I find it helpful to look up FAA docs and read what's considered
airworthy for production aircraft. Here goes.
Flexible hose are perfectly satisfactory, if proper material, and
mandatory firewall fwd where flexing is possible. Problem is likely
the hose barbs and spiral clamps. FAA recently tightened the rules
for hose and fittings from "approved" to a demonstration of
"suitability." In govermentese, that means if Mil Spec and AN stuff
is no longer automatic approval, it's hard to see where the barbs and
spiral clamps would be accepted. It doesn't seem to bother Rotax,
though, whose docs have no bold print warnings about injury or death
like elsewhere.
Per FAA, metal lines have several issues, in comparison to (suitable)
flexible hose. Properly securing them to prevent chafing or any
movement, including under G loads, is important, with Adel type clamps
of course. But where it is secured to a structure with different
expansion coeff due to temperature (like fiberglass), the technique is
joggle bends where determined necessary.
Significant is that metal lines must be electrically bonded to a metal
airframe structure. Otherwise what's then necessary for lightning
protection is beyond the scope of homebuilding. And finally
crashworthiness standards exist only for transports, as in air
carrier.
For my trigear, I have a liberal supply of automotive hose (internal
braid). Since auto fuel is recommended, I can't see where installation
of same in an airplane means it won't perform as in a car, where there
seems to be no problems. The internal braid hose is easier to
inspect, and easy to partially slit to remove, meaning replace rather
than reinstall old hose, a potential source of problems. Everything
is in the tunnel, with large access panel on the fuselage bottom.
Regards,
Fred F.
|