Replacement 3000 psi Line: Bad Flare, Misalignment and a Blue Stain
A 3/8 in OD 2024-T3 hydraulic line on a transport aircraft is being replaced after chafing was found. The technician cuts the new tube to length with a hacksaw, deburrs, and flares it with the workshop's 37 degree flaring tool, whose die is worn; the resulting flare shows chatter marks and slight ovality. He reuses the original AN818 nut, connects it to the AN815 union, and finds the tube sits about 1.5 mm off centre and has to be pulled into place. The nearest support clamp is 450 mm away on a straight run next to the engine-driven pump; the manual's support spacing table is not consulted. System pressure is 3000 psi. A blue stain is already visible at the B-nut after a ground run.
1. Explain how each defect contributes to the leak, and what the blue stain tells you.
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The 37 degree flare is a controlled metal-to-metal interference fit. The tube cone and union cone must be concentric and matched in angle, and the nut must apply axial preload without distortion. Each defect undermines one of those conditions:
- Hacksaw cut: burrs and a non-square end get trapped under the flare, and swarf enters the system.
- Worn flaring tool: chatter and ovality mean contact occurs on high spots only, so the seal line is not continuous around the circumference.
- 1.5 mm misalignment pulled into place: the tube is preloaded in bending before the nut is snugged, so the nut cocks and the cones seat eccentrically. That both creates the leak path and sets up fatigue cracking at the flare.
- 450 mm unsupported span next to a pump: vibration produces fretting at the seat and tube fatigue; support spacing must come from the manual's table for tube size and vibration environment.
The blue stain is phosphate-ester hydraulic fluid weeping past the seat. On a 3000 psi system this is a fire and system-loss hazard, not cosmetic - it confirms the joint is not sealing.
2. State the corrective actions and the acceptance checks before the system is returned to service.
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Replace, do not re-work
- Scrap the tube. Do not re-flare it: the end is now short, work-hardened and not square, and 2024-T3 cannot be re-heat-treated in the field.
- Cut the new tube with a roller-type cutter (or fine-tooth saw against a square guide), deburr inside and out, and keep the end square.
- Replace or recalibrate the flaring tool before making the flare, then inspect the cone with a gauge and visually for angle, diameter, ovality, chatter and cracks. No sealant or tape on the cone - 37 degree joints seal dry.
- Fit a new nut if the threads or cone are worn, and inspect the union cone; a damaged union cone must be replaced.
Rig and support
- Align so the nut threads on by hand with no side load, removing the 1.5 mm offset; support with the specified cushioned clamps at the manual's spacing and clear of structure and hot parts.
- Torque to the manual value with a calibrated wrench (or by the specified flats method), then pressure-test the system per the manual and check for leaks at working pressure, chafing and correct fluid level.
3. The supervisor suggests simply re-torquing the B-nut and releasing the aircraft. Judge this.
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Reject it. Torque cannot compensate for an eccentric, chattered, oval flare. Adding torque increases preload on a seat that is only touching on part of its circumference; it risks overstressing the flare and cracking the tube, and the leak will return as soon as the system is hot and pressurised.
The decision to make is not 'how tight' but 'this assembly is unairworthy'. Replace the tube, correct the alignment and support, and pressure-test. If any short-term action were contemplated it would need approved data - and no approved scheme covers a badly made flare.
Also record it as a nonconformance: the worn flaring tool is the root cause and may have produced other tubes already fitted. Quarantine the tool, check recent work by the same technician, and confirm no fluid loss or contamination elsewhere in the system before release.