DG Questions

Six weeks ago my attitude indicator gave up the ghost, after slowly dying
over a period of several months. It's been replaced with a re-built unit
that's working fine.

Two days ago my directional gyro abruptly failed. It worked fine flying
into Pella, and it didn't work at all flying out. The thing wouldn't spin
up at all -- as if the gyro failed catastrophically, or, perhaps, a vacuum
line had come detached. The setting button would pop out, and the thing
would momentarily spin (10 degrees or so), but nothing would happen from
that point on.

My A&P says all the lines look fine, so the cheap fix is apparently out
(naturally). This leaves the DG -- and, of course, it's the real expensive
one with the heading bug for my autopilot...

Doesn't it seem just a little weird to lose both vacuum instruments in a six
week period? Is there any way my vacuum pump could still be putting out 5
inches of suction, and somehow sending junk into the instruments? Could the
AI failure last month have influenced the DG failure this month?

Or is it just bad luck that they've failed together? Any opinions?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

"Jay Honeck" wrote in message news:xdvnb.50697$HS4.232433@attbi_s01...

Doesn't it seem just a little weird to lose both vacuum instruments in a six
week period? Is there any way my vacuum pump could still be putting out 5
inches of suction, and somehow sending junk into the instruments? Could the
AI failure last month have influenced the DG failure this month?

Or is it just bad luck that they've failed together? Any opinions?

You do have a vacuum filter on the intakes to these things? When we lost both
our gyros (other than finding one and a $5 price tag on the top of it), we found
that the gyro inputs were just open to the crumbling insulation behind the panel.

Exactly my thought. Remember that the vacuum sucks air through the
instrument, check the inlet to see if the filter is on it. Could be just as
Ron described, insulation, dust and other crud could be getting sucked into
your gyros from behind the panel.
--
Jim Burns III

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"Ron Natalie" wrote in message
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"Jay Honeck" wrote in message
news:xdvnb.50697$HS4.232433@attbi_s01...

Doesn't it seem just a little weird to lose both vacuum instruments in a
six
week period? Is there any way my vacuum pump could still be putting out
5
inches of suction, and somehow sending junk into the instruments? Could
the
AI failure last month have influenced the DG failure this month?

Or is it just bad luck that they've failed together? Any opinions?

You do have a vacuum filter on the intakes to these things? When we lost
both
our gyros (other than finding one and a $5 price tag on the top of it), we
found
that the gyro inputs were just open to the crumbling insulation behind the
panel.

On 28-Oct-2003, "Jay Honeck" wrote:

Six weeks ago my attitude indicator gave up the ghost, after slowly dying
over a period of several months. It's been replaced with a re-built unit
that's working fine.

Two days ago my directional gyro abruptly failed.


In close to 2000 hours over 34 years I have never seen a vacuum instrument
suddenly die like that, except when associated with vacuum failure (pump or
vacuum line breakage or disconnection). I suppose it could happen, but
usually it's a slow death like you experienced with your AI.

I'd check to make sure that there is suction to the instrument, not just
visual inspection of the hoses, before I removed it from the plane.

--
-Elliott Drucker

I'd check to make sure that there is suction to the instrument, not just
visual inspection of the hoses, before I removed it from the plane.

How would you recommend going about doing that?

I've not seen what's up there for myself -- how are the hoses attached to
the instruments? How do you check for vacuum? Detach the hose and stick
your finger over it?

Thanks for the advice!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:
I'd check to make sure that there is suction to the instrument, not just
visual inspection of the hoses, before I removed it from the plane.

How would you recommend going about doing that?
I've not seen what's up there for myself -- how are the hoses attached
to the instruments? How do you check for vacuum? Detach the hose and
stick your finger over it?

You could removed the line from your panel mounted vacuum gauge and attach
it directly to the suction hose going into your DG (remove the hose
completely from the DG, don't use a "T" at first). This way you're
isolating the vacuum comming from the regulator. You may need an adapter,
because I believe they're generally use a smaller hose to the gauge than
the hose to the instruments. A decent avionics shop, or even a well
equipped maintenance shop should be able to apply a vacuum to the
instrument using a bench mounted vacuum pump to check it out. I agree
that most vacuum instruments die slowly, not suddenly.

The hoses are generally hooked to the instrument with either a barbed
adapter, or an adapter with a single "ridge" flaired into it. This type
is the type where you'd use a hose clamp. It's generally not needed with
the barbed adapter, although it's often used anyway.

The "schematic" for your vacuum system is likely the following:

+----- DG ------+
| |
filter regulator ----- vacuum pump
| |
+----- AI ------+
|
+----- vacuum gauge

direction of air flow ------------

Note that both the filter, and the firewall mounted regulator have two
ports on them. The vacuum gauge is often driven by an auxilary port
on either the AI (as pictured) or the DG. I think you can also have it
fed by a "T" off the normal lines to one of the instruments.

-- Jay


__!__
Jay and Teresa Masino ___(_)___
http://www2.ari.net/jmasino/ ! ! !

Checkout http://www.oc-adolfos.com/
for the best Italian food in Ocean City, MD and...
Checkout http://www.brolow.com/ for authentic Blues music on Delmarva

Thanks, Jay.

I have no idea how I'm going to climb under there to actually see all
this -- but if my 60 year old, 220 pound A&P can do it, so can I, dang it!
:-)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On 28-Oct-2003, "Jay Honeck" wrote:

How would you recommend going about doing that?


I really don't know how easy it is to determine the presence of suitable
suction at the end of a vacuum tube just by putting your finger over it, but
certainly your A&P should have a vacuum gauge that will remove all doubt.

There should be two hoses attached to the back of the DG. One (the air
outlet) comes from the vacuum regulator and the other (the air inlet) goes
(probably through a "T" connector) to the filter.

First remove the air inlet hose. With the engine running (vacuum gauge
registering) check if there is suction at the inlet port to the DG (where
you just removed the hose). BTW, if the DG now seems to be working, the
problem is an obstruction on the inlet side.

Then remove the air outlet hose from the DG. Check if there is suction at
the end of this hose.

If there is suction in both places your DG is bad. The airflow is not
spinning the gyro.
If there is no suction at the air outlet there is a leak or obstruction
between the regulator and the DG.
If there is suction at the air outlet but not at the air inlet your DG is
bad, but it may just be a relatively simple internal obstruction.

--
-Elliott Drucker

Jay,

Two can die close together if you lost or the previous owner lost a
vacuum pump and did not check for backwash in the lines from the
intruments to the pump. Carbon has been know to get sucked back into the
lines and make it into the instruments.

Michelle

Jay Honeck wrote:

Six weeks ago my attitude indicator gave up the ghost, after slowly dying
over a period of several months. It's been replaced with a re-built unit
that's working fine.

Two days ago my directional gyro abruptly failed. It worked fine flying
into Pella, and it didn't work at all flying out. The thing wouldn't spin
up at all -- as if the gyro failed catastrophically, or, perhaps, a vacuum
line had come detached. The setting button would pop out, and the thing
would momentarily spin (10 degrees or so), but nothing would happen from
that point on.

My A&P says all the lines look fine, so the cheap fix is apparently out
(naturally). This leaves the DG -- and, of course, it's the real expensive
one with the heading bug for my autopilot...

Doesn't it seem just a little weird to lose both vacuum instruments in a six
week period? Is there any way my vacuum pump could still be putting out 5
inches of suction, and somehow sending junk into the instruments? Could the
AI failure last month have influenced the DG failure this month?

Or is it just bad luck that they've failed together? Any opinions?



--

Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P

"Elisabeth" a Maule M-7-235B (no two are alike)

Volunteer Pilot, Angel Flight Mid-Atlantic

Volunteer Builder, Habitat for Humanity

Jay Honeck wrote:

Is there any way my vacuum pump could still be putting out 5
inches of suction, and somehow sending junk into the instruments?

No.

George Patterson
You can dress a hog in a tuxedo, but he still wants to roll in the mud.