Stainless hydraulic fittings for aviation ground support and test equipment

Where stainless steel fittings are used in aviation

• Ground support equipment (GSE): hydraulic circuits of aircraft jacks, tail and wing support systems, hydraulic ground power units, maintenance platforms and service vehicles.
• Hydraulic test benches: benches where actuators, pumps, valves and landing gear components undergo post-maintenance pressure, leak and function tests.
• Maintenance workshops: test adapters, pressure measuring points, transition fittings and service connections that are frequently connected and disconnected.
• Aviation manufacturing facilities: press and fixture hydraulics used in metal and composite part production, process cooling and auxiliary lines.

Ground support equipment works in apron conditions: rain, de-icing fluids and constant relocation expose connections to both corrosion and impact. That is why stainless steel is a practical choice for exposed hose ends and adapters, extending their service life.

Cleanliness: the invisible tolerance

On a hydraulic test bench, contamination is carried straight into the component under test. This is why, in aviation applications, the cleanliness of a fitting matters as much as its dimensions. After machining, sharp burrs are removed and thread roots and transition areas are inspected; the goal is a clean internal surface free of chips and burr residue. Before shipping, threads and sealing surfaces are closed off with protective caps so that the part stays clean until assembly.

System cleanliness is usually specified with a cleanliness code such as ISO 4406. The target cleanliness level is a property not of a single part but of the entire circuit after assembly and flushing; still, starting with clean parts shortens the path to that target. The same care applies to the hose assemblies used on test benches: the ends of made-up hoses should be kept capped, and the hoses should be flushed before they are connected to the circuit.

Fluid and seal compatibility

The hydraulic fluids used in aviation can differ from those in industrial hydraulics. Fire-resistant phosphate ester-based fluids, common in civil aircraft, attack the NBR seals that are suitable for mineral oil; test benches and ground equipment operating with these fluids should use a compatible seal material such as EPDM. With mineral or synthetic hydrocarbon-based fluids, NBR or FKM seals can be used. The stainless steel body is compatible with both fluid families; what matters is specifying the correct O-ring and seal material.

Connection standards

Although JIC 37° fittings have a similar geometry to the 37° flared fittings in aviation standards, they are not the same in terms of tolerances and documentation requirements. Using JIC fittings on ground equipment does not mean that a JIC part can be installed in place of an aviation-standard part on the aircraft.

Because components built to different standards are connected to the same pressure source on test benches, conversion adapters are used extensively. Each transition should be defined together with the thread standard and sealing method on both sides: for example, an adapter that connects a JIC 37° hose end to a BSPP port seals by cone seating on one side and by a seal on the other.

Practical notes for test bench connections

• On test adapters that are frequently connected and disconnected, use a suitable anti-seize compound to prevent thread galling, and replace worn adapters from a spare set.
• Assess test pressure separately from working pressure; fitting selection should be based on the highest test pressure the bench will apply.
• Position pressure gauge and transmitter connections so that they are protected from vibration and impact.
• If you test different component types on the same bench, standardize your conversion adapters as a fixed set so that nobody has to hunt for a new adapter for every test.

Process lines in manufacturing facilities

In aviation manufacturing facilities, hydraulic and process lines are concentrated less on the aircraft part itself than on the machines that produce it: forming presses, clamping cylinders on assembly fixtures, workholding hydraulics on machining centers and process cooling circuits. On these lines, long service life, regular maintenance and fewer production stoppages are the priorities. Stainless steel fittings reduce corrosion in areas that come into contact with coolant and moisture; fixtures, in turn, need elbows that fit into tight spaces and durable adapters at points that are frequently connected and disconnected. Using the same connection type for the same size throughout the facility reduces spare parts variety and the maintenance team's margin for error.

Lead times and custom manufacturing

Standard sizes are prepared within 1-3 business days on average depending on stock availability, and standard production items within 5-10 business days. Sample-based or custom-size requests are added to the production schedule after technical approval and planned for completion within 10-20 business days on average. Critical diameters, thread standard, sealing type and installation dimensions are mutually confirmed before production; series production begins after first article approval.