top of page

How do we Measure Airspeed and Altitude of Aircraft?

Measuring the airspeed and altitude of an aircraft it is not as simple as you might first think, and requires the consideration of a number of different factors. In this article we will introduce the different types of aircraft airspeed and altitude, why they are important and how they are measured.

Airspeed Measurement

There are 4 types of airspeed often referred to:.

  1. Indicated Airspeed(IAS): the value that is read directly off the airspeed indicator, calculated from the pitot static system.

  2. True Airspeed (TAS): the speed of the aircraft relative to the air it's flying through. At higher altitudes, true airspeed is higher than your indicated airspeed.

  3. Groundspeed (GS): the movement of the aircraft relative to the ground. It is the true airspeed with a correction for wind.

  4. Calibrated Airspeed (CAS): the indicated airspeed corrected for instrument and positional errors.

A brief overview of the principles behind which the devices work will be given below.

Indicated Airspeed.

The air speed indicator is located in the cockpit of an aircraft. It is a device for measuring forward speed of an aircraft. Airspeed is measured in knot or kilometres per hour.

On older aircraft, airspeed is usually indicated to the pilot on a graduated scale over which a pointer moves ( the first diagram). In modern aircraft, airspeed is usually indicated on a speed tape which forms part of the electronic flight instrument system display ( the second diagram).

Air speed indicators
Figure 1 - Air speed indicators