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In this video, Destin — the creator of YouTube channel Smarter Every Day — rides in an F-16 with a USAF Thunderbirds pilot. If you start watching the video at the 11:06 mark, you will see what happens when the aircraft passes through Mach 1, namely, that the altimeter reading suddenly jumps.

Destin explains that this jump in the altimeter reading is because the air pressure downstream of the normal shock wave that is impinging on the pitot-static tube of the aircraft is lower than the air pressure upstream, but by the laws of compressible airflow, this would be total air pressure, not static air pressure.

As static air pressure, which is sensed by the altimeter, is always higher downstream of a normal shock wave, why is it that the altimeter reading registers a sudden jump and not drop instead?

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The altimeter doesn't measure static air pressure, it measures actual air pressure. So, to get an accurate reading of the static air pressure, the static port has to be carefully placed so that the ram air pressure (caused by the forward motion of the plane ramming into the air) and the slipstream pressure (caused by air rushing past the static port opening) cancel each other out (or nearly so).

So, Destin is correct: The actual air pressure after the shockwave is lower, causing a jump in the displayed altitude.

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  • $\begingroup$ Hi, HiddenWindshield. Isn’t the altimeter designed to sense only static air pressure in straight and level flight? Destin was absolutely correct in saying that the total air pressure behind the shock wave is lower, but his explanation doesn’t seem consistent with the fact that the F-16’s altimeter senses air pressure from the static taps of the F-16’s nose-mounted air data probe (please refer to Page 1-139 of the F-16C/D flight manual). Also, does the jump in the altimeter reading indicate that it’s unreliable after passing through Mach 1? $\endgroup$ – Transcendental Aug 20 '20 at 9:11
  • $\begingroup$ @Transcendental The data probe acts like a tiny airplane, with its own ram air pressure from the front and slipstream pressure on the side, and stagnation point where the two cancel out. The tap for the altimeter would be on the side of the probe, as close as possible to the calculated stagnation point. (Actually the diagram seems to show two taps, one on each side, which would help keep the altimeter accurate if flying uncoordinated.) And I wouldn't call the altimeter "unreliable" past the speed of sound, just that it's going to be off by a fixed amount that varies with speed. $\endgroup$ – HiddenWindshield Aug 20 '20 at 15:30

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