Fixing up the Airpath Compass.

The Airpath compass is meant to work in a fluid
filled chamber.
A bubble is an annoyance. A leak leaves you with a
trembling indicator that won't indicate a heading
with any reliability.

Gasket and fluid kits are not that high.
From Chief etc. $20-$30
If caught short, don't try making a rear
diaphragm from a truck inner tube, and refilling
with lamp oil. The oil swells the diaphragm, and a
week later, it's leaking again.

The construction is a cast barrel with a front
gasketed glass, and a rear rubber diaphragm.
The glass face can be chipped by too enthusiastic
attempts to lift it out.

It's helpful to use a little gasket
cement on the front and back gaskets.
The filler screw has a sealing washer.It's best
filled with a throw-away syringe.
Fill to the top. Screw closed.
Turn several ways. Look for the air bubble.
Repeat. Three or four times. You finally get lucky,
with no bubble at all.
Leave it standing face down overnight, and look
for sweating. If you see any sweating, you still
have a leak.
The glass face is held onto its thin gasket by
a face plate with a cork gasket under it.
You snug the face plate down but don't over tighten,
else the inner gasket can squeeze out.
Even worse, watch out for the cover plate screws
on the adjusting magnets.
If these screws are too long, they force the face
plate away from the glass, and...it leaks again.


If the worse comes to the worst - a replacement
unit is $110-$130 from the aero suppliers.

Brian Whatcott

On Nov 8, 11:14*am, brian whatcott wrote:
The Airpath compass is meant to work in a fluid
filled chamber.
A bubble is an annoyance. A leak leaves you with a
trembling indicator that won't indicate a heading
* with any reliability.

Gasket and fluid kits are not that high.
*From Chief etc. $20-$30
* If caught short, don't try making a rear
diaphragm from a truck inner tube, and refilling
with lamp oil. The oil swells the diaphragm, and a
week later, it's leaking again.

The construction is a cast barrel with a front
gasketed glass, and a rear rubber diaphragm.
The glass face can be chipped by too enthusiastic
* attempts to lift it out.

It's helpful to use a little gasket
cement on the front and back gaskets.
The filler screw has a sealing washer.It's best
filled with a throw-away syringe.
Fill to the top. Screw closed.
Turn several ways. Look for the air bubble.
Repeat. Three or four times. You finally get lucky,
* with no bubble at all.
* Leave it standing face down overnight, and look
for sweating. If you see any sweating, you still
have a leak.
The glass face is held onto its thin gasket by
* a face plate with a cork gasket under it.
You snug the face plate down but don't over tighten,
else the inner gasket can squeeze out.
Even worse, watch out for the cover plate screws
on the adjusting magnets.
If these screws are too long, they force the face
plate away from the glass, and...it leaks again.

If the worse comes to the worst - a replacement
unit is $110-$130 from the aero suppliers.

Brian Whatcott

For a homebuilt that certainly works but for certified aircraft I have
to ask
two questions. 1. Are you a FAA certified instrument technician? If
you are not then you cannot "legally" work on the compass no matter
how dirt simple it is. 2. Did you "swing" the compass after working on
it?

One technique for filling with no bubbles is to submerge the compass
in a small can filled with fluid with the plug off. Roll and turn the
compass around until no bubbles flow out. Install the plug while the
compass is still submerged.

The official Airpath technique involves submerging the compass in a
vacuum canister. This is the primary reason that the FAA frowns on
mere mortals servicing compasses.

John Dupre'

John wrote:

/snip/
The filler screw has a sealing washer.It's best
filled with a throw-away syringe.
Fill to the top. Screw closed.
Turn several ways. Look for the air bubble.
Repeat. Three or four times. You finally get lucky,
with no bubble at all....
If the worse comes to the worst - a replacement
unit is $110-$130 from the aero suppliers.

Brian W


One technique for filling with no bubbles is to submerge the compass
in a small can filled with fluid with the plug off. Roll and turn the
compass around until no bubbles flow out. Install the plug while the
compass is still submerged./snip/

John Dupre'

That sounds like a neat method: the approved repair kit does not carry
enough fluid for that approach though.

Brian W