Understanding the Difference Between Indicated and True Airspeed

One of the most important concepts for newer pilots to understand is that the airspeed shown on the instrument panel is not always the airplane’s actual speed through the air.

The airspeed indicator displays Indicated Airspeed (IAS), which is based on pressure sensed by the pitot-static system.

As altitude increases and air density decreases, True Airspeed (TAS) becomes higher than indicated airspeed.

In practical terms:

• At higher altitudes, the airplane moves faster over the earth than the airspeed indicator suggests

• Takeoff and landing performance are still primarily based on indicated airspeed

• Navigation and flight planning rely more heavily on true airspeed and groundspeed

This concept becomes especially important during:

• High density altitude operations

• Cross-country flight planning

• Mountain flying

• Performance calculations

For example, an aircraft may indicate the same approach speed at a high-elevation airport, but the airplane is actually moving across the ground much faster due to thinner air.

This increases:

• Landing distance

• Takeoff roll

• Energy during landing

Understanding the difference between IAS, TAS, and groundspeed helps pilots better visualize how the airplane truly performs in varying conditions.

Airspeed indicators tell an important story — but they do not tell the entire story.