What caused the VSI and ALT bouce in the IMC?

[cpu]

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

[Teacherjh]

>>
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)

[Brad Z]

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

[Andrew Sarangan]

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

[Roy Smith]

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?

[rip]

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
>
>

[Nathan Young]

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

[Teacherjh]

>>
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)

[Andrew Sarangan]

(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.

[Teacherjh]

>>
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)

[Peter R.]

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

[David Brooks]

"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

[Peter Clark]

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 R.]

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

[Stan Prevost]

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
>
>
>
>
>
>
>
>
>
>

[Peter Clark]

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.

[Peter R.]

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

[Teacherjh]

>>
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)

[David Brooks]

"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

[Teacherjh]

>>
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]

"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

[Ray Andraka]

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

[Teacherjh]

>> 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)

[cpu]

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

[cpu]

>
> 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]

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

[C J Campbell]

"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.

[Teacherjh]

>>
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)

[cpu]

> >>
> 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

[Teacherjh]

>>
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)

[cpu]

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

[Teacherjh]

>>
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.
<<

I think I'd find better weather and land before I broke the glass.

Jose



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

[David Megginson]

cpu wrote:

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

In many countries (including Canada), an alternate static air source is
legally required to fly IFR. In the U.S., it may not be a legal
requirement, but it's still a very bad idea to take up a plane into IMC
without one -- you might not be able to get enough room to swing your arm
hard enough to break the VSI face, and in any case, you might be too busy
trying to control the plane.

I understand that it's a fairly cheap modification (as simple as a little
valve under the front of the panel).

> 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.

That's correct. Smashing the face of the ALT would also work, but you don't
want to risk breaking that if you're in IMC. The VSI is expendible.


All the best,


David

[Michael]

David Megginson > wrote
> I understand that it's a fairly cheap modification (as simple as a little
> valve under the front of the panel).

If you own an experimental, what you say is correct.

If you own a certified airplane, you may rest assured that the weight
of the paperwork will exceed the weight of the installed components.
A change to the pitot-static system is considered a major alteration.
If it's a manufacturer's kit or STC, it will be expensive. If there
is no STC or manufacturer's kit, it will be VERY expensive or
downright impossible because it will require a field approval, which
these days usually requires you to hire a DER/DAR.

Just one more way the FAA keeps us all safe from unproven technology.

Michael

[Michael]

"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

[[email protected]]

Michael > wrote:
: If you own an experimental, what you say is correct.

True... for a certified aircraft, the little $2.99 valve costs $299, and the
paperwork to install it costs another $300. Yet another instance of "what's safe is
not necessarily legal, and what's legal isn't necessarily safe."

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[Jon Woellhaf]

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

[Ray Andraka]

I had an alt static valve installed on my plane by an avionics shop when they had it in
for the IFR static/transponder test. They considered it a minor mod, no 337 needed,
just a log entry. Total incremental cost was less than $100.

wrote:

> Michael > wrote:
> : If you own an experimental, what you say is correct.
>
> True... for a certified aircraft, the little $2.99 valve costs $299, and the
> paperwork to install it costs another $300. Yet another instance of "what's safe is
> not necessarily legal, and what's legal isn't necessarily safe."
>
> -Cory
>
> --
> ************************************************** ***********************
> * The prime directive of Linux: *
> * - learn what you don't know, *
> * - teach what you do. *
> * (Just my 20 USm$) *
> ************************************************** ***********************

--
--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

[[email protected]]

While I don't doubt you did this, the avionics shop is in error. As mentioned
before, *any* alteration to the static system is considered a major alteration and
thus requires a field approval and 337. That's not to say it isn't tempting to put a
valve in where the encoder goes, just to say that it's not legal.

Similar to the thought that installing a NAV/COM is a logbook entry. From
what I understand, many people (A&P/IA types included) believe that's all that's
necessary, but many FSDOs consider it an alteration requiring a field-approval.

Of course, I consider it all a bunch of B.S. I'd prefer to learn and live by
the rules of Physics (which cannot be bent), rather than the rules of Man (which are
often arbitrary and capricious).

-Cory

Ray Andraka > wrote:
: I had an alt static valve installed on my plane by an avionics shop when they had it in
: for the IFR static/transponder test. They considered it a minor mod, no 337 needed,
: just a log entry. Total incremental cost was less than $100.

: wrote:

:> Michael > wrote:
:> : If you own an experimental, what you say is correct.
:>
:> True... for a certified aircraft, the little $2.99 valve costs $299, and the
:> paperwork to install it costs another $300. Yet another instance of "what's safe is
:> not necessarily legal, and what's legal isn't necessarily safe."
:>
:> -Cory
:>
:> --
:> ************************************************** ***********************
:> * The prime directive of Linux: *
:> * - learn what you don't know, *
:> * - teach what you do. *
:> * (Just my 20 USm$) *
:> ************************************************** ***********************

: --
: --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



--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************