Section 6.6

Pipes & Unions

Rigid pipes and flexible hoses, identification, 37° flared and flareless unions, and installation rules.

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Main ideas

Rigid pipes (aluminium alloy, stainless steel, titanium) and flexible hoses (synthetic rubber or PTFE with braided reinforcement) carry fuel, hydraulic fluid, oil, pneumatic air and oxygen; they are specified by outside diameter and wall thickness.
Pipe runs are identified by colour-coded bands and symbols to a standard scheme so that fuel, hydraulic, pneumatic and oxygen lines are not cross-connected during maintenance.
Rigid-pipe unions are flared (the aircraft standard single flare is 37°) or flareless (bite-type/Ermeto); a B-nut and sleeve clamp the flare/ferrule. Correct flare form and correct torque matter — over-tightening cracks the flare and causes leaks.
Flexible hose has a minimum bend radius and a lay-line to prevent twist; it shortens slightly under pressure so length allowance is needed. Fit fire-sleeves where required, clamp to prevent chafing and vibration fatigue, and observe hose life limits.
Cleanliness is critical: blank open lines with caps/plugs — a single particle can jam a hydraulic valve. Lines are bonded to dissipate static.
⚠ Exam trap: the aircraft flare angle is 37°, NOT the 45° automotive flare — mixing them leaks. And once a PTFE/Teflon hose has been bent to shape and used it takes a set: do not straighten and re-bend it, as it can crack.

Solved examples

A hydraulic line is reassembled with a fitting that will not seal no matter how hard it is tightened, and the flare looks slightly cracked. What are the likely causes?
Either the flare is the wrong angle (a 45° flare on a 37° aircraft fitting, or vice versa) so the seating faces do not match, or the B-nut was over-tightened and split the flare. The fix is to cut off the bad flare, re-flare to the correct 37° with the proper tool, and torque to spec — not to keep tightening.

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