View Full Version : IAS vs. TAS

20th May 2004, 15:29

just have a small question:

At ISA the IAS is equal to TAS

My Question: Is this also true for a high speed jet travelling above or near local speed of sound, or do I have to correct for compressibility(at ISA)?

Thx for your help!


20th May 2004, 15:43
IAS =TAS at ISA at sea level (i.e at 1013.2 mb ambient pressure). At any other pressure altitude TAS is greater than IAS. Yes there is then a correction for compressibility at high Mach Nos.

Mad (Flt) Scientist
20th May 2004, 15:46
Technically, even at ISA Sea Level IAS may not be equal to TAS, because IAS is "Indicated" and therefore includes the effects of errors in the airspeed sensing system. IAS is airframe/installation dependent (even varying between systems on a single aircraft).

Genghis the Engineer
20th May 2004, 15:46
Not always true to say that IAS=TAS.

At ISA, Sea-Level conditions, CAS = TAS

It goes....


(correct for position errors)


(correct for compressibility, not worth the bother below M=0.6)


(correct for density)


(and, of-course, correct for wind)



20th May 2004, 16:22
Thank you for your replies!

Once again:

(sea level conditions)
I want to know if CAS=TAS(at ISA) even when flying at or above M=0.6
I think it's not! Just want to confirm this.

20th May 2004, 17:26
WK - I think Genghis has answered that?

(correct for compressibility, not worth the bother below M=0.6)
(correct for density) TAS

20th May 2004, 19:51
To understand this matter we must look at how an airspeed indicator (ASI) works. Its takes in total pressure, which is the sum of dynamic pressure plus static pressure and puts this into a capsule. It also takes in static pressure and puts this on the ontside of the capsule. So dynamic plus static is trying to expand the capsule while static pressure is trying to compress it. The static pressure inside and outside the capsule cancel each other out, leaving dynamic pressure to expand the capsule. The expansion of the capsule is used to move the pointer to give an indication of airspeed. ASIs are calibrated to give an indication that is equal to TAS at low speeds at ISA msl.

As expained in previous posts every ASI is slightly different from every other ASI so the actual indication includes minor individual instrument errors.

The next type of error is caused by problems encountered in capturing the dynamic and static pressures. Unless the pitot probe is pointing directly at the undisturbed airflow the pressure detected will be different from the actual pressure. When sensing the static pressure movement of airflow around the aircraft will also cause errors. These pressure sensing errors are not constant, but change as angle of attack changes. But if we correct the indicated airspeed to cancel out these error we get Calibrated Airspeed or CAS.

The next type of error, and I suspect that this is the one that you are asking about WhiteKnight, is compressibility error. To understand this we must look at the nature of dynamic pressure.

Dynamic Pressure is the pressure exerted when a moving airstream is brought to rest on a surface. The surface must exert a force upon the air to bring it to rest, and dynamic pressure is the Newton's third law equal and opposite reaction to this force.

Dynamic pressure is equal to 1/2 Rho Vsquared, where Rho is air density and V is the TAS. If we ignore the instrument and pressure sensing errors, we IAS = CAS = TAS at ISA m,sl at reasonably low airspeeds.

But as speed increases, the dynamic pressure acting inside the pitot probe compresses the air, thereby increasing its density slightly. This in turn will increase the dynamic pressure and the indication given by the ASI. The increase in denity, dynamic pressure and airspeed indication are proportional to the TAS squared. So this compressibility error increases as airspeed increases.

This means that the ASI tends to over-indicate as airspeed increases. To get the correct value we must subtract the compressibility error from the CAS to get the equivalent airspeed or EAS.

At low speeds this compressibility error is negligible so we can say that CAS = EAS = TAS at low speeds at ISA msl. But as speed increases, the difference betweeen CAS and EAS increases. We can however say that EAS continues to equal TAS at all speds at ISA msl. But as Genghis has said the compressibility error is only significant at high speeds.

So getting back to your original question, no CAS does not equal TAS at high speeds. But EAS = TAS at all speeds (within reason)at ISA msl.

25th May 2004, 19:41
Charles E,

Been away, not avoiding you.

Dead right. I should have said "At any GREATER pressure altitude."

Still, abeler scibes than I have covered it all in detail.