FLYING ARTICLES

Airspeed

The speed of an aircraft relative to its surrounding air mass is called airspeed and is normally measured in Knots.

As you know already from school, the term "speed" relates to distance traveled per unit time.

One Knot refers to one nautical mile per hour. One nautical mile is equal to 1.151 statute miles or 1.852 kilometers.

The airspeed indicator displays the speed of your airplane or helicopter through the air by comparing ram air pressure with static air pressure. The greater the differential, the greater the speed.

Now let us take a closer look at the various definitions and meanings of the term "airspeed".

1. INDICATED AIRSPEED - IAS

This is the speed shown on the aircraft airspeed indicator and is used in pilot/controller communications.

2. CALIBRATED AIRSPEED - CAS

Calibrated airspeed is the indicated airspeed corrected for installation and instrument errors. Although these errors are minimized by the manufacturer, they cannot be totally eliminated throughout the full range of operating speeds.

3. EQUIVALENT AIRSPEED - EAS

Equivalent airspeed is calibrated airspeed corrected for the compression of the air at a particular altitude. In the higher speed ranges, an aircraft passes through the atmosphere so rapidly that the air is compressed in front of it.

This error, often called compressibility, is generally considered insignificant when operating below 200 Knots.

4. TRUE AIRSPEED - TAS

True airspeed is the calibrated airspeed or EAS as appropriate corrected for pressure altitude and temperature.

It is the airspeed of an aircraft relative to undisturbed air and is used primarily in flight planning. On a calm day, your true airspeed is the same as ground speed assuming level flight.

5. MACH

Mach is theratio of the aircraft's true airspeed to the speed of sound. For example, a speed of Mach 0.7 means the aircraft is flying at 70% of the speed of sound.

The speed of sound is variable. Depending on air temperature, relative humidity and pressure, it is approximately 1 117 feet or 340.48 meters per second at 59 degrees Fahrenheit.

By the way, Concorde crosses the Atlantic ocean with Mach 2.0 at 55 000 feet altitude.


About the Author

Ing. Guenther Geretschnig (guenther@gg-pilot.com) currently holds pilot certificates and ratings in several countries all over the world.

With more than 10 years experience in aviation, Guenther has been active as a helicopter pilot and instructor in America.

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Copyright © , Ing. Guenther Geretschnig. All rights reserved.