Understanding different types of speeds

There are 4 different types of speed we are going to be covering in this tutorial.

  • Indicated airspeed (IAS)
  • True airspeed (TAS)
  • Ground speed (GS)
  • Mach speed (M)

Indicated airspeed (IAS)

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Indicated airspeed is what we see inside the plane. This is how we judge when to rotate, when we are approaching a stall. Or any other critical flight dynamic. There is also such thing as Calibrated airspeed (CAS) which is IAS corrected for Installation errors. We don’t have that in IF, but during some flight planning it may come up.

So what exactly is indicated airspeed? It’s simply as it stated it is the dynamic pressure ”airspeed” that can be read while the aircraft is moving thru the air. So it is an exact measure of the Air Force the Aircraft is moving thru.

When we are with a Radar facility (Approach, Departure,Center) and they ask us to maintain a specific speed. They are telling us to maintain that specific IAS.


True airspeed (TAS)

True airspeed is your indicated airspeed IAS corrected for pressure altitude, and non standard temperature.
What is pressure altitude? Pressure Altitude is the altitude read on the altimeter when the Baro Barometric pressure is set to standard which is 29.92. That is also the pressure set when climbing above 17,999 MSL. That is why they are called Flight levels “FL” since it’s a standard pressure and not the actual pressure to give you a MSL Reading.
What is standard temperature?
Standard temperature is 15° c. So if we are at sea level and the Baro is 29.92, and the temperature is 15° c. Then our indicated airspeed and our True airspeed are going to be exactly the same. If any of those variables change however we get a drastically different result.

Example
We are cruising along at FL360, and the temperature is -50° c with an IAS of 280 kts. This is a pretty typical situation.

Our true airspeed would then be 520 KTAS.


Ground speed

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Ground speed is true airspeed corrected for wind. So if we had a no wind situation our ground speed, and True airspeed would equal each other. Since that is rarely the case we need to calculate our wind factor.

So if we have a 50 kt direct headwind we would have a 470 kt Ground speed. If we had a 50 kt direct tailwind we would have a 580 kt Ground speed.

As we look at other users in flight, or man an ATC facility. We see that users Ground speed. The reason for this is because a Radar will pick up the aircrafts movement across the ground. There is no equipment that relays IAS to the ground other then communication with the pilot.


Mach speed

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Mach speed is the speed in which sound travels through the air. Mach 1 is the speed of sound. A lot of us think the speed of sound is in reference to something over the ground. When actually it’s another measurement of dynamic air pressure. As such it will change with altitude, and temperature just like TAS.


That concludes the speed tutorial. As always if you have any question ask in the comments below. ✌️

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Very informational, glad you made this! Thank You!

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@MrAlaska See this link too:

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Once again an absolutly needed Topic. Thanks @Brandon_Sandstrom

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Thanks for the great explanation @Brandon_Sandstrom :)

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Thank you for the info :)

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I like these tutorials, would be great for new flyers! Keep doing them, even experience flyers (me) can learn something new!

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ATC sees only an AC’s GS and instructs pilot to maintain ‘x’ kts; you mentioned that Pilots should maintain IAS and not GS?
Have I been doing it wrong since I get to ‘x’ kts GS as I imagined it is only GS that ATC can see?

I also thought that your ground speed is always higher than your airspeed and the higher you go, the bigger the difference between the two. I believe this is because of the air density/pressure. If you have a higher air density, more air will be moving over the plane, giving you a higher airspeed. If you have a lower air density, in order to keep your airspeed the same you would increase your ground speed to keep the same amount of air flowing over the wings. This happens naturally to aircraft because the thinner the air, the less resistance there is. At 1000ft MSL for example your airspeed could be 250kts and GS 252kts, but at FL370 your airspeed could be 270kts but your ground speed might be more like 450kts. I hope I made sense. 😅

I could be wrong though…

@Brandon_Sandstrom

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In short yes you are doing it wrong. ATC can only see GS but they give instructions based on IAS.

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Yes you are right, you just need to factor wind into that as well.

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Glad you made this!
Thank you so much!

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Bookmarked! Thanks :)

Is there an easy to remember/basic/dumbed down formula for calculating KTAS?

No there isn’t a easy way to calculate true airspeed unfortunately.

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Great post. So just to be clear, if ATC requests that I maintain a certain speed, I would adjust my speed based on the IAS and not the GS?

Yes, you’d only adjust the IAS.

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Really nice explanations. Thanks!

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Thanks brotha.

You would adjust your autopilot pilot speed which would be IAS.

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