True Airspeed Explained: Why It Matters for Pilots

IAS vs. TAS: What's the Difference?

Indicated Airspeed (IAS) is what you read directly from the airspeed indicator in the cockpit. It measures the dynamic pressure of the air ramming into the pitot tube. True Airspeed (TAS), on the other hand, is the actual speed of the aircraft through the air mass.

At sea level on a standard day (15°C, 29.92 inHg), IAS and TAS are essentially the same. However, as you climb, the air becomes less dense. Because there are fewer air molecules hitting the pitot tube, your airspeed indicator will read lower than your actual speed through the air. Therefore, TAS increases relative to IAS as altitude increases.

Why True Airspeed Matters

TAS is the foundation of all navigation and flight planning. While you fly the airplane using IAS (because aerodynamic forces like stall speed are tied to dynamic pressure, not true speed), you navigate using TAS.

Without knowing your TAS, you cannot accurately calculate your Ground Speed (GS), which is TAS adjusted for wind. Without Ground Speed, you cannot accurately estimate your Time En Route (ETE) or your required fuel.

The Rule of Thumb

A common rule of thumb is that True Airspeed increases by about 2% over Indicated Airspeed for every 1,000 feet of altitude. For example, if you are flying at 10,000 feet with an IAS of 100 knots, your TAS will be approximately 20% higher, or 120 knots.

How to Calculate True Airspeed

To calculate TAS accurately, you need to know your pressure altitude, your indicated (or calibrated) airspeed, and the outside air temperature (OAT). By correcting your IAS for non-standard temperature and pressure, you arrive at your TAS. Modern flight planning tools handle this instantly, ensuring your cross-country navigation is built on accurate data.