Servicing hydraulic and pneumatic systems

Seven procedures. Procedure 0 is not optional and is not a formality.

Procedure 0 — Relieve and confirm (R1, R2)

1. Shut the system down per the manual.
2. Relieve pressure by the manual's method. This is usually a specific action, not simply waiting.
3. Confirm on a gauge that pressure is zero — including any accumulator, which holds pressure independently of the pump (AF-F Concepts §1).
4. Discharge the accumulator's fluid side if you are working on it or downstream of it.
5. PPE for the fluid, and containment.

Check — say what the gauge reads, and which gauge. If there is no gauge on the circuit you are opening, you have not confirmed anything.

Failure — opening a component downstream of a charged accumulator. The system is off, the pump is stopped, and the energy is still there.

Procedure 1 — Identify the fluid and the seals (S1, S2, S15)

1. Read the placard and the manual for the approved fluid. That is the authority.
2. Check what is actually in the container you intend to use — label, color, and whether it has been decanted.
3. Confirm the seal material is compatible with that fluid before fitting anything (AF-F Concepts §2).
4. Never mix families. If a system's fluid is in doubt, that is a finding.

Check — name the fluid specification, not its color. Color is a cross-check on a correctly labeled container, not an identification.

Failure — topping up from an unlabeled can. The aircraft flies fine, and every seal in the system begins failing weeks later.

Procedure 2 — Install seals and backup rings (S3, S16)

1. Confirm the part number of the packing against the component manual, and check its cure date.
2. Inspect the groove and the mating surfaces — burrs, scratches and corrosion will cut the seal.
3. Protect the seal on entry: thread protectors, installation tools, tape over sharp edges.
4. Lubricate with the system fluid unless the manual says otherwise.
5. Fit the backup ring on the low-pressure side of the packing — the side away from the pressure it must resist extrusion toward (AF-F Concepts §3).
6. Assemble without twisting the packing in its groove.
7. Torque per the manual and operationally check.

Check — state which side is the pressure side before fitting the backup ring. If you cannot, you cannot fit it correctly, and it will look fitted either way.

Failure — a nicked seal on installation. It seals on the bench and weeps in service.

Procedure 3 — Service an accumulator (S7)

1. Procedure 0 first, and discharge the fluid side completely.
2. Check the gas precharge with the fluid side discharged — a precharge read with fluid pressure present is meaningless.
3. Compare against the manual's precharge, at the stated temperature.
4. Charge with the specified gas — usually dry nitrogen, never oxygen and never shop air.
5. Recheck after the charge has stabilized.
6. Restore the system and check operation.

Check — the fluid side must be fully discharged before you read the precharge. A reading taken with fluid pressure present tells you about the system, not the accumulator.

Failure — charging with the wrong gas. Compressed air introduces moisture and oxygen; oxygen with hydraulic fluid is a fire.

Procedure 4 — Service a reservoir (S8)

1. Check the level in the condition the manual specifies — system pressurized or not, gear and flaps in a stated position, accumulator charged or discharged. The level moves with all of these.
2. Clean around the filler before opening.
3. Add the approved fluid only, to the specified level.
4. Check for leaks and record.

Check — confirm the configuration the level is specified in. A reservoir filled to the mark with the gear up will overflow when the gear comes down.

Procedure 5 — Service a filter (S6)

1. Procedure 0.
2. Note the bypass indicator before removal — a popped indicator is a finding, and it means unfiltered fluid has been circulating.
3. Remove, catching fluid.
4. Inspect the element for debris, and identify what you find (PP-G §1 — the same reasoning in oil).
5. Fit a new element with new seals, torque per the manual.
6. Reset the indicator, restore the system, check for leaks.

Check — look at the element before you throw it away. It is the only evidence you will get about what is happening inside the system, and metal in it is a finding regardless of the indicator.

Procedure 6 — Remove and install a component (S4, S9, S12)

1. Procedure 0.
2. Cap every line and port as it is opened (GEN-F Procedures Procedure 1).
3. Support the component; note the orientation and any shims or clocking.
4. Fit new seals per Procedure 2.
5. Install, torque per the manual, holding fittings with a second wrench (GEN-D Procedures Procedure 6).
6. Operational check: pressurize, check for leaks, check function through full travel or full range.
7. Purge air if the system was opened (Procedure 7).

Check — an operational check is part of the installation, not a separate job. A component fitted and not checked is a component you have not finished installing.

Procedure 7 — Purge air (S11)

1. Follow the manual's bleeding sequence — order matters, and the highest point or the furthest actuator is usually first.
2. Operate the system through full travel repeatedly to move air to the bleed points.
3. Bleed until the fluid runs clear and free of bubbles.
4. Maintain reservoir level throughout — running it dry draws in more air and you start again.
5. Check operation for firmness and consistency.

Check — operate the system several times and confirm the response is consistent. Spongy or varying response means air remains (AF-F Concepts §4).

Failure — letting the reservoir run low during bleeding. It undoes the whole procedure and it is easy to do.

The checks, collected

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