What caused the VSI and ALT bouce in the IMC?

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.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

The lesson learned here was to turn on the alternate air intake if the
airplane was equipped with one. Or from the book, "break the VSI
glass" in such condition. Well, I did not do it this time (other
than turn on the pitot heat, OAT was 35F). Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).

Any thoughts are welcome.

-cpu

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
--
(for Email, make the obvious changes in my address)

I've experienced wild fluctuations in IMC due to water droplets blocking the
single static air source of the 172. The 182, which has 2 ports on either
side of the cowling, doesn't seem to do this. Nevertheless, engage the
alternate static air and the problem should go away. Don't break any gauges
unless you were going to replace them anyway.

"cpu" wrote in message
om...
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.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

The lesson learned here was to turn on the alternate air intake if the
airplane was equipped with one. Or from the book, "break the VSI
glass" in such condition. Well, I did not do it this time (other
than turn on the pitot heat, OAT was 35F). Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).

Any thoughts are welcome.

-cpu

This clearly has to be an instrument error, because a 200' oscillation
four times per second will produce several thousand G's. Are you certain
that the rate of oscillation was actually 4Hz? It would be pretty hard to
even see the altimeter needle if it was oscillating that fast. Another
possibility is the static tube that feeds the VSI and ALT could have a
blockage that was rattling around when you experienced the chop. But that
still doesn't explain why the ASI was not bouncing at the same rate.




(cpu) wrote in news:26751d41.0404181215.35b668d9
@posting.google.com:

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.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

The lesson learned here was to turn on the alternate air intake if the
airplane was equipped with one. Or from the book, "break the VSI
glass" in such condition. Well, I did not do it this time (other
than turn on the pitot heat, OAT was 35F). Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).

Any thoughts are welcome.

-cpu

In article ,
(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.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

Water in the static line?

Unless the blockage rattling around was after the ASI and before the VSI
and ALT. Interesteing idea.

Andrew Sarangan wrote:
This clearly has to be an instrument error, because a 200' oscillation
four times per second will produce several thousand G's. Are you certain
that the rate of oscillation was actually 4Hz? It would be pretty hard to
even see the altimeter needle if it was oscillating that fast. Another
possibility is the static tube that feeds the VSI and ALT could have a
blockage that was rattling around when you experienced the chop. But that
still doesn't explain why the ASI was not bouncing at the same rate.




(cpu) wrote in news:26751d41.0404181215.35b668d9
@posting.google.com:


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.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

The lesson learned here was to turn on the alternate air intake if the
airplane was equipped with one. Or from the book, "break the VSI
glass" in such condition. Well, I did not do it this time (other
than turn on the pitot heat, OAT was 35F). Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).

Any thoughts are welcome.

-cpu

On 18 Apr 2004 13:15:07 -0700, (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.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

The lesson learned here was to turn on the alternate air intake if the
airplane was equipped with one. Or from the book, "break the VSI
glass" in such condition. Well, I did not do it this time (other
than turn on the pitot heat, OAT was 35F). Next time if it happened,
I will break the glass (or turn on the alternate in a equipped
airplane).

My Cherokee had an old altimeter that oscillated when in IMC.
Replaced it and no problems since.

In your case since both the altimeter and the VSI are oscillating, its
something in the static system. You should try having it blown out.
Some planes have drains to empty water out of the static system, not
sure about the 172.

-Nathan

Are you certain
that the rate of oscillation was actually 4Hz? It would be pretty hard to
even see the altimeter needle if it was oscillating that fast.


4 Hz is not that fast.

Jose

--
(for Email, make the obvious changes in my address)

(Teacherjh) wrote in
:


Are you certain
that the rate of oscillation was actually 4Hz? It would be pretty hard
to even see the altimeter needle if it was oscillating that fast.


4 Hz is not that fast.

Jose


If an altimeter is going through 400' of altitude change in 250ms, that
is definitely fast.

If an altimeter is going through 400' of altitude change in 250ms, that
is definitely fast.


Yes, but not too fast to see.

Jose

--
(for Email, make the obvious changes in my address)