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.