Ias Tas

[Diabolo]

Hello,

How to find TAS base on IAS and TAS to IAS.

Many thx

[Genghis the Engineer]

Correct for PEC (graph in POH) from IAS to CAS

Correct for compressibility to EAS (don't bother if below 0.5M), use whizz wheel or aircraft may have a graph in POH for this too.

Correct for density to TAS (either use whizz wheel, or multiply by square root of sigma, the relative density).

G

[Keith.Williams.]

I suspect that you mean divide by root sigma Genghis.

If you multiply by root sigma then the TAS at any given CAS will decrease as altitude increases.

[Diabolo]

example FL330 ISA condition and IAS 480kts.
What would be the TAS?

THX

[Keith.Williams.]

Using a CRP5 I get about 745 Kts TAS.

If all you want is general trends then at 40000 ft pressure altitude in the ISA, the relative density is about 1/4 so the TAS is about twice the CAS. (TAS = CAS / sqr root of 1/4)

If you are not interested in the numbers, but simply want the general effects, just draw four straight lines starting at a common point and fanning out as they move upwards from the bottom of a sheet of paper. Mark these lines from left to right, EAS, CAS, TAS and Mach.

To test the effect of climbing at constant CAS for example, rotate the sheet so that the CAS line is vertical. The EAS line will now slope to the left as it moves up the sheet. This indicates that EAS decreases with increasing altitude. The TAS and Mach lines slope to the right indicating that these values increase as altitude increases.

For a descent just look at how the lines slope as you move down the page. Sloping left means decreasing values and sloping right means increasing.

By rotating the sheet so that the appropriate line is vertical you can test the effects of climbing or descending with constant EAS, CAS, TAS or Mach.

You can also modify the lines to test the effects of inversions and isothermal layers.

For an inversion draw the four lines fanning out as before, but label them EAS, CAS Mach and TAS from left to right.

For an isothermal draw three lines fanning out from the bottom. Label the first line EAS, the second line CAS and the third line Mach and TAS.

[Old Smokey]

I beg to differ -
example FL330 ISA condition and IAS 480kts.
What would be the TAS?

Pht = 33,000 : CAS = 480 : TAS = 732.5 : Mach No. = 1.2592 : EAS = 423.6

I get the feeling from the basic nature of the question that the original poster is at the training stage, i.e. fairly slow aircraft and at or below 10,000 feet. If so just use the CAS and Density Height calculation available from any old 'garden variety' pilot computer to obtain TAS.

The resultant airspeed is actually more correctly referred to as Density Air Speed (DAS), but at Mach Numbers <0.5 and below 10,000 feet, the error is negligible.

If I'm wrong in assuming the status of the question poster, then there's much more to be said.

[Diabolo]

Thank you guys for your replies.

[Intruder]

Quickie rule of thumb: add 2% to airspeed for each 1,000' altitude.

example FL330 ISA condition and IAS 480kts.

480 * 166% = 797 kts

Not as accurate as the "real" formula or whizz wheel will give you, but close enough for realistic speeds and altitudes.

[MkVIII]

Intruder,
No offence intended, but why settle for second best? The RIGHT answer is acheivable just as quickly as the "best guesstimate"?

Yes, I am a perfectionist when it comes to figures.

No offence or ridicule intended.

[Old Smokey]

Diabolo,

I just revisited this site to see any developments and saw from your profile that you operate B777 (I hadn't checked that before). For heaven's sake don't use the DAS suggestion that I made, it belongs to 'low and slow'. If you want more info, I and many others would be happy to comply, but in the meantime just read the TAS directly from the top of your B777 PFD.

Oh what a wonderful day in so many ways if "they" could standardise on EAS as primary airspeed indications for aircraft, for this and countless other scenarios.

[Intruder]

why settle for second best? The RIGHT answer is acheivable just as quickly as the "best guesstimate"?
Back when I flew single-seat fighters with round dials, it was often "too hard" to dig out the whiz wheel. Besides, they taught us the rule of thumb in flight school, so it must have been "good enough"...

Of course, 30 years ago I didn't have GPS, FMS, INS, or any of those goodies...

[Diabolo]

Merci Intruder this was what I was looking for.

Thx a lot

Be on the safe side