Let’s now define the following airspeeds: Calibrated Airspeed, Equivalent Airspeed, Indicated Airspeed, Mach Number, and True Airspeed.

Calibrated Airspeed is indicated airspeed corrected for position installation error.

Equivalent Airspeed is calibrated airspeed corrected for compressibility. Equivalent airspeed will always be lower than calibrated airspeed.

Indicated Airspeed is the airspeed shown on the airspeed indicator.

Mach Number is the ratio of aircraft true airspeed to the speed of sound. The speed of sound is directly related only to temperature. At sea level, on a standard day of 59°F; 15°C the speed of sound is about 660 Kts. Low temperature and high altitude, decreases the speed of sound.

True Airspeed is equivalent airspeed corrected for temperature and pressure altitude. At speeds below 200 knots, true airspeed can be found by correcting calibrated airspeed for temperature and pressure altitude. A rule of thumb is: True airspeed is 2% more than calibrated airspeed per 1,000 feet altitude above sea level.

The airspeed indicator is connected to both the static ports and the pitot tube. The airspeed indicator is color-coded to help the pilot immediately recognize the important airspeeds and ranges of airspeed. The color codes are: White Arc, Green Arc, Yellow Arc, Red Radial Line, and Blue Radial Line.

White arc – The white arc is the flap operating speed. The bottom of this white arc is VS0. VS0 is the power-off stalling speed with the gear and flaps down. Going up along the airspeed indicator we come to the Green Arc. VS1 is where the white arc meets the green arc, or is commonly referred to as the bottom of green arc, or the power-off stalling speed with gear and flaps up.

Green arc – The Green Arc is the normal operating range. The very top of the green arc, is the maximum structural cruising speed, commonly known as VNO.

Yellow arc – The yellow Arc is the caution range. The aircraft should not be flown in this speed range in rough air. The Yellow Arc extends from the top of the green arc to the red line.

Red radial line – The Red Radial Line is the never-exceed speed, or VNE.

Finally the Blue radial line is best rate-of-climb speed for a twin-engine airplane with one engine inoperative.

The design maneuvering speed VA is the maximum speed at which the aircraft can be safely stalled, VA is not color-coded on the airspeed indicator. This maneuvering speed is noted in the Aircraft Flight Manual and is often marked on a placard on the instrument panel. So the question is, if VA for an aircraft is not known, what is a generally accepted rule of thumb for a safe maneuvering speed? The answer is 1.7 times the normal stalling speed.