How Fast Are You Flying?

Photo by Jim Koepnick

Not long ago I was behind a Piper Saratoga at the same altitude—6,000 feet– and flying the same course. The controller asked each of us for our airspeed. The Saratoga pilot reported 170 knots which is what was showing on the airspeed indicator in my Baron. Actually it was 172 or 173 knots, but I rounded down to 170.

Our responses were clearly making no sense to the controller who asked again, and got the same answer from both of us. A Saratoga is a fine piston airplane, but is substantially slower than the Baron. The controller was watching me gain on the airplane ahead and trying to figure out how to keep us apart. He could assign me a slower airspeed to maintain separation, but he needed to know each of our airspeeds in order to issue an effective speed restriction to me.

I was reporting indicated airspeed to the controller which is what controllers want when they ask for your airspeed. The Saratoga pilot was possibly reporting his groundspeed, or perhaps an optimistic estimate of true airspeed, or maybe even the cruise speed he read in a brochure for the airplane. But we were not both indicating the same airspeed.

Airspeed comes in several forms including indicated, true and calibrated. In real life flying we care about indicated airspeed because that’s what makes the airplane fly, and true airspeed, because that’s how fast we move through the air. And controllers only want us to report indicated airspeed because that is the value that keeps airplanes apart.

Indicated airspeed is the air pressure recovered by the pitot tube. This air pressure—called Q—is the air flow that the wing and tail of the airplane experience, and it is that air pressure that provides lift, and controllability. Calibrated airspeed is indicated airspeed corrected for errors in the pitot-static system due to location of the ports on the airframe. In most airplanes the calibrated and indicated airspeeds are quite close together so as pilots we don’t need to be concerned with calibrated airspeed except, perhaps, when using the alternate static system where errors can be significant.

Indicated airspeed only equals the true airspeed when the conditions are standard day temperature with sea level air pressure. Any change from ISA (international standard atmosphere) up or down increases or decreases the true airspeed compared to the indicated. At high altitude the difference between indicated airspeed and true airspeed can be huge, as much as 200 knots or even more. In the non-oxygen altitudes for piston airplanes true airspeed is typically 10, 20, 30 or so knots faster than indicated airspeed. The warmer the air temperature the greater the spread between indicated and true airspeed at a given altitude.

The reason we reference indicated airspeed when maneuvering, or taking off or landing, is because it is a consistent measure of the airflow available to create lift. For example, if you are taking off or landing at a high elevation airport on a hot day the airplane needs to fly at the same indicated airspeed as when it is at lower elevation airports. But at the hot and high airport your true airspeed—and thus your groundspeed on the runway—will be much faster than at a lower airport so you need more runway.

The reason controllers only want us to report our indicated airspeed when they ask for our airspeed is because they only care about relative airspeed between the airplanes being separated. Our true airspeed doesn’t matter to a controller because airplanes he is separating are on the same altitude so the difference between indicated and true will be the same. If both pilots maintain the same indicated airspeed the gap between them will not close.

So, indicated airspeed is what we fly by, and what we tell controllers. True airspeed is what we need to know to flight plan effectively. And that brochure speed is what we need to have at hand for those post flight discussions over a beer.

 

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8 Responses to How Fast Are You Flying?

  1. Rob P says:

    I think since the advent of the GPS, that the only numbers a VFR pilot (and maybe an IFR pilot too, for that matter) really care about are the Indicated airspeed (which is all the airplane cares about) and the Ground speed (which is all the pilot really cares about) or velocity made good (VMG) along your track. True airspeed is good for bragging rights, but otherwise isn’t very useful.

  2. Charlie Gibbs says:

    Even with GPS, there’s still a use for true airspeed: by comparing it with the ground speed given by the GPS (and eyeballing your drift), you can get an estimate of winds aloft.

  3. 30+ ATC says:

    Another thing we controllers are concerned with is radius of turn. As indicated airspeed increases, so does radius of turn. That greatly influences a controller’s ability to anticipate separation and to properly time the issuance of turns to intercept a course (final) when vectoring in the terminal environment. Radius of turn is a major reason for pilots to comply with the 200 knot speed limit of 91.117 (b) below the lateral limits of (and within VFR corridors through) Class B airspace, and 91.117 (c) below 2,500 feet AGL within 4nm of the primary airport of a Class C or Class D airspace area. (It’s a good rule to follow even outside those regulatory limits when being vectored for final.)

    The 200 knot speed limit also applies to and is particularly important for departing aircraft in congested areas such as the San Francisco Bay area where departures are routinely turned immediately after departure in order to avoid arrival traffic flows to adjacent airports. The charted departure procedure designs are predicated on aircraft speed; non-compliance by going “too fast” causes aircraft to turn wide and conflict with those other nearby traffic flows.

    As a side note, flying any procedure that requires more than a ninety-degree turn at a high airspeed will increase the distance travelled and the time required to complete the turn, thereby increasing the flight time and fuel burn to destination as compared to the same procedure flown at a lower airspeed. The effect is greatest for procedures that require course reversal, or in the case of procedures like the LOUPE departure from SJC, two course reversals before proceeding toward the destination. Although it may be of little consequence for piston singles, for jets, especially when the airspeed is allowed to exceed the speed at which the flight director limits bank angle, the time and fuel spent flying rapidly “away” from the desired course and the time and fuel expended “backtracking” that same distance again after the turn can be significant. This is one case where slowing down will get you to your destination sooner and actually increase your fuel mileage.

    And, yes, we controllers do anticipate the effects of wind and altitude on the groundspeed we see vs. the indicated airspeed you report.

    • Scott Stevens says:

      30+
      Thank you for the look into the ATC side of the picture. I often wonder why I am vectored in a particular sequence on departure. Sometimes the routing seems counterproductive – your insight is appreciated.

      Scott S
      P46T

  4. DEL says:

    How wrong could the IAS be? As wrong as wrongness!

    I own and fly an ICP Savannah Classic LSA. This airplane has full-span leading edge slats and its pitot tube opening is some 5 inches upstream of the slat leading edge, midspan of the left wing. The airspeed indicator is well calibrated, as far as I know.

    Occasionally, I practice stall recovery procedure, just to keep in shape. That’s not really a stall recovery, because under no circumstance would that airplane stall, according to the normal definition of the term. (The nose may point into the sky, the power may be on or idle, the IAS may be as small as you want; but the nose would not drop; instead, the whole thing just sinks — slowly, nicely and controllably. And it resumes normal flight with just a little push on the stick and with an insignificant loss of altitude.)

    What’s relevant to the subject matter concerning this bahavior is that throughout the high AOA phase the IAS indicator shows zero — not a small number, not something close to zero, but an absolute zero — zilch, nada, the round figure.

    Well, it can’t be right, can it? But if you consider the expected airflow pattern just upstream of a slat at high AOA, it makes sense — the streamlines there are almost perpendicular to the wing chord, hence also to the pitot tube. Thus the pitot tube experiences not the stagnation pressure but the local static pressure. The airspeed indicator, supposed to measure the difference between the stagnation and static pressures, measures instead the difference between the static pressure and the static pressure — zero!

    The point I want to make is this: Airspeed indication should always be taken with a grain of salt — respect it but suspect it. Under particular circumstances it may be as wrong as wrong may be.

  5. Timothy Meyer says:

    Pitot Static Airspeed Indicator. As a recently new private pilot, I remember my instructor mentioning a few things about airspeed and how it is derived. Under normal flight conditions, the Pitot tube is pointing almost directly into the direction of the relative wind of the airplane. Under slow flight or forward slips, consider the direction of flight or travel of the airplane vs. the direction of the relative wind. Hence my instructor impressed me that when flying a forward slip, with full rudder and ailerons cross controlled, do not pay any attention to the airspeed indicator, just keep the nose pointed down almost to a point of exaggeration to prevent going into a stall / spin accident. Also depending on the location of the static tube on the airplane (left side in a C172) the airspeed indicator will give very different airspeed indications if forward slipping to the left vs forward slipping to the right relative to the pressure on the static port during the slip.

    Tim

  6. Roger M. Derby says:

    Please! What we fly is Calibrated Airspeed. Only that corresponds to the dynamic pressure on the airframe/wings/control surfaces.
    If you’re lucky, the IAS will be close enough, but it is NOT the same, and there is no way ATC can correct IAS to CAS.
    I spent ten years with General Dynamics/Lockheed/Lockheed Martin doing Software Quality Assurance on air data computers, and have about 1400 hours of pushing my C172 around in both IMC & VMC.

  7. Jordan says:

    “Indicated airspeed is the air pressure recovered by the pitot tube. This air pressure—called Q—is the air flow that the wing and tail of the airplane experience, and it is that air pressure that provides lift, and controllability.”

    Mac… I think it needs to be clarified that the airflow that the wing and tail experience is actually ‘calibrated airspeed’. To be even more correct it’s ‘equivalent airspeed’ but I understand that you’re trying to write to a mostly GA audience and EAS doesn’t apply much in the GA crowd.

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