KIAS, Ground speed and MACH

What altitude is the plane at? That matters.

The speed of sound varies with temperature. At altitude, the temperature is much lower and thus the speed of sound is lower also. At a typical cruise altitude of +FL350, the temp. is approximately -55 C. Mach .81 at that temp is about 464 Knot TAS. The difference between TAS and GS is wind component. IAS and GS have no relationship. The difference between IAS and TAS is the result of changes in air density at different altitudes. Approx. rule of thumb is to and 2% per thousand feet of altitude to IAS to approx. TAS.

CRJCapt wrote:

The speed of sound varies with temperature. At altitude, the temperature is much lower and thus the speed of sound is lower also. At a typical cruise altitude of +FL350, the temp. is approximately -55 C. Mach .81 at that temp is about 464 Knot TAS. The difference between TAS and GS is wind component. IAS and GS have no relationship. The difference between IAS and TAS is the result of changes in air density at different altitudes. Approx. rule of thumb is to and 2% per thousand feet of altitude to IAS to approx. TAS.

CRJCapt, very informative answer. You were actually right, the flight is at FL350, where, if I use your 2%/1000 feet approximation, I would get TAS = 465 knots, which is exactly MACH .812. Now TAS-GS = 98knots, is that the wind speed? Seems a bit too much to me ...

Thanks a lot!

BTW, check the picture out http://www.airliners.net/open.file/0943177/L/ It is very nice.

For FL350, 98 kts is a high wind but happens and sometimes is higher. The picture you provided makes sense also in the aspect that they are West bound, this normally will give you a headwind thus the lower GS. TAS is the the speed of the aircraft thru the air and GS is the speed of the aircraft over the ground. The only thing that causes a difference in these two values is wind. 🙂

Here are the forecast winds for KROA(winds aloft)
FT---3000---6000------9000----12000----18000---24000---30000----34000----39000
ROA 9900 9900+15 0407+09 3311+05 3031-07 2946-18 277233 278543 258952

Thats at FL340 270 degrees at 85 kts with a temp. of -43 C

Basically KIAS is the speed relative to the air, and GS is the speed relative to the ground.

If you are on the ground, and the air is traveling at a speed of 10 knots (headwind) your KIAS will be 10 knots, but your GS will be 0.

So yes, the wind is the difference between KIAS and GS

SeanGa wrote:

Basically KIAS is the speed relative to the air, and GS is the speed relative to the ground.
If you are on the ground, and the air is traveling at a speed of 10 knots (headwind) your KIAS will be 10 knots, but your GS will be 0.
So yes, the wind is the difference between KIAS and GS

Actually it's the difference between True Air Speed and Ground speed, not IAS. TAS is the speed relative to the air. In your example, IAS and TAS would be about the same but at altitude they would be very different.🙂

that I can really use....!!!! Thank you all contributors. I enjoyed the read. Now I can understand a lot more about the theory of flight. I had the basics but this was excellent...!!!!

🍻