Question about Airspeed vs Groundspeed

a reply to: Zaphod58

But I saw the term "computed airspeed".

What do you think is meant by that?

a reply to: RogueWave

Computed airspeed is used by the computer. It takes indicated airspeed and adjusts for pressure errors. The computer uses that adjusted speed to find calibrated airspeed, and other required adjustments.

a reply to: Zaphod58

So does this result in a True Airspeed reading or does this involve another step?

a reply to: RogueWave

It's been a lot of years since I've dealt with airspeed, but I think computed airspeed is used for calibrated airspeed, not true.

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.

Is the airspeed that is recorded by a black box, true airspeed?

a reply to: Zaphod58

It's been a lot of years since I've dealt with airspeed, but I think computed airspeed is used for calibrated airspeed, not true.

Ok, so can you estimate the difference between calibrated airspeed and true airspeed, at cruising altitude?

a reply to: RogueWave

Ok I found this,

Modern aircraft instrumentation use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS. A very simple rule of thumb is to add 2% to the calibrated airspeed for every 1000 ft of altitude.

www.skybrary.aero...

a reply to: RogueWave

There are too many variables without a manual to show the error factor of the instruments. It will almost always be lower than true though.

a reply to: Zaphod58

I thought that the error factor of the intruments had already been taken into account with calibrated air speed.

a reply to: RogueWave

It has been, but without knowing the error factor you can't calculate the difference between calibrated and true without actually being up at altitude and using the computer to do it.

a reply to: Zaphod58


That is not true, if you have calibrated airspeed you can calculate true airspeed with a formula, or like I posted already, add 2% per 1000 ft of altitude.

a reply to: RogueWave

What numbers are you looking at? Altitude and either true or calibrated.

a reply to: Zaphod58

I asked this,

Ok, so can you estimate the difference between calibrated airspeed and true airspeed, at cruising altitude?

You answered with this,

There are too many variables without a manual to show the error factor of the instruments. It will almost always be lower than true though.

But the answer to my question has nothing to do with instrument error. I was asking what the difference is between calibrated and true airspeed.

Instrument error has already been taken into account to get the calibrated airspeed.

I already answered my own question,

Modern aircraft instrumentation use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS. A very simple rule of thumb is to add 2% to the calibrated airspeed for every 1000 ft of altitude.

a reply to: RogueWave

There is no set difference between CAS and true. That's a rule of thumb that will get you close but not exact. It's between 1.5 and 2, although if you want to be exact the formula is a lot more complicated.

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?

Indicated airspeed, IAS, is what the instrument shows.

CAS Calibrated airspeed is that value adjusted for various tiny installation effects or errors, and is usually something less than 2 or 3 knots.

True airspeed TAS is IAS adjusted for altitude and temperature, OAT, outside air temperature.

The groundspeed is the final result of the TAS adjusted for wind velocity and direction, obviously factoring in the course of the aircraft.

The higher one is flying, the greater the gap between IAS and TAS. Up in the flight levels, 25000 feet or so, the difference is 100 knots or more between IAS and TAS. At sea level, in theory at least, and depending upon temperature and pressure, IAS and TAS will be about the same. Even at 7 or 8000 feet in a smaller aircraft, there is usually a 15 knot difference, with TAS always being higher than IAS

a reply to: Salander

So is it possible that there is a 200 knot difference between calibrated air speed and true airspeed, at cruising altitude?

originally posted by: Salander

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?

Indicated airspeed, IAS, is what the instrument shows.

CAS Calibrated airspeed is that value adjusted for various tiny installation effects or errors, and is usually something less than 2 or 3 knots.

True airspeed TAS is IAS adjusted for altitude and temperature, OAT, outside air temperature.

The groundspeed is the final result of the TAS adjusted for wind velocity and direction, obviously factoring in the course of the aircraft.

The higher one is flying, the greater the gap between IAS and TAS. Up in the flight levels, 25000 feet or so, the difference is 100 knots or more between IAS and TAS. At sea level, in theory at least, and depending upon temperature and pressure, IAS and TAS will be about the same. Even at 7 or 8000 feet in a smaller aircraft, there is usually a 15 knot difference, with TAS always being higher than IAS

And pressure. Everything is calibrated at standard temperature and pressure. 29.92" of mercury at sea level and 59*f if I recall and you adjust from there. So if it is standard temp and pressure at sea level the 2% per 1k of altitude formula is close. If the pressure is lower than standard or the temp higher than standard then your calculation will be off where your supposed indicated is higher than your actual.

it's possible to have zero ground speed or negative ground speed. i've come close with a 172 in slow flight.

a reply to: bigx001

I've seen a couple people that did manage to get negative ground speed. They were rather proud of that.

a reply to: RogueWave

Well of course many things are possible, but that's beside the point.

CAS is more of a theoretical value than a practical and useful number. IAS or TAS is a practical value that one uses all the time in flight. The difference between CAS and IAS is commonly very small, less than 2 knots as I recall.

To answer your question, it depends upon one's 'cruising altitude'. 200 knot difference seems extreme to me, and might be seen by the SR71 or other aircraft flying at very high altitudes.

At operating altitudes for most jet aircraft in the Flight Levels, something like 100-150 knot difference would be more common IMO.