1. Junior Member
Join Date
Jul 2008
Posts
81

## TAS to IAS

I have a print out from a previous thread showing how to calculate TAS/GS from the IAS, but is there a formula for calculating IAS from the TAS/GS?
When I do a flight plan it shows the GS (kts) estimate and I would like to figure out what the IAS is so I can set it prior to taking off.
Thanks,

____________
Bob

2. thinman,

Post the forumula your software is using and I should be able to come up with the reverse algebra that will answer your question.

3. Junior Member
Join Date
Jul 2008
Posts
81
Hi Mike,

Not using software. The printout I have is an answer I received from a prior post concerning IAS vs GS. The formula is as follows.

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS

The example was:

At 4,000ft 140kts IAS = ((140x0.02) x 4) + 140 = 151kts TAS

By the way, this was supplied by Alastair back around 22 Jul 08.

____________
Bob

4. thinman,

I'm going to show the derivation of what you want so that other people will kind of understand what to do when similar situations arise in the future ...

xxxxxxxxxxxxxxxxxxxx

This is an algebra problem. In essence you have asked that we solve your equation for IAS as a function of TAS. Let's do it step by step as follows ...

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS <------- your original equation

TAS = IAS + (IAS x 0.02 x Altitude) / 1000 <------- rearranging and simplifying

1000 x TAS = (1000 x IAS) + (IAS x 0.02 x Altitude) <----- multiply both sides of the equation by 1,000

1000 x TAS = (1000 x IAS) + 0.02 x IAS x Altitude <------ remove parentheses because no longer needed

Oops, and now we see a problem ...

Unless I'm mistaken this equation can only be solved for IAS iteratively. That is, plug in a series of guesses for IAS until the resulting value of computed TAS is very close to the target value of TAS. Such an iterative solution can be set up in almost any spreadsheeting system. It can also be done by hand ...

Trouble is, whether by spreadsheet or by hand, this now becomes a problem in iterative numerical analysis and while I certainly could create the necessary algorithm given enough time (adaptation of Newton-Raphson), I instead used Google to search for

"ias from tas"

This resulted in the following link into a thread in an Avsim forum ... http://forums.avsim.net/dcboard.php?...d=36083&page=6 and one of the posts in this thread says "Found a formula to calculate IAS by myself on this site: http://williams.best.vwh.net/avform.htm#Intro." (I take "by myself" to mean "by hand".)

If you search within that second link for "IAS" you will come to a discussion about how to do it. Frankly, I haven't read the discussion. If you can't figure it out, let me know and over a several-day period I'll design an iterative hand solution that will do the job. If you're looking for a programmatic solution you could then use the hand solution as a guide to writing the code or spreadsheet formula.

But I'm intensely curious, thinman. Why are we doing this? Are you doing some kind of panel programming?

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I wish I had better news,
But I'll develop the by-hand solution myself if you really really really need me to,
But it would take me several days because this would be both difficult and low priority,
Last edited by xxmikexx; 08-27-2008 at 06:06 AM.

5. Originally Posted by thinman
I have a print out from a previous thread showing how to calculate TAS/GS from the IAS, but is there a formula for calculating IAS from the TAS/GS?
When I do a flight plan it shows the GS (kts) estimate and I would like to figure out what the IAS is so I can set it prior to taking off.
Thanks,

____________
Bob
Keep in mind that the 2% per 1,000 feet to convert IAS to TAS is just a rough guesstimate when conditions are standard, that is, 59º F, 0% humidity, 29.92" Hg at sea level, and the lapse rate (temperature drop) is 3.5º F/1,000 feet of altitude change.

Any variation from those conditions throws off your calculations, which is why pilots use a flight computer (E6B, a circular slide rule or its electronic equivalent) to do that calculation. So with that rule of thumb you could be off by as much as 10-15%, maybe more under some conditions, though you may be very close, again depending on the existing conditions.

I have trouble visualizing a reason to do that conversion in reverse, unless you have the sim set to show TAS on your indicator, in which case you're losing the benefits of the airspeed indicator, which is to show stall speed, climb speed, never exceed speed, etc. corrected for the change in air density. We HAVE to use IAS in the real world, and only use TAS to figure times for navigation and fuel burn -- it's useless otherwise. Actually, we care more about ground speed than we do TAS, since GS is what actually affects our navigation and times. TAS is just an intermediate value used to get ground speed.

Oh, yes -- you keep saying TAS/GS -- they're only the same if you have NO wind whatsoever at your altitude. And that 2% rule gives you only a rough TAS. You must then take wind at your altitude into account to figure ground speed.
Last edited by lnuss; 08-26-2008 at 10:24 PM.

6. Originally Posted by thinman
Hi Mike,

Not using software. The printout I have is an answer I received from a prior post concerning IAS vs GS. The formula is as follows.

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS

The example was:

At 4,000ft 140kts IAS = ((140x0.02) x 4) + 140 = 151kts TAS

By the way, this was supplied by Alastair back around 22 Jul 08.

____________
Bob
I dont think that's right,
TAS is also compensated for tempature as well, and I don't see a
tempature varible in the equation......
Also keep in mind you need to know wind direction at the altitude your flying at...
IAS/TAS. If you have a side wind, you may have to use the pathergian therom and
vector calculations.
-Jonathan

7. bstolle Guest
Jonathan,

It might be a good idea to check the other responses before posting.
Especially the one right above yours from Inuss
Furthermore in aviation there is no "side wind"....

8. Originally Posted by bstolle
Jonathan,

It might be a good idea to check the other responses before posting.
Especially the one right above yours from Inuss
Furthermore in aviation there is no "side wind"....
I beg to differ. Not only is there a "side wind", there are also "butt winds" and "jib winds" respectively.

9. Are we forgetting CAS -- calibrated air speed? Do we care?
Last edited by xxmikexx; 08-27-2008 at 06:07 AM.

10. bstolle Guest
Tig, sorry but my english isn't good enough to get these.
But I assume you can experience buttwinds only if you are flying a boeing peashooter

Mike,

No we are not forgetting CAS, but 'we' don't care is there isn't any daily use for it....

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