Indicated Airspeed Versus True Airspeed | 5 Types of Airspeed

In this article, we’re going to compare indicated airspeed with true airspeed and help you get a deeper understanding of the difference between the two.

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.

As a knew pilot I know understanding the difference between indicated and true airspeed can be challenging so let’s take a look at the two.

But before we do, I do want to make sure you’re aware of the acronyms that are often used when looking at airspeed.

• KIAS – Knots Indicated Airspeed
• KTAS – Knots True Airspeed
• KCAS – Knots Calibrated Airspeed
• KEAS – Knots Equivalent Airspeed

Types of Airspeed Indicators

Traditional round gauges are found in many aircraft. The pointer moves clockwise to indicate an increase in airspeed. The instrument may be calibrated to show statute miles per hour, knots (nautical miles per hour), or sometimes both. 

A true airspeed indicator has a dial for inputting the current pressure altitude and outside air temperature (OAT). True airspeed (TAS) is displayed on the instrument only when the correct inputs have been made. 

Modern glass cockpits have Primary Flight Displays (PFDs) that incorporate a tape-style airspeed indicator on the left-hand side. The tape scrolls up and down to indicate changes in airspeed, with the current speed centered and in a highlighted box. 

Trend lines show changes and indicate what the airspeed will be six seconds in the future. 

The computer calculates true airspeed and displays it at the bottom of the airspeed tape. Since glass cockpits are also connected to GPS, ground speed is usually shown on the PFD as well. 

Airspeed Corrections

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.

As a side note, when flying a holding pattern, all max holding speeds are listed 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.

Now that you understand the difference between indicated airspeed vs true airspeed, take a look at how fast commercial airplanes fly. 

Types of Airspeed Indicators

Traditional round gauges are found in many aircraft. The pointer moves clockwise to indicate an increase in airspeed. The instrument may be calibrated to show statute miles per hour, knots (nautical miles per hour), or sometimes both. 

A true airspeed indicator has a dial for inputting the current pressure altitude and outside air temperature (OAT). True airspeed (TAS) is displayed on the instrument only when the correct inputs have been made. 

Modern glass cockpits have Primary Flight Displays (PFDs) that incorporate a tape-style airspeed indicator on the left-hand side. The tape scrolls up and down to indicate changes in airspeed, with the current speed centered and in a highlighted box. 

Trend lines show changes and indicate what the airspeed will be six seconds in the future. 

The computer calculates true airspeed and displays it at the bottom of the airspeed tape. Since glass cockpits are also connected to GPS, ground speed is usually shown on the PFD as well. 

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.

How Do Airspeed Indicators Work?

The airspeed indicator is part of the aircraft’s pitot-static system. The system includes the static port, which measures air pressure outside the aircraft that changes with altitude, and the pitot tube, which is pointed forward and measures the air pressure that increases as speed increases. 

The airspeed indicator is a simple pressure instrument that measures the difference in pressure between the static port (stationary or not moving air pressure) and the pitot tube (dynamic or moving air pressure). 

As the aircraft flies faster and faster, the pressure coming into the pitot tube increases, and the instrument shows higher speed. The static port keeps the reading more accurate, allowing the instrument to function at any altitude.

Digital glass cockpits have a computer module that serves the function of the pitot-static system and works in the same basic ways—there are still physical pitot tubes and static ports. 

But the computer takes digital measurements instead of mechanical pressure readings. 

The computer that translates it all is the Air Data Computer (ADC).

Airspeed Indicator Markings

Important airspeeds for the aircraft are permanently marked on the airspeed indicator using color arcs and lines. 

When referring to speed markings, ‘lower limits’ refer to slower speeds, while ‘upper limits’ refer to faster speeds. These speeds are shown regardless of whether you’re looking at a round gauge or tape-style airspeed indicator.

The white arc shows the flap operating range. It starts at the slowest speed you can fly with flaps extended, otherwise known as stall speed “dirty” or Vso, and goes up to the fastest the plane can fly with the flaps extended or Vfe. 

The green arc shows the normal operating speeds. It starts at the slowest speed the plane flies, or stall speed “clean” (Vs1), and goes up to the maximum normal operating speed, or Vno. Vno is also called the maximum structural cruising speed.

The yellow arc is referred to as the caution range. It should only be entered in smooth air. It starts at Vno and goes up to the red line.

The red line is the never-exceed speed, or Vne. Vne is an aircraft limitation—flying faster than this speed will damage the aircraft. 

Multi-engine aircraft have two more lines depicted on their airspeed indicators. These are safety speeds for when one engine has failed. 

The lower red line is for Vmc, or the minimum controllable airspeed.

Vyse is the best rate of climb with a single engine. 

There are quite a few important V-speeds that are not depicted on the airspeed indicator, like landing gear operating limits (Vlo and Vle), best angle and best rate of climb (Vx and Vy), and maneuvering speed (Va). 

These speeds are placarded elsewhere in the cockpit, usually near the appropriate control.