Airspeed indicator
5 min read

Say your airspeed. Seems like a simple question. And it’s one controllers often ask when separating in trail airplanes in busy airspace. But there’s nothing simple about airspeed.

There are at least four kinds of airspeed—indicated airspeed (IAS), calibrated airspeed (CAS), true airspeed (TAS) and Mach. Each value has significance to pilots.

But, back to the controller’s request to “say airspeed.” The answer to that really is simple. The controller wants to know your IAS in knots.

Indicated airspeed is what we read on the airspeed indicator. In light airplanes, particularly those built before pilot’s operating handbooks were standardized by the major manufacturers in the mid-70s, the airspeed indicator is often marked in mph. Larger airplanes, and more recently built piston airplanes, show IAS in knots. And some instruments are marked with both values.

Airspeed indicator

Which airspeed do they mean?

Calibrated airspeed (CAS) is what you get when you correct IAS for errors in the pitot-static system and the instrument itself. These errors are sometimes called “position errors” because the location of the pitot tube and static ports are the source of most of the error.

The difference between CAS and IAS is usually quite small. Particularly in level flight. But with flaps down, or maybe gear extended, the errors can grow. The errors are not always linear and can change with speed and altitude.

The good news is that many of us now fly with electronic digital air data computers (ADC). The ADC can be programmed to correct for most position errors so the IAS we see is very close the CAS.

So CAS is really how fast the airplane is moving through the air. Right? No, not usually. Any change in air density caused by temperature, barometric pressure or altitude, will alter the CAS/IAS. That’s why we need true airspeed (TAS).

TAS is a very close representation of how fast the airplane is moving through its local parcel of atmosphere. If there is zero wind, TAS would accurately reflect how fast we are moving over the ground.

I’m sure you remember from private ground school that TAS is CAS corrected for air temperature, air pressure and altitude. The “wiz wheel” can perform that calculation, but now an ADC does it automatically and continuously. An important side benefit of that TAS calculation is that with GPS giving us accurate ground speed, and with a reliable heading from the attitude heading reference system (AHRS), we can continuously see the wind direction and velocity presented on any modern nav display.

Of course, finding the true air temperature—also called static air temperature—so that one can calculate TAS is no easy matter in flight because of ram rise. What in light airplanes we call outside air temperature (OAT) is really ram air temperature (RAT), but that’s a topic for another discussion. In reality RAT and SAT are very close at piston airplane speeds so it’s an issue mostly for turbine airplane pilots.

To fly with precision we need to know TAS for accurate flight planning, and CAS/IAS to keep the airplane within its operating envelope.

Without knowing TAS it would be impossible to plan a flight. TAS corrected for wind equals ground speed, and that is the speed that determines how much fuel we’ll need. Not much in flying is more critical than accurate fuel planning.

But IAS—let’s just assume that the airplane limitations are presented in that value—is what keeps us flying, and also keeps us from breaking the airframe.

IAS is often called Q pressure, because it represents the pressure the airplane experiences moving through the air. So it’s the pressure represented by IAS that creates lift, but also is what generates airframe loads. Too little IAS pressure and the wing can’t lift. Too much Q pressure and the wing can break.

Mach airspeed

At high altitude, Mach becomes the speed that matters.

The convenient thing about IAS is that its effect on the airplane doesn’t change with air temperature, or pressure or altitude. In one-G flight at the same weight the airplane will stall at the same IAS no matter the air temperature or pressure. So, for example, Vref landing approach speed will be the same IAS at the same weight no matter the elevation of the runway, or air temperature. And the Vne red line airspeed is still the structural airspeed limit no matter air density.

At higher altitudes the effects of Mach—the speed of sound—can change the behavior of airflow and thus the flying characteristics of an airplane. That’s why high flying airplanes have a Mach limitation called Mmo for maximum Mach operating limit. Above a certain altitude—typically in the twenties—Mmo replaces Vmo as the airspeed limit.

So, when a passenger asks how fast we’re flying I’d pick the greater of two speeds—TAS or ground speed.

When the controllers ask for airspeed they want to hear your IAS in knots. And if you get to fly faster airplanes the controller will take the guesswork out of the question and specifically ask for your speed in knots or Mach.

Bottom line on speed is that what we all want more groundspeed because that’s what gets us from A to B.

Mac McClellan
13 replies
  1. David Ward Sandidge
    David Ward Sandidge says:

    Mac, good review. Thanks for submitting this. Hey, I had an idea for you. Sometime, I would like to read from you a comprehensive review (with perhaps a catchy mnemonic phrase which enables one to easily remember the detrimental effects of icing) of various pitot/static systems and their components. Thanks in advance.

      Mac MCCLELLAN says:

      Good idea, David. I’ll go to work on that for next fall. Most of the country is coming out of the lower altitude icing season now–I hope.
      Thanks for the kind words,
      Mac Mc

  2. Brian
    Brian says:

    Yes, it seem this topic seems to come up often. As a CFI many students ask me what speed matters. In the “old days” all most GA airplanes had available was IAS directly read off the airspeed indicator. many of those legacy airplanes have not been updated with much of anything else so IAS has always been the speed to report to ATC.
    But I guess we can blame modern technology for this confusion. The computers in our airplane can now calculate all of the “speeds” by using GPS and OAT.
    Seems the more tools we have , the more complicated it gets!
    I gets all of this provides job security for CFIs!! Thanks for a good article.

  3. Jean-Francois Reat
    Jean-Francois Reat says:

    I don’t follow your logic. Since TAS is a closer representation of how fast the airplane is moving through its local parcel of air, then why would the controllers be interested in the less accurate IAS? They aren’t concerned with our flight envelope or V speeds.

      Mac MCCLELLAN says:

      Hi Jean-Francois,
      The reason controllers want to know our airspeed as indicated in knots is because they are trying to separate airplanes in trail now–or soon will be–at the same altitude. When flying at the same altitude in close proximity all airplanes operate in the same atmospheric pressure at the same temperature so the indicated airspeed provides the same speed reference through the air. And it’s assigning speeds using the same reference that allows controllers to keep us apart in trail. IAS does the job without need for any of the calculations required to determine TAS.
      Mac Mc

  4. Stephen Phoenix
    Stephen Phoenix says:

    For your bottom line you suggest we all want more ground speed. A friend of mine once told me, “If I didn’t like flying, I would get a faster airplane”. I think that there is logic in that statement also.

      Mac MCCLELLAN says:

      Hi John,
      The global aviation system mixes the metric and English measurement systems, but the nautical mile is the constant for distance measurement, and the knot for speed.
      Russia and China, and perhaps a few smaller nations I’m unaware of, have used meters to measure altitude. Russia is changing to feet, but I don’t believe China has begun to move away from meters for altitude.
      However, much of the world uses millibars for the altimeter setting instead of the inches hg used in the U.S. Also, runway distances in most of the world will be reported in meters. And weight will be measure in tonnes. And even the U.S. switched to C instead of F to measure air temperature. If you’ve been flying long enough you can remember when here in the U.S. we reported temperature at the surface in F, but in C above the surface.
      A reason the nautical mile and knots have persisted instead of kilometers is that the nautical mile is a derivative of the lat-long navigation system. A nautical mile is a one minute arc of longitude. Since we map in lat-long, it makes sense to use the nautical mile and knot.
      So, I’m sure some light airplane makers around the globe mark their airspeed indicators in kph, just as some here use mph. But in the ATC system it’s still knots of speed and feet of altitude that prevail.
      Mac Mc

      • John Opalko
        John Opalko says:

        Just a minor quibble, Mac.
        You state that a nautical mile is equal to one minute of longitude. That’s true only at the equator because the meridians (lines of longitude) converge at the poles. So, the length (in nautical miles) of a one-minute arc of longitude is proportional to the cosine of your latitude.
        One nautical mile is generally considered to be one minute of latitude, because this is relatively constant all over the globe. There is variation from place to place because the Earth isn’t a perfect sphere, but it’s close enough for practical purposes.
        I’ll see myself out and go back to my Cave of Pedantry now…

  5. Bill Eudy
    Bill Eudy says:

    About thirty years ago I bought a small instructional paperback you wrote about flying. It was literally a pocket-sized book about 4” x 3” with hundreds of charts and illustrations covering dozens of topics of importance to pilots. Things like light signals from the tower, runway markings, radio procedures, compass deviation, navigation, instrument flying, etc. I loved that little book, read it numerous times and even kept it in my flightbag. Have yo ever updated it?

      Mac MCCLELLAN says:

      Hi Bill,
      Thanks for the kind words. But I’m afraid the days of print for guides and data are over. It’s so much easier to keep electronic information current that printing it doesn’t make the same sense it did those many years ago.
      I was lucky to be part of the good old days of aviation magazines at Flying. Richard Collins was even luckier than I was in his timing. But long ago we both agreed our careers and the aviation print business came to an end at about the same time. That’s why Richard was enthusiastic about Air Facts Journal. An online publication such as this has many advantages over print, but one that Richard and I appreciate most is the ability to get instant feedback from readers. So thanks again for your comments.
      Mac Mc


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