If the airspeed indicator has inputs regarding air density (static port) and temperature. Why isn't Indicated airspeed from the indicator simply the same as True airspeed


Well, indicated airspeed is the most important speed a pilot needs to know, so it makes sense to have that in the most obvious position.

IAS can be thought of as the pressure that the wings are experiencing - therefore all of the stall, manoeuvring, flap extension, and landing speeds etc are based off indicated airspeed.

True airspeed is really only important for flight planning and navigation. Given that more and more aircraft have GPS nowadays to give the groundspeed, the need for the pilot to know the TAS in flight is further reduced.

  • $\begingroup$ Thank you. This is what i thought at first, came to the conclusion that TAS must not that important for flying it self but more for navigation. But then i read more and as the answer from Martin here below suggests, the IAS will always show the same speed at any height. But we know that for example stall speed and other flight performance is heavily affected by the thin air at for example 20,000 feet. Then you might stall at a speed you deemed safe when at a lower altitude. $\endgroup$ – John Dec 3 '17 at 16:09
  • $\begingroup$ @John Your example is incorrect. Let's say a wing stalls at 50k IAS. As long as your IAS remains above 50, you are not going to stall regardless of altitude, temperature, air density, etc. Let's assume a perfect day of 0 wind and a perfect standard lapse rate with standard temperature from ground level to FL200. If you take off at sea level and climb to FL200 with the same IAS the entire way, you're fine. Your ground speed will be increasing the entire time but your wing doesn't care about ground speed, just IAS. $\endgroup$ – rmaddy Dec 3 '17 at 16:50
  • $\begingroup$ Thank you rmaddy. Can you explain to me why the IAS would show the same indications at FL20 as at 2,000 feet? I was taught that the airspeed indicator measured how many air molecules were thrusting against the pitot and at very high altitudes that would be a lot less than near the ground. Hence, the indicator would show a lot less IAS as you go higher (not unless you keep up with it by going faster). $\endgroup$ – John Dec 3 '17 at 17:23
  • $\begingroup$ @John (please put a @ before a person's username when replying.) Yes, the air at FL200 is less dense than the air at 2000'. You see the same IAS because, relative to the ground, the plane is moving faster so the airspeed indicator sees the same IAS. You seem to be falling into the common confusion of air speed and ground speed. Don't get them mixed up. $\endgroup$ – rmaddy Dec 3 '17 at 17:23
  • $\begingroup$ @rmaddy if a plane has an for example an 120IAS and is climbing at a fixed GS of 500kts (let's say it keeps that GS constant), at FL200 the IAS would be lower but still at the same GS ? $\endgroup$ – John Dec 3 '17 at 18:20

In its basic form, the airspeed indicator measures pressure differential: between the dynamic pressure (from the pitot tube) and the static pressure (from the pressure port). As it only sees differential, it couldn't possibly tell "true" airspeed. That would require, as you say, factoring air density, which is usually not available to the pneumatic instrument.

From a practical point of view, the aircraft flies in terms of indicated air speed: in the sense that the indicated stall speed will be the same at all heights, where a speedometer showing true airspeed will require the pilot to make a calculation to know what the stall speed (or any other speed related to the movement of the airframe through the air) is. There is a single exception to this, as far as I know, which is the VNE: since the maximum speed is determined not by direct flying characteristics of the wing but by stress and vibration, it depends on true airspeed and not on the indicated one; so, at height, the pilot needs to be aware that the VNE shown in the airspeed indicator is not the real one. At the extreme, we have the "coffin corner", the height where the (true) stall speed coincides with the VNE, and it becomes impossible to fly the aircraft without either stalling it or destroying it.

  • $\begingroup$ TAS is as i understand, simply IAS but corrected for air density. The static port measures air pressure, but what is the difference between air density and air pressure? Are they not both just a measurement of "thickness of air"? $\endgroup$ – John Dec 3 '17 at 16:17
  • $\begingroup$ You could have some kind of barometer in the aircraft (in fact, the altimeter is one), and use it for your purpose; but it is not very useful information for the pilot, as I say in my answer. In the usual setup of the speedometer, the static port is used to take the difference with the dynamic pressure. Note that with altitude, both the static and the dynamic pressures change. $\endgroup$ – Martin Argerami Dec 3 '17 at 16:27
  • 1
    $\begingroup$ @MartinArgerami Perhaps the better answer is "it depends". I'm a private pilot and an ultralight pilot. Vne is indicated for the aircraft I've flown. Doing a google search on "is vne indicated or true" supports this but there do seem to be cases where Vne may be tied to true airspeed. So the best answer is "Check the POH of the aircraft in question". $\endgroup$ – rmaddy Dec 3 '17 at 19:28
  • 1
    $\begingroup$ @John Pressure and density are interdependent, but are two seperate things. One is a property of a substance and the other is a form of energy. Imagine a sealed, rigid container with air inside. There is a given mass of air molecules in a given volume. Density is mass/volume. If you heat the air, the mass and volume will not change, so density stays the same. But since you added energy, the pressure will increase. Pressure is caused by molecules banging into each other and the more heat you add, the faster they move and the more they bang into each other. $\endgroup$ – TomMcW Dec 4 '17 at 23:33
  • 1
    $\begingroup$ Say the container is now flexible, like a balloon. When you heat the air, pressure will increase, but since the container can now expand, the density will decrease. So density, pressure, temperature and the makeup of the air (humidity, etc) are all separate things that are interdependent on each other. The pressure of air is created by the weight of the column of air above it. If you increase the temperature of a parcel of air the molecules will spread out lowering density at the same pressure. $\endgroup$ – TomMcW Dec 4 '17 at 23:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.