What caused the VSI and ALT bouce in the IMC?

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

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.

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

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

What model C172?

It is an old but nicely maintained 1974 C172 in our flying club, with
(Mph in ASI), but with no alternate air intake. When I encountered
this problem, I was thinking that I should fly our club's C182 or
ArrowIV in this soupy weather. Fortunately, I had my friend who is a
student pilot to take care of radio dial rolling. That helped a lot.

I am sure VSI did not have oblivious bounce because I tried to slow
down to attempt decreasing the oscillation. But it apparently did
not help too much; probably decreased the range from +-250 to +-150.

Thank you guys for provide valuable information. I will forward this
to our club's forum.

-cpu

"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).

You had water in the static line. It is a common problem, especially when
doing things like penetrating thunderstorms. Opening the alternate static
port can help the problem.

If the alternate static port does not fix the problem, then smashing the
gauges will not help, either.

If the alternate static port does not fix the problem, then smashing the
gauges will not help, either.


Besides which the gauge you would smash is the one you're tring to fix.

Jose

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

If the alternate static port does not fix the problem, then smashing the
gauges will not help, either.


Besides which the gauge you would smash is the one you're tring to fix.


1. The plane does not have a alternate static port.

2. I thought smash only VSI will help all the static based instrument
such as ALT, ASI because the static system are all connected. Once
the air bleed through the VSI, it will propergate to the other
instruments through the static connection. I think I am right on
this.

-cpu

2. I thought smash only VSI will help all the static based instrument
such as ALT, ASI because the static system are all connected. Once
the air bleed through the VSI, it will propergate to the other
instruments through the static connection. I think I am right on
this.


Yes, you are right on this. If knowing altitude becomes critical and you don't
trust the "average" reading, then yes I would smash the VSI without
heasittaion. But if I believed I could trust the average reading, especially
if I had nearby altitudes clear, I'd leave it alone.

Jose

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

Thanks Jose, I was in the cruise flight and gyro looked OK. Yeh, I
won't break the glass in that situation. However, as you mentioned,
if in the non-precision approach or even on the ILS, if I encountered,
I would probably do:

1. Abort the approach and flight to the miss.
2. Trouble shooting the VSI and ALT on the hold.
3. If I have to break the glass, then do it on the hold, and test it.
4. If things get restored, then go ahead do the approach again.
5. Otherwise, fly to some better weather and land.

-cpu