What is the difference between indicated airspeed and true airspeed

True airspeed is the speed your plane is actually moving through the air. But since you can't actually see air molecules, it's easier to think of it as the speed your plane is moving past the clouds at your altitude. Often times, your true airspeed and your indicated airspeed are different. That's because your airspeed indicator doesn't measure speed, it measures pressure.

Your airspeed indicator reads accurately at sea level in standard conditions. But as soon as you start to change air density, whether you have non-standard temperature or pressure, or as you start to climb, your airspeed indicator doesn't report accurate speed. That's because your airspeed indicator reports a slower speed than true airspeed as density decreases, based on altitude and air temperature changes.

Because of that, you can't directly measure true airspeed, but you can calculate it with an E6B some analog airspeed indicators have a window to compute true airspeed as well.

If you used indicated airspeed to calculate flight times on a cross country, you'd be pretty far off on your time and fuel burn estimates. That's why true airspeed is so important. Because true airspeed is the speed your plane is actually passing through the air, it's used as the basis for your cruise performance calculations. First, you need to figure out your calibrated airspeed.

There's a chart in Section 5 of your POH to figure that out, using indicated airspeed. Second, you need to calculate the air density against calibrated airspeed. When you factor in air density, you get true airspeed. Using an E6B to compute true airspeed works well below knots. But as you speed up, compressibility becomes a factor. At slow speeds, the air molecules basically get out of the way as your plane passes through them. But as you speed up above knots, the air in front of your plane starts to compress, and that can make your calibrated airspeed inaccurate.

When you factor compressibility in calibrated airspeed, you get equivalent airspeed. But there's a problem with that. Equivalent airspeed isn't easy to compute in the cockpit, as you can see in the diagram below. But that's where the glass panel cockpit comes into play.

At speeds below about knots, it uses the same calculation as your E6B. But as you start to get faster than about knots, it calculates your Mach number, which factors in compressibility. Then, it converts that into true airspeed. Here's the difference in indicated and true airspeed that we found. You can see that we had an increase of 45 knots of true airspeed at a constant airspeed climb of knots indicated airspeed.

At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds, with nose high pitch attitudes. If there is no wind it is also the same as ground speed. Want to learn more about how your airspeed indicator works? Click here. Become a better pilot. Subscribe to get the latest videos, articles, and quizzes that make you a smarter, safer pilot.

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