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Why Are True Airspeed And Indicated Airspeed Different?

True airspeed and indicated airspeed are rarely the same, but why? We explain in our latest video...


What Is 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.

Why Is True Airspeed Different Than Indicated Airspeed?

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).

Why Does True Airspeed Matter?

If you used indicated airspeed to calculate flight times on a cross country, you'd be pretty far off on 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.

How Do You Get From Indicated Airspeed To True Airspeed?

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.

You can use an E6B to do the calculation, but you can also use the performance charts in your POH.

What About Compressibility?

Using an E6B to compute true airspeed works well below 100 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 100 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 100 knots, it uses the same calculation as your E6B.

But as you start to get faster than about 100 knots, it calculates your Mach number, which factors in compressibility. Then, it converts that into true airspeed.

How Much Does True Aispeed Change With Altitude?

On average, true airspeed increases about 2% per 1,000' of increase in altitude, but the actual change depends on temperature and pressure.

To demonstrate the difference, we flew a constant airspeed climb of 125 KIAS from Rocky Mountain Metro Airport, to Flight Level 240. Here's the difference in indicated and true airspeed that we found.

  • 6,700' @ 125 KIAS: 142 KTAS
  • 9,000' @ 125 KIAS: 147 KTAS
  • 10,300' @ 125 KIAS: 150 KTAS
  • 15,000' @ 125 KIAS: 160 KTAS
  • 18,500' @ 125 KIAS: 170 KTAS
  • 24,000' @ 125 KIAS: 187 KTAS

You can see that we had an increase of 45 knots of true airspeed at a constant airspeed climb of 125 knots indicated airspeed.

True Airspeed Increases As The Air Gets Thin

You can see that as the air gets thinner, true airspeed increases significantly. And assuming no wind, the faster your true airspeed, the faster you'll get to your destination.

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