What caused the VSI and ALT bouce in the IMC?

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?

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

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

"Peter R." wrote in message
...
cpu ) wrote:


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.

Does engaging the alternate air shut the external static source off? If not,
it's only a partial test.

-- David Brooks

Does engaging the alternate air shut the external static source off? If not,
it's only a partial test.


Hope so - you use it when the primary air is hosed.

Jose

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

"Teacherjh" wrote in message
...

Does engaging the alternate air shut the external static source off? If
not,
it's only a partial test.


Hope so - you use it when the primary air is hosed.

One of us must have cut out too much of the context.

Because you can never know too much about your plane's systems, you engage
the alternate air one day while the static port is working fine and note the
offset. Now you are ready with a known correction, when the time comes that
the primary air is hosed. Is this a valid test?

-- David Brooks

Because you can never know too much about your plane's systems, you engage
the alternate air one day while the static port is working fine and note the
offset. Now you are ready with a known correction, when the time comes that
the primary air is hosed. Is this a valid test?


Only if engaging the alternate cuts off the primary. Because in that case,
when you engage the alternate air, you now have the situation you tested
(alternate air ONLY). If, OTOH, the perversities in the design are that the
primary air is left connected, then you do not have a valid test. Consider the
following scenario: You've done the test, and find that there is a 75 foot
difference with alternate air.

Then one day you are flying in the soup, you see a saucer shaped apparition,
you are boarded by little green men who take the cowling off, reroute the
engine exhaust into the static port plumbing, phone home, and vanish. Before
looking for Area 54 in the AF/D, you notice the altimeter shows you
underground.

Ok, pull the alternate air. Now you're still underground, but with a 75 foot
difference. You're still hosed because the hosed primary air is still part of
the system.

So, the alternate air had better cut off the primary air!

If it does (as it should) then the test is valid and useful.

Jose


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

"David Brooks" wrote
Does engaging the alternate air shut the external static source off? If not,
it's only a partial test.

I have never seen an alternate static source on a light GA airplane
that shut the external static source off. Every one I have seen
simply vents the static system to the cabin. It is intended ONLY as a
means to deal with static source blockage, such as by ice, not with
intermittent effects such as streaming water.

Michael

Michael wrote, "I have never seen an alternate static source on a light GA
airplane that shut the external static source off. ..."

Several posts in this thread reminded me of an incident that happened to me
several years ago a few weeks before I got my PP certificate. I intended to
fly from Front Range Airport (FTG) in Colorado (where I was then based) to
Centennial (APA) to meet my instructor. I'd made this short solo flight many
times before.

Before taking off, I called Unicom and got the altimeter setting: 30.10. I
set the altimeter and observed that it read 5495 feet. Field elevation is
5500 feet so that checked.

I performed a normal takeoff. Airspeed alive, rotate at 55 KIAS, climb out
at 80 KIAS.

A few seconds later I glanced at the altimeter and saw it still read 5500
feet! It had always read about 6000 feet at this point. Did I forget to set
the altimeter? I called Unicom and requested the altimeter setting. It was
still 30.10. I verified the setting: 30.10. I leveled off at what looked
like pattern altitude -- 6500 feet. The altimeter still read 5500 feet and
the ASI still read 80 KIAS. It should have been about 120 KIAS by now.

I knew I couldn't possibly have a blocked static port -- the 182 has two --
because no one else flew the plane and I hadn't washed it or performed any
maintenance and it had been in the hangar since the last flight when
everything worked perfectly and I checked both ports during preflight and
they were clear.

Even though I _knew_ the static ports couldn't possibly be obstructed, I
pulled the knob to open the alternate static source.

The ASI immediately showed 120 KIAS and the altimeter immediately showed
6800 feet.

I completed the flight to APA without further incident.

Once on the ground, I told my instructor about the problem I'd had. We
investigated and soon found the problem.

The alternate static source knob on the 182Q is normally in. To select the
alternate static source -- a short tube that opens to cabin pressure -- you
pull the knob fully out. During preflight, as I had done many times before,
I pulled the alternate static knob out then pushed it back in to verify
normal static source was selected.

This time, however, without my noticing it, the little plastic trim piece
that surrounds the knob, which had been loose since I bought the plane, fell
down a fraction of an inch when I pulled the knob out. When I pushed it back
in, the knob contacted the trim piece and stopped about half way between
normal static and alternate static. That half way position turned out to be
an undocumented Static Blocked position!

I think it's ironic and amusing that the very device that was intended to
prevent a blocked static line actually caused the blockage.

Jon

On Mon, 19 Apr 2004 10:05:58 -0400, Peter R.
wrote

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.

Greetings,

I seem to recall checking two static ports in my 172SP - one just aft
of the cowling on the left side, and one aft of the door, left side.
Since it's got two holes, are they calling this a single port because
they plumb to the same line inside, or has an additional port been
added? Would blocking one of the two (since the line itself is still
vented to the outside) cause this, or would a blockage have to be
forward of the forward static port (where the lines merge) to cause a
problem?

Random related question, is there some reason why are both ports on
the same side of the aircraft?