What caused the VSI and ALT bouce in the IMC?

cpu ) wrote:

Yesterday I flew a cessna 172 in the hard IFR. When I penetrated
apparently a heavy cumulonimbus rain cloud area, 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 (3 to 4 times
per second). It lasted for about 10 minutes until I passed that area.
The AI and airspeed was relatively stable in such light to moderate
chops condition.

What model C172?

Last year I flew several flights in a C172SP in rainy weather and noted the
same behavior you did. After speaking with a few pilots more knowledgeable
than I, I learned that this issue is commonly caused by the aircraft's
single static port becoming temporarily blocked by streaming water. For me,
engaging the alternate air was SOP during wet weather.

If you haven't already, read the POH about alternate air altimeter errors
and note the conditions (vents open/closed, heat on/off, etc) in the POH
where Cessna documented the error.

You also can engage the alternate error on a VFR day at altitude and see
the altimeter difference yourself. In the SP I flew, the error was about
70 feet higher with the alternate air engaged.


--
Peter