What caused the VSI and ALT bouce in the IMC?

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

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



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

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

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

"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:
: 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$) *
************************************************** ***********************

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

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

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$) *
************************************************** ***********************