Fuel line Fire Protection

[mophie]

Hi

regulation require fireproof (15min @ 2000f) lines and fittings in designated firezones (engine compartment). Does anyone knows if aluminum alloy fittings are accepted by the FAA?

Thanks for info!

Rob

 

[WKTaylor]

mophie...

What is the melting temperature of most aluminum alloys?

Regards, Wil Taylor

 

[RPstress]

An aluminium fitting would not normally be acceptable. The Jetstream 32 horsecollar was arguably a fitting, and was a solid bent bar of aluminium. It passed the test of the 2000 F burner on it with load. However, that was to Part 23 and was in the late 70s or early 80s, and I don't think you'd get away with it even to Part 23 these days, let alone Part 25. (It didn't melt because the burner flame is a limited size, and it was able to conduct the heat away to the cool(er) areas.)

You can get away with an aluminium sheet as a fire barrier IF it has adequate cooling air flowing over the back face. Aluminium has also been used as a fire barrier if coated with intumescent paint or similar. However, it might be problematic to get the authorities to accept such coatngs these days (you have to prove that the coating won't come off, etc.), and also some of the materials are toxic. NB: carbon/epoxy can be a weight and cost effective alternative to titanium for lightly loaded firewalls.

For fittings, fire zones usually use titanium and stainless steel. However, the ATP had some carbon/epoxy struts in the nacelle, and these were protected by stainless steel and insulation. So, you might get away with aluminium if it was adequately protected. Usually the added expense and testing of such things means you're better off with titanium, in spite of the astronomical raw material cost (I'm a bit out of date, but last time I checked 6-4 was above $60/lb).

 

[mophie]

Would alloy fittings at least be accepted as fire-resistant (5 min. @ 2000°F)?

 

[RPstress]

I think that if it was even possible you'd have to prove it with a test, possibly including loading. (I don't think a structural/thermal analysis would be acceptable.) With/without loading may depend on exactly what the fitting is doing. Anything not very robust (the horse collar I mentioned was about 3" square) or small will likely fail. From memory the burner flame is about 8" across. Even a chunky part which is fully inside the flame will probably fail.

 

[WKTaylor]

Mophie...

If You can pass the rigorous SAE AS1055 testing with aluminum, more power-to-You. Be advised... the test involves a fully functional system line at operating pressures/temperatures/fluid flow-rates [with the specified flammable fluid]; and air-flow on outside. Then hit it with the specified fire.

Thick aluminum fittings might survive... but I doubt that aluminum tubing will.

If this is for a UAV anything goes. Man-rated air vehicles require serious attention to possibility of engine-bay fire. Wouldn't it be a shame to pass the test... but burn-up a real acft after a 6-minute in-flight fire? If aluminum is mandatory, then consider installing a fire-sleeve per AS1072.

Note: I investigated a mishap acft that had a small fire under and aluminum oxygen line. After several minutes, during an emergency descent, the O2 line ruptured, feeding the fire to "blow-torch" intensity. The Copilot burned to death in seconds; the pilot ejected with major burns and a barely functional, smoldering parachute (survived).

Regards, Wil Taylor

 

[Boathouse]

If you have detection, suppression and a shutoff valve on the firewall that is fireproof you may well get away with aluminum tubes / fittings for fuel. Rational being that you detect the fire, shut off the supply of flammable fluid and suppress the residual fire all within the fire resistant 5 minute window.

Engine oil is different since you cannot shut off the source of flammable fluid i.e. if a line breaks the engine will, even if shut down, windmill all the oil it can on to the fire.

My advice would be to talk to a DER with the relevant delegations to get a direction in which to go. The FAA website has a listing of all their current consultant DER's with addresses and delegations.

Good luck and if you do talk to a DER, let us know what he says!!

Regards

Boathouse