View Full Version : IAS versus TAS

7th Feb 2002, 18:31
A question from a PPL who is not satisfied with what a read in my PPL textbooks.

My question concerns the, probably well-known story about increasing TAS with increasing altitude. Now, I know that, with increasing altitude, the air density gets lower, so the IAS falls behind of the TAS. In other words, you'd have to fly faster (higher V) to compensate for this. To stick to the formula: you have to increase Velocity to compensate for the lower Rho (the symbol should be somewhere on the computer, but don't ask me where), and if you do that right, the result will be the same IAS, but with a higher TAS. Ram-air pressure has to stay the same.. .I was always perfectly happy with this answer, untill I realised that inside the altimeter there is also the static pressure. Now, it seems to me that, with increasing altitude, the static pressure will go down as well, probably with the same rate as the ram-air pressure (doesn't air density play a role also in the static pressure?). So, I thought, basically the pressure differential between both sides of the diaphragm will not change - the air comes from the same source, doesn't it?. .To put a long story short: with increasing altitude Rho will go down, but doesn't that go for the ram-air pressure as well as for the static pressure? So, how come that you fly a higher TAS with the same IAS?. .I hope this question is clear (I'm Dutch, not English), and that there's anyone who is able to shed some light on this.. .Thanks in advance, hope to hear from all of you.

7th Feb 2002, 20:03

I think you are right. The static pressure falls at the same rate the ram pressure falls. However they maintain their same relative proportions as long as you maintain the same IAS. So the ASI reads the same despite the fact you are actually going faster through the air.

And that's why you need to convert IAS to TAS.

8th Feb 2002, 00:50
Eagle 1,

For the sake of simplicity, the explanation below ignores the finer distinctions between IAS, RAS, CAS and EAS. They don't change the sense of the overall effect, but simply adjust the magnitude of the changes a little.

An airspeed indicator produces an indication (IAS) that is proportional to dynamic pressure. For any given dynamic pressure, the IAS will be constant regardless of altitude. So climbing at constant IAS means climbing at constant dynamic pressure.

Dynamic pressure is equal to the TAS squared, multiplied by half the air density. But density decreases with increasing altitude. So for a given IAS, the TAS must increase, such that the rate of increase in TAS squared, just balances the rate of decrease in density, thereby maintaining a constant value of dynamic pressure. At 40000 feet ISA for example, the density is about a quarter its sea level value. So the TAS squared must be 4 times the IAS. This means that the TAS at 40000 ft is approximately twice the IAS.

The ASI cannot detect dynamic pressure directly, so it takes total pressure (dynamic plus static) and feeds this to one side of a capsule. By feeding static pressure to the other side, the instrument subtracts static from total, to leave dynamic. As altitude increases, the static pressure decreases, but this decrease is felt on both sides of the capsule. So the overall effect of reducing static pressure is nil.

So the overall effects are:

a. At any given IAS the dynamic pressure is constant at all altitudes.. .b. As altitude increases at any given IAS the TAS increases.. .c. Although static pressure decreases with increasing altitude, this does not affect the relationship between IAS, dynamic pressure.

[ 07 February 2002: Message edited by: Keith Williams. ]</p>