In this article, we’re going to compare indicated airspeed with true airspeed and help you get a deeper understanding of the difference.
Like its partner in crime, the altimeter, your airspeed indicator gives you vital information about the plane you’re flying. Unfortunately, they both like to keep their secrets, and it takes a clever pilot to solve all the mysteries.
The good news is that the airspeed indicator always provides a pretty reliable picture of how much air is flowing over the wing. There aren’t many things more important than this!
When it comes to keeping your dinner reservation for that $100 hamburger at the end of a long cross country, you need more information than the airspeed indicator provides. You’ll have to factor in the altitude you’re flying, the temperatures, and even the wind speed and direction.
Here’s a look at the steps you’ll take along the way, from reading the instrument to calculating your ETA. Don’t let that hamburger get cold!
- Indicated airspeed – read off the instrument
- Calibrated airspeed – indicated airspeed corrected for instrument errors
- Equivalent airspeed – calibrated airspeed corrected for high-speed compression errors
- True airspeed – calibrated or equivalent airspeed corrected for pressure altitude and temperature
- Ground speed – true airspeed corrected for wind
Indicated Airspeed – IAS
Indicated airspeed is read off the face of the instrument. It’s the easiest to get since it’s right in front of you, but unfortunately, it’s also the least accurate.
Most of the errors come from installation errors in the instrument system itself. The pitot tube on the wing is fixed in place. The wing changes its angle of attack during flight, so the pitot tube is only pointed perfectly into the relative wind at one time.
The pitot tube is installed to give you the most accurate reading most often—so it’s closest during cruise flight.
Even though it’s the least accurate number, you should still have faith in it. For most purposes in the cockpit, it’s the indicated airspeed that you should live by. All of the critical V-speeds that you need to know, like rotation, approach speed, and airplane limitations, are referenced in KIAS.
Calibrated Airspeed – CAS
To correct that instrument error, most aircraft manufacturers provide a table in the POH. This table will list airspeeds in knots of indicated airspeed (KIAS) and provide the solution in knots calibrated airspeed (KCAS). The angle of attack and the flap settings will have the greatest effect on CAS.
Equivalent Airspeed – EAS
Equivalent airspeeds are often skipped over for small training aircraft. Correcting for equivalents means removing the error caused by high-speed air pressed against the leading edges and pitot tube. It’s mostly a factor on larger jet aircraft and usually ignored in small training planes.
True Airspeed – TAS
True airspeed takes these instrument readings and corrects them for air density. An aircraft can move faster when the air is thinner, but this won’t appear on the airspeed indicator, because when the air is thinner, so is the air entering the pitot tube.
To find true airspeed, you’ll need an E6B flight computer. The correction is made by inputting your pressure altitude and temperature. If this sounds familiar, it’s because it’s the same things you need to do to find density altitude.
Some aircraft have TAS built into their airspeed indicators. If your airspeed dial has a selector knob, then you can program in the pressure altitude and temperature to correct it to show true airspeed.
Of course, electronic flight displays can do all of this automatically. On the G1000, for example, TAS is displayed underneath the indicated airspeed tape.
True airspeeds increase with altitude. Here’s an example of a 120-knot airplane at different altitudes (pressure altitudes and standard temperatures).
- Sea level, 15º C, 120 KIAS, 120 KTAS
- 5,000 feet, 5º C, 120 KIAS, 130 KTAS
- 10,000 feet, -5º C, 120 KIAS, 140 KTAS
- 15,000 feet, -15º C, 120 KIAS, 151 KTAS
- 20,000 feet, -25º C, 120 KIAS, 164 KTAS
- 25,000 feet, -35º C, 120 KIAS, 178 KTAS
Ground Speed – GS
True airspeed is the final step in the chain to figuring out how quickly the plane moves through the atmosphere. But, of course, there is one more step to figure out how long it will take to reach your destination.
When planning cross countries or finding your ETA, you need to know your ground speed. This is your true airspeed corrected for the effects of the wind. To do it, you need to know the winds aloft at your altitude and use the backside of your E6B calculator.
There are no analog instruments that display ground speed inside the cockpit. DME or RNAV navigation systems can do it, but it will only work in certain situations if they use VOR information.
But GPS systems show ground speed—a handy tool to have in the cockpit.
Types of airspeeds are covered in the Airspeed Indicator section of Chapter 8: Flight Instruments in the FAA’s Pilot’s Handbook of Aeronautical Knowledge.