Fabricating and installing fluid lines

Eight procedures. Guided first attempts are in GEN-D Hands-on; this is the procedure once you can hold the tools.

Procedure 0 runs before all of them.

Procedure 0 — Make the system safe (R1, R2, R3, R4)

1. Establish what fluid is in the system and read its safety data sheet. PPE follows from that, not from habit.
2. Depressurize by the manual's method, and confirm it — a gauge reading zero, an accumulator discharged, a system that has been left to bleed down. Confirmed, not assumed.
3. Position the system as the manual requires — gear down and pinned, controls in a stated position, power off.
4. Have containment ready. Fluid will come out.

Check — say what pressure the system is at and how you know. "It should be down by now" is not an answer.

Failure — opening a charged system. R4's injection injury is not a theoretical hazard, and the fluid does not have to be visible to be dangerous.

Procedure 1 — Cut and prepare tube (S1, part 1)

1. Measure and mark, allowing for bends and for the length taken up by flares.
2. Cut square, with a tube cutter, advancing the wheel a little at a time. Forcing it work hardens and ovalises the tube.
3. Deburr inside and out. Inside burrs restrict flow and shed into the system; outside burrs prevent the sleeve seating.
4. Check the cut is square and the tube is round at the end.

Check — look through the tube. A ridge you can see is a burr you have not removed, and it will be in a hydraulic system for the rest of the aircraft's life.

Failure — a rushed cut. Every downstream defect in the flare traces back to this step.

Procedure 2 — Bend tube (S1, part 2)

1. Confirm the minimum bend radius for that tube size and material, from the manual.
2. Use the correct bender for the size. Support the tube fully in the former.
3. Bend smoothly, in one motion where possible.
4. Inspect for flattening, kinking, wrinkling and scoring.

Check — measure the ovality at the bend. A bend that has flattened beyond the allowed percentage is scrap. Judging it by eye passes bends that are out of limits.

Failure — bending tighter than minimum radius because the run needs it to fit. The run is wrong, not the radius.

Procedure 3 — Flare a tube (S1, part 3)

1. Confirm the flare angle required — 37° for aviation. Confirm the tool is the matching angle (GEN-D Concepts §2).
2. Slide the sleeve and nut on first. Every mechanic has flared a tube with the nut still on the bench.
3. Clamp the tube at the correct projection above the die.
4. Form the flare smoothly, without over-forming.
5. Inspect: even wall thickness, no cracks, no splits, correct diameter, square to the tube.

Check — offer the flare to the fitting cone and look at the contact. It should seat on the face, evenly, all the way round. A flare that contacts on a line rather than a face is the wrong angle or is not square.

Failure — nut left off. Discovered after the flare is formed, and the tube must be cut back and re-flared, which is why step 2 exists.

Procedure 4 — Fabricate a flareless connection (S8)

1. Preset the sleeve in the presetting tool or the fitting, per the manual, with the tube bottomed in the fitting.
2. Inspect the preset: the sleeve has bitten into the tube, and there is a visible ridge of metal raised ahead of the sleeve.
3. Assemble to the fitting, finger tight with the tube bottomed.
4. Tighten by the specified turns or flats past finger tight — from the manual, not by feel (GEN-D Concepts §5).

Check — the sleeve must have bitten. Run a fingernail along the tube ahead of the sleeve; you should feel the raised ridge. No ridge means no bite, and no bite means no seal at pressure.

Failure — tightening "until it feels right". Under-tight leaks at pressure only; over-tight cuts the tube.

Procedure 5 — Fabricate a flexible hose (S7)

1. Identify the hose and fittings by part number against the manual. Hose types are not interchangeable and are rated for different fluids, pressures and temperatures.
2. Cut to the length in the assembly instructions — which accounts for the fitting insertion depth, so it is not the same as the installed length.
3. Cut cleanly, without fraying the braid.
4. Assemble the fitting to the specified insertion depth and torque.
5. Proof test where the specification requires it.
6. Label the assembly — date, part number, and the assembly's identification.

Check — measure the finished assembly against the required length, fitting face to fitting face. Getting this wrong is discovered at installation, with the aircraft open.

Failure — using a hose rated for one fluid in a system carrying another. The part number is the check, not appearance.

Procedure 6 — Install a rigid line (S2)

1. Offer the line up without forcing it. A line that must be sprung into place is the wrong line — it will be preloaded for its whole service life and will fail at a fitting.
2. Start both fittings by hand, both ends, before tightening either.
3. Tighten to the specified torque, holding the fitting body with a second wrench so no torque is transmitted into the line or the component (R7).
4. Install clamps at the specified positions and tighten them last.
5. Apply a witness mark where required (GEN-D Concepts §4).

Check — before tightening, let go of the line. It should sit in position on its own, with both fittings started. If it springs away, it does not fit and tightening will not fix it.

Failure — torquing a fitting with a single wrench, so the reaction goes through the line into the component. Cracked castings and cracked flares both come from this.

Procedure 7 — Install a flexible hose (S3)

1. Route it as the installation drawing shows, respecting bend radius, separation and slack.
2. Watch the lay line while tightening. Hold the hose fitting with a second wrench so the hose does not turn.
3. Tighten to specified torque.
4. Check the lay line again after tightening.
5. Clamp as specified. Apply witness marks where required.

Check — the lay line must run straight. If it spirals, the hose is twisted; slacken, untwist and re-tighten (GEN-D Concepts §3).

Failure — a hose installed with twist locked in. It looks right, it passes a casual inspection, and it fails early.

Procedure 8 — Inspect lines and hoses (S4)

Rigid line: chafe, cracks — especially at flares and bends — corrosion, dents and flats, security of clamps, correct separation, witness marks intact, evidence of leakage.

Flexible hose: the lay line straight, cover condition, cracking, blistering, soft spots, exposed braid, leakage at fittings, correct bend radius, chafe, and age or life limit where the hose carries one.

Check — run a hand along the hose (system depressurized) feeling for soft spots. A hose degrading internally can look perfect and feel wrong.

Failure — inspecting hose only where it is easy to see. Failures concentrate at fittings and at bends, which are usually the least accessible parts of the run.

The checks, collected

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