View Full Version : What is True Ground Speed?

11th May 2004, 13:17

I`m little confused;
I know TAS, GS, IAS,CAS,EAS etc. but I dont`t have a clue about the "True Ground Speed"!

What is it, I think it's quite simple, can somebody help me?

Thank you guys


11th May 2004, 13:29
Never heard of it???

Maybe its since you are flying above the earths surface (ground) you are flying a bigger curve than if you were on the ground for the same angular displacement from the earth's centre, so for the same track miles on the ground you fly a longer distance at altitude...

Therefore your speed to cover that distance in the same time must be greater than if you had to fly that angle at the earth's surface.

Managed to confuse myself too...

Notso Fantastic
11th May 2004, 14:04
If you fly around the world at 35,000' (or 6nms altitude), how much further do you travel than doing so on the surface? On the back of a virtual fag packet, difference in circumference is 2xPie (R1-R2), ie 2 x Pie x 6=38 miles. In terms of circumference of 25,000 miles, about 0.15% difference.

I've not heard of 'True Ground Speed'. Did he confuse it with 'True Airspeed'?

11th May 2004, 21:32
Only a guess but;

True ground speed would be the actual speed over the ground.

Indicated ground speed would be the ground speed indicated by something like an INS system which could be different from true (actual) groundspeed towards the end of a long flight but not by very much.

Other than that, the person could be miximg up TAS and GS or TAS and IAS.



12th May 2004, 04:00
Bit like

DEAD Reckoning or

DED Reckoning.

What is it these days?

Then there is g or G for acceleration and cg or CG for centre of gravity

How did they ever become G or CG?

White Knight
12th May 2004, 04:17
Oi you - who's taken my moniker???

The original "White Knight":cool: :cool:

Capt Fathom
12th May 2004, 06:23
GS = The GS you had on the last sector, as told to everyone in the bar. This speed increases proportionally with alcohol consumed.

True GS = Actual GS achieved.

12th May 2004, 10:04
Thank you for your replies;

I think I do not confuse the difference between TAS, GS! I just give you a quote of OAT-Notes:

Distance to accellerate/General Principles Take Off:

"From Netwon's laws of motion, the distance required for a body to reach a speed `V` from rest with a constant acceleration 'a' is : Vē/2a

This speed is True Ground Speed, wheras the lift off speed is an IAS."


John Farley
12th May 2004, 17:16
Perhaps the author could have called it Actual Ground Speed - which it seems to me is what he actually (sorry) had in mind??

12th May 2004, 18:29

I'm looking in the FAR AIM 2004, every Jeppeson manual and handbook, and my own notes looking for true ground speed...I'm just not finding anything about it.

There's indicated airspeed, calibrated airspeed, equivalent airspeed, true airspeed, ground speed, all the bloody V-speeds...stall, manoeuvre, NE, NO...uh...I'm running out of places to look.

Is it a term that is used out of the U.S. with which I am not familiar?

I'll keep looking.