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

"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

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?

Peter Clark ) wrote:

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?

Only the SPs that have a dual-axis AP have the secondary static port behind
the door on the pilot's side. The SPs without the dual-axis AP only have
the one port forward the door.

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

I do not know why.


--
Peter

But why didn't it affect the airspeed indicator?

"Peter R." wrote in message
...
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

On Mon, 19 Apr 2004 14:11:33 -0400, Peter R.
wrote:

Peter Clark ) wrote:

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?

Only the SPs that have a dual-axis AP have the secondary static port behind
the door on the pilot's side. The SPs without the dual-axis AP only have
the one port forward the door.

OK, another proof of the saying "you learn something new every day" -
thanks. Mine has the NavII with MFD and dual-axis with alt preselect.

Stan Prevost ) wrote:

But why didn't it affect the airspeed indicator?

It may have, but the problem was much more noticeable and distracting with
the VSI and the altimeter, at least to me since those instruments were in
my scan on the ILS more so than the airspeed indicator.

--
Peter

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)