9.2 Flexible Hoses, Installation Geometry & Serviceability

9.2 Flexible Hoses, Installation Geometry & Serviceability

Flexible hoses accommodate relative movement, vibration, installation tolerance, and connections that rigid tubing cannot serve safely. Flexibility does not make routing arbitrary. A hose can fail through pressure, temperature, chemical incompatibility, ageing, twist, minimum-bend-radius violation, abrasion, fire exposure, bad fitting assembly, or movement beyond its designed envelope.

Identification and selection

Use the exact part number or an approved equivalent. Verify the hose construction, size, pressure class, temperature range, fluid compatibility, electrical or fire-protection features, and end fittings against current data. External similarity is not proof of interchangeability. A replacement also needs the correct release documentation, storage history, and life status.

Elastomeric, thermoplastic, metal, and PTFE-lined hoses behave differently. Some assemblies have calendar or installation lives; others are controlled by condition, cycles, or a maintenance programme. PTFE resists many fluids and temperatures, but the complete assembly can still be limited by braid damage, fitting condition, liner degradation, fire sleeve, storage, contamination, or the approved programme. PTFE therefore does not mean unlimited life.

Routing and motion

First place the end fittings in natural alignment. Do not use the hose as a torsion spring to force a fitting into position. A continuous lay line makes twist easier to see, but acceptance is based on the approved installation. Respect the specified minimum bend radius; a tighter bend can flatten the bore, overstress reinforcement, and concentrate strain near a fitting.

Slack is task-specific. Too little can place tension on fittings or prevent full travel. Too much can whip, chafe, trap fluid, foul controls, or contact heat. The required allowance depends on length, pressure change, temperature, component movement, and hose construction. Generic percentage rules are training illustrations, not universal acceptance criteria.

Route away from exhausts, hot air, electrical arcing, sharp edges, moving controls, and areas where leaked fluid would create a hazard. Use the specified clamps, stand-offs, sleeves, grommets, and fire protection. A clamp should support the assembly without crushing it. Check the complete movement envelope, including landing-gear travel, engine movement, steering, flight-control travel, and maintenance access where applicable.

Inspection

Clean sufficiently to inspect without driving contamination into the system. Look for wetness, staining, blisters, soft or hardened areas, cracking, exposed or broken braid, flattening, kinks, pulled fittings, corrosion, damaged fire sleeve, chafe, and heat effects. Check security and evidence of relative movement at clamps and fittings. The maintenance data determine allowable condition and disposition; do not repair a pressure hose with tape, an improvised sleeve, or an unauthorised fitting adjustment.

A leak is not always at the point where fluid appears. Clean, dry, operate or pressurise only as authorised, and trace the source while respecting fluid-injection hazards. Never use a hand to search for a high-pressure pinhole leak.

Assembly and testing

Hose fabrication is performed only with approved materials, tooling, instructions, and authorisation. Cleanliness caps, cut quality, fitting insertion, crimp dimensions, angular alignment, and identification are controlled. Proof testing or leakage testing uses the specified fluid or gas, pressure, ramp rate, hold time, temperature, and acceptance criteria. The assembly is restrained or enclosed because stored pressure can release violently. Oxygen-system hose preparation has additional cleanliness controls.

After installation, remove protective caps at the latest practical point, prevent contamination, torque fittings by the specified method, fit locking devices, restore clamps and guards, and perform required leak and functional checks. Record part identity and life data as required.

The reliable exam answer is the data-led one: avoid twist, respect the stated bend radius, provide only the movement allowance specified, protect against chafe and heat, and apply the component’s actual life and test requirements.

Worked installation check

Suppose a replacement hose connects a pump on a vibration-mounted assembly to a fixed bulkhead union. Before tightening, confirm that both fittings line up naturally and that the hose part number and fire protection match the illustrated parts data. Position the hose so the lay line is not spiralled. Move the supported assembly through the range required by the task and observe the hose near each end fitting, clamp, adjacent structure, and heat source. The hose must not become taut, fold, rub, or drive a side load into the union. Install clamps in the stated positions and confirm they support rather than flatten the hose.

After torquing and locking the fittings by approved data, restore the system and carry out the specified leakage and functional test. A stain that reappears after cleaning is investigated at its true source; tightening a fitting beyond its prescribed value is not a safe response. Reinspect the hose under operating pressure and after movement if required, because pressure can change length or stiffness. Finally record identity and life information and confirm that no cap, plug, tool, or cleaning material remains in the system. This sequence tests configuration, motion, security, and leakage rather than relying on appearance at one static position.

Installation Inspection Matrix