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I know that if you fly at a low altitude and at a high altitude at the same TAS, your IAS will be lower at the high altitude than at the low altitude. Why is this though? Wouldn't the static vent account for the change in pressure? I have heard it has to do with the change in density but why would this effect the system as it measures pressure not density? Maybe because ram air pressure (measured by the pitot tube) is affected by density?

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As you state, IAS is simply based upon the difference between total pressure and static pressure. For low subsonic conditions:

$$ p_t = p_s + p_q = p_s + \frac {1}{2} \cdot \rho \cdot V^2$$

$\rho$ is the air density, which is a function of air temperature: $\rho = \frac {p_s}{R \cdot T}$. For indicated airspeed, the density at sea level $\rho_{SL}$ at 15 degC is taken:

$$ V_i = \sqrt {\frac{2 \cdot (p_t - p_s)}{\rho_{SL}}}$$

However, the actual dynamic pressure measured by the pitot tube is caused by actual density.

  • When temperature decreases, density increases.
  • So at colder temperatures, some of the dynamic pressure is due to higher $\rho$
  • Therefore, the $V_{ind}$ bit decreases. The pitot only measures dynamic pressure and does not know what is due to temperature

Air is colder at higher altitude if we stay in the troposphere - climb to over 33,000 ft and the IAS does not change anymore.

enter image description here

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The anemometer is not a true speed instrument, but returns the value of the dynamic pressure at the Pitot tube, that can be correlated with the airspeed only for a given density. Thus, at higher altitudes, (lower air density) airspeed has to be higher in order to generate the same dynamic pressure at the Pitot tube, and the opposite is true for lower altitudes (higher density) where airspeed has to be lower in order to generate the same dynamic pressure...

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Your airspeed is measured as a difference in pressure between the pitot (ram air) and static ports. As you go up in altitude the amount of air entering the pitot tube decreases even though the true airspeed stays the same. This reduces the "pressure" seen by the airspeed indicator and therefore decreases the delta seen between the two ports, indicating a lower airspeed.

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    $\begingroup$ But wouldn't the static port also see less air and compensate for it? Why does the pitot tube sense this but not the static port? $\endgroup$
    – Murey Tasroc
    Aug 9, 2017 at 0:20
  • $\begingroup$ of course, they both sense less air. But the delta between them is also less. Hence a lower IAS will be indicated $\endgroup$
    – Radu094
    Aug 9, 2017 at 7:45
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    $\begingroup$ @Murey Tasroc The values at both static and Pitot are both absolute pressures, the sum of static and dynamic pressures. At the static port, the dynamic pressure is zero, and the static pressure is the same as the static pressure at the Pitot. Hence, and as the anemometer gives you the difference in absolute pressures between the pitot and the static port, that difference amounts to the dynamic pressure only... $\endgroup$
    – xxavier
    Aug 9, 2017 at 7:54
  • $\begingroup$ True airspeed does not stay the same as altitude increases unless power is reduced in which case the airplane is flying at a higher angle of attack and a lower indicated airspeed. $\endgroup$
    – John
    Oct 15, 2019 at 7:12
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This bothered me as well with most of the explanations I found online, until I remembered that the pitot tube measures dynamic PLUS static pressure. This makes sense when you remember that the pitot tube is also open to the outside air pressure just like the static vent. As such, at higher altitudes the static vent will receive less pressure due to the drop in static air pressure, but the pitot tube will receive even less for two reasons; the drop in both static and dynamic air pressure.

Hope that helps.

This youtube video helped me when trying to answer your question:

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No math is needed to explain.

If you fly at 100 knots indicated and climb from 1,000ft to 5,000ft with the same power your indicated airspeed will remain about the same.

In order to fly at the same true airspeed, you’d have to reduce power. So true airspeed is only constant if you make changes to your flight configuration to make it constant- it’s really not constant at all. Even if you stay at the same altitude, if the temp changes, it will change.

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