Topic Author
Posts: 2968
Joined: Mon Mar 07, 2005 4:52 am

IAS, GS, And TAS Relation

Sat Jun 16, 2007 8:26 am

Hello, all.

I've been playing x-plane a lot, and I have an odd question.

Right now, for example, I am flying LGA-SXM. My Indicated Air Speed is 297 knots. There is a 21 knot wind at 170, and yet my ground speed is 461 knots. My TAS, which I don't know what it stands for, is 482 knots.

How are GS and TAS related to IAS and how are they calculated? I always thought GS was the sum (or, in this case, with a head-wind, the difference) of IAS and wind speed, but apparently that's wrong.

On a related note, if my IAS is 297 at 33000 feet, why is my Mach number .82, where as at sea level at the same IAS, the Mach number would be much less? I understand it has to do with air pressure and temperature and the like, but I'm confused as to WHY all that makes a difference.


TIS Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
Posts: 813
Joined: Sun Oct 10, 2004 4:16 pm

RE: IAS, GS, And TAS Relation

Sat Jun 16, 2007 9:38 am

True airspeed (TAS) is your actual speed relative to the air.

Ground speed is TAS plus tailwind, so in your example you seem to be flying 170 directly into that 21 knot wind (or that 21 knots is the headwind component, not the total wind).

The problem is, it isn't that easy to actually calculate the TAS by measuring atmospheric qualities on the airplane, so they display Indicated Airspeed (IAS), which is a relationship between the total pressure and static pressure. You can read about the relationship between different types of airspeeds in introductory aeronautics books by people like Anderson or Shevell, or even Wikipedia (although I haven't read that article so have no idea how accurate it is, but assume really gross errors would have been corrected by now), or if you prefer a lot of equations without much explanation, maybe this site will be interesting for you.

Quoting ANITIX87 (Thread starter):
On a related note, if my IAS is 297 at 33000 feet, why is my Mach number .82, where as at sea level at the same IAS, the Mach number would be much less?

Two reasons:

1. At sea level for an IAS of 297 you would have a much lower TAS.
2. At sea level the speed of sound is much higher, so even at the same TAS you would have a lower Mach number -- the speed of sound is proportional to the square root of temperature, and temperature drops linearly from sea level to around 36000 feet.
7 hours aint long-haul

Who is online

Users browsing this forum: TZTriStar500 and 2 guests

Popular Searches On

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos