What caused the VSI and ALT bouce in the IMC?

"Teacherjh" wrote in message
...

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....
(snip LGM scenario)

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

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

Does it, in your spam can? In mine? I probably have to go home and study the
systems, but I admit I can't remember this detail in the POH. Or is there
maybe some regulation behind your use of "should"? Inquiring minds want to
know.

-- David Brooks

I know mine does not. It just opens the static line to the cockpit.

David Brooks wrote:



Does it, in your spam can? In mine? I probably have to go home and study the
systems, but I admit I can't remember this detail in the POH. Or is there
maybe some regulation behind your use of "should"? Inquiring minds want to
know.

-- David Brooks

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

Does it [cut off the primary], in your spam can?

Dunno. Have to check. But it had better if it's to be of use.


Or is there
maybe some regulation behind your use of "should"?


No. I am not from the FAA, nor do I play one on TV.

Jose

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

Well, it is about 3 to 4Hz (3 to 4 full swings from -250 to +250 FPM
or the other way per second). You basically can't track it. (Other
than approaximate its "main value", which is about 0 fpm.

The ALT basically fall back to 6000 ft (assigned altitude) after swing
to the high point (6200 ft) so you can see where you at without a
problem.

No obivious change to ASI...

No alternate air on this old 172, the ASI has MPH instead of KtH. But
it is a nice plane.

(Teacherjh) wrote in message ...

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


Yes, but not too fast to see.

Jose

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

rip wrote in message ...
Unless the blockage rattling around was after the ASI and before the VSI
and ALT. Interesteing idea.


I'm sure that ASI did not have oblivious oscillation like the VSI or
ALT's big needle movement. I guess the reason behind it was because
of the fraction. (if not because of place of blockage)

What I mean by fraction is that it might take more air pressure to
move the ASI needle than to move VSI and ALT needles. In that case,
ASI needle oscillation will be not that obivious to observe.

BTW, one of my instructors told me today that he guesses the
oscillation might be caused by the water plus the engine vibration
(that makes the blockage on and off).

-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)