# How does a true airspeed indicator work?

How does a True Airspeed Indicator work?
I am aware of how the air speed indicator works, but how is the density input being provided to this instrument?

• This answer contains some math and background on how TAS can be calculated/displayed that might be helpful. Feb 12, 2016 at 1:45

For a basic True Airspeed Indicator (TAS Meter) like the one shown below, you simply dial your pressure altitude into the top window using the knob (aligning it with the appropriate temperature mark at the bottom of that window), and then read your true airspeed in the other window at the bottom.

Your pressure altitude can be obtained by setting your altimeter to 29.92 and reading the indication, the air temperature can be obtained from an outside air temperature (OAT) gauge (or you can use one from the Winds Aloft forecast if your aircraft does not have an OAT gauge), and the combination of the two gives you your density altitude.
The meter itself is simply a mechanical slide rule, performing the same functions as you could do manually on an E6B flight computer.

The indicator above is showing a true airspeed of about 202 MPH, at a pressure altitude of 10,000 feet and a temperature of -15 degrees.

For a nice video tutorial check out this one from the University of Oklahoma.

• So TAS is in knots not M.P.H? Does the label "TRUE SPEED KNOTS" mean that white scale is in KNOTS? What a confusing indicator! I hope ones without MPH are available... Feb 12, 2016 at 8:40
• @Andrius That was a typo in my answer, but it depends on the instrument: in this illustration TAS is in MPH (the outer scale is MPH, and the true airspeed arc is on the outer scale). There are also indicators that only display knots, or that have knots on the outer scale & MPH on the inner scale. (I've never seen one, but I suppose its possible to have a design with both scales on the TAS arc as well, the correspondence between the two is linear...) Feb 13, 2016 at 17:59
• @voretaq7, the picture has a slight problem that it does not indicate which are the separately moving components well. The white windows clearly are, but apparently the inner scale should be as well and there is absolutely no indication of that in the image. Feb 13, 2016 at 22:48
• @JanHudec The inner scale does not move in the unit illustrated, it's fixed and will always display indicated airspeed in knots. I've replaced the image with a better one that makes this easier to see (also from Wikimedia Commons - the generic "airspeed indicator" ironically had a better image than the "true airspeed indicator") Feb 13, 2016 at 23:47
• @voretaq7, ok, now I see. The labels are rather confusing though. Because the inner scale is indicated speed in knots, but the label closest to it says "true speed knots". The main ,outer, scale is, somewhat unexpectedly, in mph. And only the white scale is actually true airspeed (in mph again). Feb 14, 2016 at 18:05