lower than your actual speed through the air. For every 1,000 feet of altitude, TAS increases approximately 2% above IAS (at constant indicated airspeed).

Why is true airspeed important for flight planning?

TAS is essential for calculating ground speed, flight time, and fuel requirements. When you add or subtract wind from your speed, you use TAS - not IAS. Filing a flight plan with IAS instead of TAS would result in incorrect time and fuel estimates.

How does temperature affect true airspeed?

Warmer temperatures result in less dense air, increasing TAS for a given IAS. Colder temperatures have the opposite effect. This is why the TAS calculation includes outside air temperature. On a hot day, your TAS will be higher than on a cold day at the same altitude and IAS.

What is calibrated airspeed?

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position errors. These errors vary with aircraft configuration and attitude. CAS is found in the POH calibration chart. TAS calculations technically use CAS, though many pilots use IAS for simplicity at cruise.

What is the 2% rule for true airspeed?

A quick mental calculation: TAS increases approximately 2% for every 1,000 feet of altitude above sea level. At 10,000 feet, TAS is roughly 20% higher than IAS. For example, 100 KIAS at 10,000 feet equals approximately 120 KTAS.

True Airspeed Explained - IAS vs TAS for Pilots

Types of Airspeed

IAS Indicated Airspeed

What your airspeed indicator shows. Used for aircraft handling and V-speeds.

CAS Calibrated Airspeed

IAS corrected for instrument and position errors. Found in POH.

TAS True Airspeed

Actual speed through the air. Used for navigation and flight planning.

GS Ground Speed

TAS adjusted for wind. Your actual speed over the ground.

The 2% Rule

Quick Mental Math

TAS increases approximately 2% per 1,000 feet of altitude.

Altitude	IAS	Approx TAS	Increase
Sea Level	100 kt	100 kt	—
5,000 ft	100 kt	110 kt	+10%
10,000 ft	100 kt	120 kt	+20%
FL250	250 kt	~400 kt	+60%

Why TAS Matters

Flight Planning

Use TAS to calculate ground speed, flight time, and fuel. IAS would give incorrect estimates.

Wind Calculations

Ground speed = TAS ± wind. You can't add/subtract wind from IAS.

Altitude Selection

Higher altitude = higher TAS = faster ground speed (with favorable winds).

Fuel Efficiency

Optimize altitude for best TAS vs fuel flow tradeoff on longer flights.

Remember

Fly the airplane with IAS (stall speeds, V-speeds, approach speeds). Plan the flight with TAS (navigation, fuel, time estimates).

True Airspeed Explained