Fluid Lines and Fittings
Fluid-line questions are about materials, flares, and the handling rules that prevent leaks and failures.
Tube versus pipe. Tubing is sized by outside diameter and wall thickness; pipe is sized by nominal inside diameter. Aircraft hydraulic, fuel, and oil lines are tubing. Common aluminum alloys are 1100 and 3003 for low pressure, 5052 for good formability and corrosion resistance, and 2024 or 6061 where higher strength is needed. Corrosion-resistant steel (304, 321) and titanium serve high-pressure and high-temperature systems. Never substitute a lower-strength tube.
Flares and fittings. Aviation flare fittings use a 37-degree flare; the 45-degree flare is automotive and will not seal against an AN fitting. A good flare is smooth, centered, and free of cracks or thinning at the lip. Flareless (ferrule) fittings use a sleeve; overtightening crushes the tube.
Flexible hose. A hose has an inner liner, reinforcing braid, and cover, and is marked with size, cure date, and pressure rating. Respect the minimum bend radius, support the run, and never twist the hose to align a fitting.
Traps. Questions ask for bend radius measured to the centerline, whether thread lubricant is allowed (only as specified, and never pipe dope on flare seats), and why aluminum and steel fittings mated in a wet area suffer galvanic corrosion.
Rule of thumb. Bend radius is never less than the manufacturer's chart, roughly 3× outside diameter for flexible hose and 2–3× for annealed tubing. The maintenance manual always governs.