Of primary concern are fittings that connect a hose or tubing from the
engine (a real mover and shaker) to any fixed point, like the firewall, for
instance. A fuel line running from a device on the firewall (perhaps the
gascolator or maybe the boost pump) to the engine is definitely a prime
candidate for steel fittings at both ends. As I understand it, the main
problem here is the low-frequency stressing of the fittings due to relative
movement between the engine and airframe. For peace of mind (and safety), it
is prudent to opt for high-strength (steel) fittings where such flexure is
imposed, and that goes for the hose-end fitting as well.

The cylinder head oil return fittings (aluminum on Lycs) run between points
on the engine (not between engine and a fixed dynamic ground), and are more
likely to be affected by resonant vibration. However, they are so short that
their resonant frequency is probably too high to be excited by any vibration
the engine might produce. Since there's not much of a history of failure (if
any) over the time that Lycomings have been using Al for these fittings,
that's a pretty good life test that probably proves the point.

On the other hand, Al fittings connected to an engine, and in particular
ones that connect to an unsupported array of hoses or tubes, are suspect
unless you can prove - by vibration testing - that they have no adverse
resonant responses.

You can test them in a qualified lab (not cheap or easy) or in the air in
flight (easy but possibly deadly). IMHO, using steel fittings is a whole
bunch cheaper, easier and definitely safer. On my early LNC2, I used steel
for all gas and oil fittings that could possibly flex or be stressed by a
shaking hose and purchased hoses with SS end-fittings from Sacramento Sky
Ranch (which also included integral fire-sleeve). Why take a chance?

Dan Schaefer