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Fluid lines, like everything else on your airplane,
deserve careful consideration before application. There are several
technologies available and each has its advantages and disadvantages.
There is no single solution that works best in all situations.
Aluminum hard line is the lightest and cheapest. It is easy to
fabricate and, with careful installation and chafe protection, will
last the life of the airframe. Aluminum hard line is suitable for
hydraulic, brake, fuel and vent lines aft of the firewall where both
ends are stationary.
Stainless steel hard line is three times heavier than aluminum but
still lighter and cheaper than hose. It is well suited for high
vibration and higher temperature applications where where both ends
are stationary or nearly stationary. I used stainless hard line
between the gear legs and brake calipers by bending the tube into a
540 degree loop so it would act as a spring. I also used it firewall
forward for the on firewall plumbing and the on engine plumbing. I used
-4 fire sleeve to insulate fuel supply and distribution lines to reduce
hot soak fuel vaporization.
Teflon/ stainless braid/ silicone hose (Stratoflex 124J) is the best
solution for firewall to engine and nose gear actuation cylinder
connections. It has an infinite service life and should be replaced on
condition. Sacramento Sky Ranch has an excellent policy where they will
fabricate, clean and pressure test a hose assembly and send it to you
with an unconditional satisfaction guarantee. They will replace it if
it is the wrong length even if you told them to make it the wrong
length. Weight is 6 to 10 times that of aluminum but less than rubber
hose. Over the service life of the aircraft, these are the most
economical hoses.
DH Instruments test hose is the best solution for the flexible portions
of the brake lines (gear leg and petal cylinders). They have been
exhaustively covered in previous threads.
Aeroquip 6xx series (synthetic rubber / stainless over braid) with
field terminable ends is an aesthetically attractive hose that is well
suited for hot rod show cars but has limited application in aircraft,
particularly composite aircraft. The only apparent advantage of this
hose is that the components (hose, fittings, fire sleeve and clamps)
can be purchased for ~60% the cost of a similar assembled, cleaned and
tested 124 assembly. Disadvantages include; stray over braid wires
(AKA "Meat Hooks"), ID Bore contamination from manufacturing, finite
service life, abrasive outer braid, tendency for the fitting to skive a
flap out of the hose ID during assembly, poor termination consistency
due to inexperienced assembler and a tendency to kink. You are giving
up a lot to save 40% on a dozen hose assemblies. Assembly labor time is
comparable to the aluminum hard line fabrication time. These hoses are
the supermodels of the hose world, pretty to look at but expensive to
maintain, a bitch to live with and no ability to age gracefully. That
shiny over braid will grind through any painted, composite or aluminum
surface it touches. Did I mention it is conductive? Great fun near
anything with electricity flowing.
Nylaflow and Poly-flow non-reinforced tubing is suitable for pitot
static system, door seal pressure system, pilot relief and low pressure
02 (not to be confused) plumbing. Non-reinforced aquarium tubing should
NOT be used in the brake system. It WILL fail.
One additional tip that expands on the hose pressure test, during
building it is handy to be able to test the various valves and
actuators. This can be done by connecting the pressure supply line to
compressed (shop) air. This lets you check gear and flaps range of
motion and geometry safely without flooding the shop with red oil
(5606). A secondary advantage is that you can bubble leak check all the
fittings before putting oil in the system. Be sure your shop air is dry
and adding a few drops of air tool oil will keep the valves and
actuators lubed.
Regards
Brent Regan
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