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Date: Fri, 11 Feb 2005 11:10:24 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: Aluminum fittings
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<<I hear people state that we shouldn't use aluminum fittings on the
engine...

I don't get it - Lycoming uses many aluminum fittings on the engine.
Why is it okay for them to do it but not us?  Is their aluminum
better?  Are the locations less likely to cause fatigue?  One person
tried to tell me the fittings could melt.
Aluminum fittings on my Lycoming (non-turbo) include:
* oil return / drains from cylinder heads
* prop governor fitting (the line is SS, not the fitting)
* breather hose
* dipstick housing (actually, I think it is plastic)
And I am fairly certain that the parts that come with an FAA
approved spin-on oil filter kit include many aluminum fittings.

I agree it is important to support the hoses and pipes that lead from
any fitting - but where is AL okay and where is it not okay?  Do the
engine manufacturers have any published comments on this?
Matt>>

Very good questions.  My answer is a little bit of a guess and maybe it will
draw out more educated responses:

Lycoming uses aluminum cylinder head drain line fittings where vibration is
likely to be the highest of any location.  Why?  I assume that is "okay"
because they have a lot of experience with no vibration failures at that
location.  If I attached hardware of a different weight to the same location
I would probably use steel just because I don't want to do extensive
vibration surveys.  I'm sure "their" aluminum isn't better (same AN
fittings).

As I understand it their prop governor fitting is supposed to be steel.
There were some failures of the aluminum lines and I believe the fix was to
change to steel lines - and the steel fitting was specified at the same time
even though the fitting itself wasn't the problem.  If my engine had a steel
fitting there (I checked) I would replace it with steel.

I think the problem with an aluminum line on the turbo housing is that in
the case of an exhaust leak the high-pressure jet of exhaust could actually
melt the fitting (speculation?) and then there would be a high pressure jet
of oil spraying into the exhaust for a guaranteed fire.  The hose is
protected with fire sleeve so it has some life available.

An oil drain has nothing attached to it and the manufacturers can be assumed
(oh, yeah?) to have done their homework to make sure there are no vibration
modes that will cause a problem.

My worry is mostly regarding fatigue failures at the pipe thread that often
is used to attach the fitting to the engine.  Aluminum fittings without pipe
threads, like those on the diaphragm fuel pump, are not so susceptible to
fatigue.  In any event, I wouldn't worry much about a typical fitting
attached to the engine and connected to a standard flexible hose as a
fatigue issue would be very unlikely.  The problem comes from hanging extra
weight on a fitting, like building the fitting out with nipples and
couplings.  Then there is more than ordinary weight all supported by the
pipe thread into the engine.  The resonant frequency of such an assembly is
probably unknown and could be excited by normal engine operating conditions.
There are some places where an error in building resulting in cracked parts
is just a maintenance issue, but a cracked or broken oil line fitting is a
big, big deal.

Gary Casey