View Full Version : Subtract from IAS or TAS...

18th Jul 2001, 16:00
Tooling around in the sky recently, I was trying to calculate the wind. Happens sometimes...

Anyway, I usually get the a/c generally pointed into wind and took the Ground Speed from the IAS. The thought occurred to me though that perhaps I should be taking it away from the TAS... (not a huge issue at the levels I'm at, but that's not the point)
I've come up with a number of reasons to do each, so basically I haven't come to a rock solid answer (but I'm leaning heavily towards TAS)/

Comments/answers anyone?


18th Jul 2001, 17:56

if i understand your doubt well...

always GroundSpeed=Tas +/- longitudinal wind componet (ie HeadWind/TailWind)


19th Jul 2001, 02:46
Hi Turbofan

The wind neither knows nor cares about your Indicated Air Speed, the effect the wind has on your drift and groundspeed is dependent on your actual air speed through the air, your TRUE Air Speed.

IAS is greatly affected by air density, and to a lesser extent position error, and can vary by hundreds of knots from TAS in aircraft at high altitude.

As you imply, on a standard day, if you’re flying around in a light aircraft at 100kts IAS, at 1,000 ft AMSL, your TAS will be within 2kts of your IAS, hardly worth concerning yourself about.

However, consider a large jet cruising above you at FL400 at M0.85. Broadly speaking it will have a TAS of 485 kts and an IAS of 245 kts. Assume it is flying directly into a headwind of 125 kts, what do you think its groundspeed is?

Yes, 360 kts and not 120 kts. ;)