The AD that wasn't.

With all the AD's for things with a very low likelihood of occurrence, you
really have to wonder why this (from Cessna Pilot's Association) isn't one.
It should be and you should check your engine immediately.

Many engines, particularly Lycomings, have a pressurized oil passage opening
into the flange for the vacuum pump mount. This was to supply oil to a
corresponding hole in the old wet pumps. With a dry pump, the hole is
simply blanked off by the gasket. The pump manufacturer sometimes supplies
a cork gasket or one of cork and one of rubber. If a cork gasket is used,
the only thing preventing the oil from being pumped out of the engine under
oil pump pressure is a 1/8" inch section of 1/6" cork. Not exactly what you
would bet your airplane or your life on. A number of planes have already
invented new landing fields when this little smidgen of cork let go.

Look carefully where your vacuum pump mates to the engine. If you see cork,
it should be replaced immediately.
--
Roger Long

Gee, I don't know ... Is it better to have a 1/8" section of 1/16" (sic) rubber
instead of cork? Could it be that there is no AD because there has never been a
failure? Somehow this one doesn't get me nearly as worried as you seem to be.

Best regards,

Steve Robertson
N4732J 1967 Beechcraft A23-24

Roger Long wrote:

With all the AD's for things with a very low likelihood of occurrence, you
really have to wonder why this (from Cessna Pilot's Association) isn't one.
It should be and you should check your engine immediately.

Many engines, particularly Lycomings, have a pressurized oil passage opening
into the flange for the vacuum pump mount. This was to supply oil to a
corresponding hole in the old wet pumps. With a dry pump, the hole is
simply blanked off by the gasket. The pump manufacturer sometimes supplies
a cork gasket or one of cork and one of rubber. If a cork gasket is used,
the only thing preventing the oil from being pumped out of the engine under
oil pump pressure is a 1/8" inch section of 1/6" cork. Not exactly what you
would bet your airplane or your life on. A number of planes have already
invented new landing fields when this little smidgen of cork let go.

Look carefully where your vacuum pump mates to the engine. If you see cork,
it should be replaced immediately.
--
Roger Long

As one who has done a fair amount of engineering in the past 30 years, I can
assure you that it is way better to have the higher tensile strength and
forgiving rubber holding this pressure than cork. Cut a thin slice out of
the next cork you take out of a bottle of wine and then pull on it and a big
rubber band of the same thickness. This was a secondary seal, not intended
to take any significant pressure, that is having oil pressure put directly
on it from inside the gasket itself.

According to CPA, there have been numerous failures. Nobody seems to have
died yet which is probably why no AD.

--
Roger Long

"Steve Robertson" wrote in message ...
Gee, I don't know ... Is it better to have a 1/8" section of 1/16" (sic) rubber
instead of cork? Could it be that there is no AD because there has never been a
failure? Somehow this one doesn't get me nearly as worried as you seem to be.

A far more common failure is the failure to install any seal on some of those
accessory mounts.

Steve Robertson wrote:

Gee, I don't know ... Is it better to have a 1/8" section of 1/16" (sic) rubber
instead of cork?

It really doesn't matter. On the dry pump, there's solid metal on the other
side of that hole. There's no way it's gonna leak.

George Patterson
Brute force has an elegance all its own.

No it's is very likely to leak and cork is clearly the culprit. Cork
gaskets are not made of solid cork but particles glued together. The hole
is stopped by the cork or rubber gasket. True, it can't blow out because
the gasket is backed up by the solid metal of the pump flange. What almost
certainly happens is that oil works it's way between the cork particles and
makes a small crack. It may even have an effect on the glue holding the
gasket together. The hydraulic pressure pushes the gasket both ways. Any
oil that then gets through to the interface between the metal and the gasket
lubes the sliding surface. The gasket blows out sideways and the oil blows
out of the engine.

It's not so much that the rubber is stronger or a better gasket overall but
that it is less likely to separate and let oil and pressure into the gasket
itself. Putting pressure on a gasket in this way is rather unusual and cork
gaskets have all the characteristics that would make them failure prone even
though perfectly adequate in other situations.
--
Roger Long
It really doesn't matter. On the dry pump, there's solid metal on the
other
side of that hole. There's no way it's gonna leak.

George Patterson
Brute force has an elegance all its own.

The gaskets we use on that pump are a black composition material; very
strong and solid. Haven't seen cork in a long time, and never a rubber
gasket.
I agree that any cork gasket there should be replaced with a new
composition gasket.
As far as ADs that aren't, there's another one that should be: the
horizontal stabilizer forward spar centre section cracks and could
fail. Cessna has a service bulletin on it, but sooner or later someone
will die because it wasn't mandated. Those who push down on their
stabs to turn the airplane are almost certainly going to have cracked
stab spars.
Dan

Umm...

Whats the difference between applying enough force by hand over the tail
spar to raise the nose (by pivoting on the main gear) and using enough
force in flight to push the tail down? How is one more likely to cause
cracks than the other?

Dave

Dan Thomas wrote:

The gaskets we use on that pump are a black composition material; very
strong and solid. Haven't seen cork in a long time, and never a rubber
gasket.
I agree that any cork gasket there should be replaced with a new
composition gasket.
As far as ADs that aren't, there's another one that should be: the
horizontal stabilizer forward spar centre section cracks and could
fail. Cessna has a service bulletin on it, but sooner or later someone
will die because it wasn't mandated. Those who push down on their
stabs to turn the airplane are almost certainly going to have cracked
stab spars.
Dan

Dave S wrote in message ...
Umm...

Whats the difference between applying enough force by hand over the tail
spar to raise the nose (by pivoting on the main gear) and using enough
force in flight to push the tail down? How is one more likely to cause
cracks than the other?

Dave

The stabilizer is designed to counter the force couple generated by
the centre of lift being behind the C of G. This distance is only a
few inches, while the distance from the CG to the main gear contact
point on the ground is much more than that. Worse, the CG is normally
farther forward when the pilot moves the airplane that way, since the
aircraft is more or less unloaded. The forces required to lift the
nose on the ground are therefore higher than they are in flight, are
concentrated in a small area, and are assymmetrical. I have seen
pilots pushing down on the stab well out on it, not right against the
fuselage, and the loads can become much too high.
We have replaced stab nose ribs crushed by this maneuver, and have
replaced an entire stab due to centre-section cracking. In their
Service Bulletin that addresses this issue, Cessna specifically warns
against using the stab to move the airplane.
I once ferried a 172, and in its subsequent acceptance inspection
we found that the stab spar was broken all the way through the web and
both flanges. Only the skin was holding it together. Any abrupt or
high-G maneuver would have failed it. And this aircraft had just had
an annual "inspection!"

Dan

Dan Thomas wrote:

The gaskets we use on that pump are a black composition material; very
strong and solid. Haven't seen cork in a long time, and never a rubber
gasket.
I agree that any cork gasket there should be replaced with a new
composition gasket.
As far as ADs that aren't, there's another one that should be: the
horizontal stabilizer forward spar centre section cracks and could
fail. Cessna has a service bulletin on it, but sooner or later someone
will die because it wasn't mandated. Those who push down on their
stabs to turn the airplane are almost certainly going to have cracked
stab spars.
Dan

Dan Thomas wrote:

The gaskets we use on that pump are a black composition material; very
strong and solid. Haven't seen cork in a long time, and never a rubber
gasket.

Well, since I was under the cowl yesterday anyway, I checked. Thin black line,
so it's not cork.

George Patterson
Brute force has an elegance all its own.