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Spencer Suderman recently did a world record 81-turn inverted flat spin, and dropped over 21 thousand feet while doing so. The entire thing is documented on youtube. The spin starts at 3 minutes into the clip. Looking at the footage, in the beginning of the spin, and also (although to a lesser extent) at the end of the spin, the altimeter is unwinding very unevenly, being almost steady for a second and then quickly unwinding 3-500 feet before steadying (or actually increasing the first couple of turns) for another second. Why is that? It seems unreasonable to me that it would actually be leveling out.

It's worth noting that in the middle of descent, when spin is perfectly flat, it's unwinding evenly.

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    $\begingroup$ turbulence over the static ports I think $\endgroup$ – ratchet freak Mar 18 '14 at 13:46
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The altimeter works by measurements of the pressure in undisturbed air, also called static pressure. A static port is placed, usually on the side of the hull, perpendicular to the airflow.

During a spin, the airflow will not flow evenly past the static port, but will occasionally flow directly onto the static port. This will cause a rise in measured pressure, leading the altimeter to believe it is lower than it actually is. This cause fluctuations in the indicated altitude.

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    $\begingroup$ Some aircraft have multiple static ports to help average out this kind of situation (it's generally intended to deal with the uneven pressure when flying in a slip, but conceivably it would help in this situation as well). $\endgroup$ – voretaq7 Mar 19 '14 at 1:47
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DeltaLima's answer is correct, but he doesn't say why there are oscillations. If you watch the horizon, you will see that initially and at the end of the spin the roll angle oscillates, whereas in the middle of the spin the roll angle is steady. The uneven altimeter correlates nicely with this roll oscillation. I assume that the static ports are at the rear fuselage, and during a spin the airflow will have a noticeable sideward component there. This will influence the pressure on the static port, and an oscillating change in this sideward component causes the halting movement of the altimeter.

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