FS2000: Differences Between True Air Speed and Indicated Air Speed (264616)

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True Air Speed

True air speed is the actual speed at which an aircraft moves from one point to another. True air speed can be affected by the altitude of the aircraft, but in most cases the effect is minimal because most aircraft fly at a maximum altitude of seven to nine miles. Some large aircraft can fly at higher altitudes. For example, the Concorde can fly at a maximum altitude of approximately eleven miles.

Indicated Air Speed

The air speed displayed on the Air Speed Indicator on the instrument panel in your aircraft is the indicated air speed. Indicated air speed is the speed at which an aircraft moves through the air, and it can be affected significantly by altitude and wind direction.

The Air Speed Indicator works by comparing the air pressure inside the Pilot Tube to the air pressure inside the aircraft cabin. The Pilot Tube is a hollow probe that points directly forward into the oncoming air.

Changes in altitude can affect indicated air speed. As the altitude of the aircraft increases, the air pressure in the Pilot Tube decreases, causing the indicated air speed to decrease.

Assuming no effects from wind, the following table lists the true air speed in nautical miles-per-hour (knots) for an aircraft traveling at 400 knots at altitudes between 1000 feet and 25000 feet.

Altitude	Indicated air speed	True air speed
1000 feet	400	405
4000 feet	400	421.7
8000 feet	400	445
10000 feet	400	457.2
15000 feet	400	489.1
18000 feet	400	510.1
22000 feet	400	539.4
24000 feet	400	554.7
25000 feet	400	562.2