Question about Airspeed vs Groundspeed

So planes have an airspeed indicator which indicates speed relative to the surrounding air mass.

This is then calibrated to True Airspeed(TAS), taking into account instrument error and pressure differences.

Then in order to get Groundspeed, wind speed is added or subtracted from TAS.


Please correct me if I am wrong.

So what is the average margin between indicated airspeed and True Airspeed?

originally posted by: RogueWave
So planes have an airspeed indicator which indicates speed relative to the surrounding air mass.

This is then calibrated to True Airspeed(TAS), taking into account instrument error and pressure differences.

Then in order to get Groundspeed, wind speed is added or subtracted from TAS.


Please correct me if I am wrong.

So what is the average margin between indicated airspeed and True Airspeed?

Not really informed on this myself, but I assume wind speed is probably the biggest factor

Edited.... Autocorrect.... It loves me so much it just wants my time....

a reply to: RogueWave

You would have to look at the owners manual for it is different for each aircraft at a given altitude and temperature due to inherent installation errors of the pitot system. Think Calibrated airspeed.

a reply to: 727Sky

Ok, but can you give an estimate? I assume such a margin would not exceed 50 km/h right?

And so it is correct that groundspeed = true airspeed minus/plus wind speed?

Btw, the plane of interest is a 777.

I imagine this is the type of question one could type into Google and have an answer within seconds.

a reply to: Iamnotadoctor

Please provide it then. You have one minute. Why speculate about the availability of such an answer if it takes only a few seconds to find it?


I made this thread to verify if my interpretation of google info was correct, and to get an answer to the specific question I asked, which I couldn't find on google.

a reply to: RogueWave

Just message " zaphod" he is the ATS resident expert on all things aviation, I bet he can answer this in great detail while asleep.

BTW he is also the aviation forum moderator

originally posted by: RogueWave
a reply to: 727Sky

Ok, but can you give an estimate? I assume such a margin would not exceed 50 km/h right?

And so it is correct that groundspeed = true airspeed minus/plus wind speed?

Btw, the plane of interest is a 777.

At 37,000+ feet I may be only indicating 200 knots or even 220 but my true airspeed can be 440 or 460 knots across the ground under no wind. Mach speed would be around .80 or .82 which is more relevant from an operators stand point. The flight management system automatically computes your ground speed an true airspeed along with buffet margins etc etc. with the available input data derived from the dual GPS receivers. I don't have any owner manuals any more but like someone else indicated you can probably find what you are looking for with some creative searching of the internet.

I think GPS has taken most of the hard work out of working out true position and speed in relation to the earth's surface.

Wasn't always like that though and aviators still to know how to do it the old manual way because tech can be fickle.

Ground speed is the horizontal speed of an aircraft relative to the ground. An aircraft heading vertically would have a ground speed of zero. Information displayed to passengers through the entertainment system often gives the aircraft ground speed rather than airspeed.

Ground speed can be determined by the vector sum of the aircraft's true airspeed and the current wind speed and direction; a headwind subtracts from the ground speed, while a tailwind adds to it. Winds at other angles to the heading will have components of either headwind or tailwind as well as a crosswind component.

An airspeed indicator indicates the aircraft's speed relative to the air mass. The air mass may be moving over the ground due to wind, and therefore some additional means to provide position over the ground is required. This might be through navigation using landmarks, radio aided position location, inertial navigation system, or GPS. When more advanced technology is unavailable, an E6B flight computer may be used to calculate groundspeed.

Ground speed is quite different from airspeed. When an aircraft is airborne the ground speed does not determine when the aircraft will stall, and it doesn't influence the aircraft performance such as rate of climb.

I am bored this morning. Hope this helps

www.aerospaceweb.org...

The relationship between ground speed, airspeed, and wind speed can be written in the following form:

If the wind moves in the same direction as the aircraft, the term Vwind will have a positive sign. If the wind blows in the opposite direction as the plane's motion, Vwind will be a negative number.

Effect of tailwind on ground speed
Effect of tailwind on ground speed

This important distinction between ground speed and airspeed can be better understood by observing the above example. Let's assume that the aircraft's cruise speed, or its speed relative to the local air, is 500 mph. However, the air itself is not motionless but moves at a speed of 100 mph in this example. Let us also assume that the plane is traveling in the same direction as the wind such that it experiences a tailwind of 100 mph. Since the wind and vehicle both move in the same direction, their velocities add together, and the ground speed becomes 600 mph.

Now let's look at the opposite problem, as illustrated below. In this example, the wind still blows at the same speed with respect to the ground, but its direction is reversed. Now the plane travels against the wind and experiences a headwind of 100 mph. The aircraft is still moving at the same cruise speed of 500 mph with respect to the air, but the difference in the direction of motion of the air means that the wind speed must be subtracted from the plane's airspeed in order to determine the ground speed. The ground speed now becomes only 400 mph.

Mod Note: IMPORTANT: Using Content From Other Websites on ATS – Please Review This Link.

a reply to: 727Sky

The flight management system automatically computes your ground speed an true airspeed along with buffet margins etc etc. with the available input data derived from the dual GPS receivers.

So the term "computed airspeed" is the same as "true airspeed"?

Ground speed is always fun. There was recently a 747 I think it was, that was heading East crossing the Atlantic. Going that direction, you have a tailwind normally, so according to their ground speed they were flying at about mach 1.12 or something like that. Their airspeed on the other hand was something like 0.85, nowhere near supersonic.

a reply to: 772STi

Thanks for your input.

Thank you all for your input.

a reply to: RogueWave

Zaphod will be by shortly to give you all the answers you require.

a reply to: Zaphod58


Yes, I just read that article.

Anyways thanks for posting because it can be used later on to prove a point.

a reply to: RogueWave
Your welcome. I work on jet engines for a living, but I'm am avid flight SIM enthusiast, I really just did some copy and pastes for you. If your able to download infinite flight on your phone/tablet it will help you learn real quick how it works.

Have a good day.

a reply to: RogueWave

So the term "computed airspeed" is the same as "true airspeed"?

Can anyone confirm this?

a reply to: RogueWave

It's actually calibrated airspeed, and no. Calibrated airspeed is indicated airspeed, corrected for errors. True airspeed is airspeed against the air mass.

flighttraining.aopa.org...

There is a pretty good breakdown there.

a reply to: 727Sky

At 37,000+ feet I may be only indicating 200 knots or even 220 but my true airspeed can be 440 or 460 knots across the ground under no wind.

Do you mean this difference is explained by backwind?