What caused the VSI and ALT bouce in the IMC?

the VSI and altimeter
started to oscillate and bounce +/- 250 FPM (ALT oscilated 200~300 ft
up and back). The rate of bounces was about 3 to 4 Hz


I've never seen it, but that won't stop me from speculating.

The VSI and altimiter share the static port, the airspeed indicator uses that
and the pitot tube. So, if it is an erronious reading, I'd suspect the static
port. However, if the static pressure is varying like that (due to rain
getting in?) I'd also expect the airspeed to show something, and you say it was
relatively stable. So I'm not convinced of this.

It could be that you were running through waves of actual pressure differences
which were being accurately measured. At typical cruise in a 172 you'd be
doing something like forty feet in a quarter of a second. Maybe thirty if you
had slowed down. If you were flying through up and down drafts, that might
explain it (i'd expect pressure differences which would drive the flow).

Did you experience accelerations (measured by the Mark 1 Anatomical Sitting
Sensor) that were in sync with the oscillations of the pressure instruments?

I doubt that you actually changed altitude by 200 feet up and back four times
per second however.


Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).


I'd use alternate air if equipped, but I wouldn't break the glass just yet.
It's not clear to me that it would help, and you don't yet have an emergency
situation. I'd advise ATC and probably also request higher if terrain was a
consideration and ice wasn't, just to ensure clearance. And yes, turn on the
pitot heat, but the problem (if any) is in the static system so pitot heat
wouldn't do anything.

Now if I thought that the average of the altitude and VS indicated was suspect,
then I would reconsider breaking the glass.

Jose
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