Chapter 4: Fluid Lines and Fittings

Chapter: Fluid Lines and Fittings

Overview

This chapter covers the installation, inspection, maintenance, and repair of aircraft fluid lines and fittings. Fluid lines are the lifelines of an aircraft, carrying hydraulic fluid, fuel, oil, and pneumatic pressure to critical systems. The integrity of these lines is paramount to safe flight. This material addresses the standards set forth in FAA Advisory Circular (AC) 43.13-1B, which is the definitive guide for acceptable methods, techniques, and practices for aircraft maintenance. The content focuses on the classification of lines, material selection, fabrication techniques, inspection criteria, and the identification of defects, all of which are essential knowledge for the Aircraft Maintenance Engineer (AME).

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Key Concepts

1. Classification of Fluid Lines

Aircraft fluid lines are broadly categorized into two types: rigid tubing and flexible hose. Each has specific applications, materials, and installation requirements.

• Rigid Tubing: Used in areas where lines are relatively straight, fixed, and not subject to significant vibration or relative motion between components. They are typically made from aluminum alloy, corrosion-resistant steel, or titanium.
• Flexible Hose: Used in areas subject to vibration, where components move relative to each other (e.g., engine-to-airframe connections), or where space constraints make bending rigid tubing impractical. They consist of an inner tube, a reinforcement layer (braid), and an outer cover.

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2. Rigid Tubing: Materials and Selection

The selection of material for a rigid fluid line is critical and must be made in accordance with the aircraft manufacturer's maintenance manual or AC 43.13-1B. Using an unapproved material is a violation of 14 CFR 43.13(a), which mandates the use of acceptable methods and practices.

• Aluminum Alloy: The most common material for general-purpose hydraulic, pneumatic, and fuel systems.
• 5052 and 6061-T6: These alloys are widely approved for general fluid line use due to their good corrosion resistance, formability, and strength.
• 2024-T3: This alloy offers a high strength-to-weight ratio and is also used in fluid lines. However, it is not universally approved for all applications (e.g., some brake systems) due to its susceptibility to stress corrosion cracking in certain environments. The mechanic must verify its approval for the specific application.
• Corrosion-Resistant Steel: Used in high-pressure systems or areas with high temperatures or where abrasion resistance is required.
• Titanium: Used in high-temperature and high-corrosion environments, offering excellent strength and light weight, though it is more expensive and harder to work with.

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3. Rigid Tubing: Inspection Criteria and Defect Limits

The inspection of rigid tubing is a key task during scheduled maintenance. AC 43.13-1B provides specific limits for acceptable defects. A line must be rejected and replaced if a defect exceeds these limits.

Dents:

• A dent in the straight section of a tube is acceptable if it does not exceed 20 percent of the tube's outside diameter.
• A dent that is located in a bend is not acceptable and requires replacement, regardless of depth.
• A dent that reduces the inside diameter by more than 10 percent is not acceptable, as it can restrict fluid flow and cause pressure drops.
• A deep crease or kink is a severe defect and is grounds for immediate replacement.

Nicks, Scratches, and Scores:

• Minor surface damage, such as nicks or scratches, is acceptable if the depth does not exceed 10 percent of the tube's wall thickness.
• These minor defects can be smoothed out with a fine file or crocus cloth to remove stress risers, provided the remaining wall thickness is not reduced below the minimum allowable.
• A score or nick deeper than 10 percent of the wall thickness is unacceptable and requires tube replacement.

Cracks and Bows:

• Cracks: Any crack in a rigid tube is unacceptable. Cracks create stress concentrations that can propagate under pressure and lead to catastrophic failure. Welding or stop-drilling is not an approved repair for fluid lines. The tube must be replaced.
• Bows or Bends: A tube must be free from any bow or curvature that is not part of the original design. A bow can cause undue stress on fittings and lead to fatigue failure. A tube with a bow must be rejected and replaced.

Flattening at Bends:

• When a tube is bent, some flattening is inevitable. The maximum allowable flattening is 10 percent of the original diameter. This is measured as the difference between the maximum and minimum diameters at the bend. Excessive flattening restricts flow and weakens the tube.

Chafing and Abrasion:

• Chafing that removes material from the tube's surface compromises its structural integrity. Lines with visible abrasion, grooves, or wear must be replaced. The cause of the chafing (e.g., a misaligned clamp) must be investigated and corrected to prevent recurrence.

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4. Rigid Tubing: Fabrication and Bending

Fabricating a new rigid line requires precision and adherence to standards.

Bend Radius:

• The minimum bend radius for a tube depends on its material and diameter.
• For aluminum alloy tubing bent with a hand bender, the minimum bend radius is three times the tube's outside diameter. This prevents excessive work hardening and cracking.
• Using a radius smaller than the minimum will cause distortion, flattening, or weakening of the tube.

Flaring:

• Flared fittings are the most common method for joining rigid metal lines. The flaring process creates a sealing surface at the end of the tube.
• AN (Army-Navy) fittings require a 37-degree flare. This specific angle is critical for proper seating and a leak-tight seal.
• A 45-degree flare is used for automotive or other non-aviation fittings and is not compatible with AN fittings.
• A properly formed flare must be smooth, concentric, and free of cracks, tool marks, or scoring.
• A cracked flare is a structural defect. The damaged section must be cut off and a new flare formed, provided sufficient tube length remains. If the line is too short, the entire line must be replaced.

Flareless Fittings:

• Flareless fittings use a sleeve that bites into the tube to create a seal. They are often used in high-pressure systems.
• If, after initial torquing, the sleeve has not fully gripped the tube, the fitting must be disassembled and inspected. The sleeve may be defective or the tube damaged. Over-torquing to force a grip is not acceptable. The sleeve should be replaced if necessary before re-tightening.

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5. Flexible Hoses: Installation and Inspection

Flexible hoses are more susceptible to damage from environmental factors and improper installation than rigid lines.

Installation Practices:

• Bend Radius: The minimum bend radius for a flexible hose is 10 times the outside diameter of the hose. Bending a hose tighter than this can cause kinking, restrict fluid flow, and lead to premature failure.
• Natural Curvature: A hose should be installed so that its natural curvature is followed. Twisting the hose during installation can cause premature failure and restrict flow.
• Routing: Hoses must be routed to avoid contact with structure, control cables, or other components. Chafing can lead to failure. If contact is unavoidable, a protective sleeve may be used, but the best practice is to re-route the hose.
• Length: The hose must be long enough to allow for the required bend radius without placing undue stress on the fittings.

Inspection and Service Life:

• Age: Hoses have a finite service life, regardless of visual appearance. Internal degradation may not be visible externally. Most manufacturers recommend replacement of rubber hoses at intervals of 5 to 10 years from the date of manufacture. Age alone is a valid reason for replacement.
• Chafing and Damage: A hose must be replaced if the outer cover is cut or chafed to the extent that the reinforcement braid is exposed. The braid is critical for the hose's pressure rating, and any damage can lead to failure under pressure.
• Swelling and Soft Spots: A swollen or soft spot in a hose indicates internal degradation, often due to fluid breakdown, heat, or age. This can lead to rupture and requires immediate replacement.
• Blisters or Bubbles: A bubble or blister in the outer cover is a sign of liner or reinforcement deterioration. The hose must be replaced.
• Heat Damage: Hoses routed too close to heat sources (e.g., exhaust manifolds) can show signs of heat damage, such as discoloration and hardening. This degrades the material and requires replacement. Lines should be routed with adequate clearance, typically at least 2 inches from exhaust manifolds.

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6. Fittings and Connections

Proper fitting selection and installation are essential for a leak-free system.

AN Fittings:

• AN fittings are sized by the outside diameter of the tube in 1/16-inch increments. For example, a 3/8-inch OD tube is 6/16-inch, so the fitting is designated AN-6.
• Correct identification is critical for proper fit and safety.

B-Nuts:

• A B-nut is the coupling nut used to secure a flared tube to a fitting.
• A small leak at a B-nut is often due to insufficient torque. The correct first action is to check and tighten the B-nut to the manufacturer's specified torque value.
• If a B-nut is extremely tight and will not break loose, the correct technique is to use a correctly sized wrench and a sharp tap with a mallet to break the corrosion or torque holding the nut. Using a cheater bar, heat, or penetrating oil is not recommended due to the risk of damage, fire, or contamination.

Compression Fittings:

• Compression fittings are not approved for aircraft fluid lines. They require precise tube dimensions to form a proper seal. An undersized tube will not seal correctly and can leak or fail under pressure.

Seals and O-Rings:

• A persistent leak at a tight fitting often indicates a damaged O-ring or seal. Seals should be inspected during assembly and replaced if damaged.

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Important Regulations and Procedures

• 14 CFR 43.13(a): This regulation requires that all maintenance be performed using acceptable methods, techniques, and practices. This includes using approved materials and following the standards in AC 43.13-1B.
• 14 CFR 43.9: This regulation requires that maintenance records include a description of the work performed, including part numbers and material specifications for replacements. A logbook entry that lacks this information is incomplete and does not comply with regulations.
• AC 43.13-1B: This Advisory Circular is the primary reference for acceptable methods, techniques, and practices for aircraft inspection and repair. Chapters 7, 9, and 10 contain the specific guidance for fluid lines and fittings.

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Common Relationships and Troubleshooting

• Pressure Drop: A rapid pressure drop in a hydraulic system after shutoff indicates a leak. Common causes include loose fittings, damaged flares, or cracked hoses. The system must be inspected to locate and rectify the leak.
• Spongy Brake Pedal: A swollen or soft spot in a flexible hose can cause a spongy brake pedal. This indicates internal degradation and requires hose replacement.
• Chafing: Chafing is a common cause of line failure. It can occur on both rigid tubes and flexible hoses. The correct action is to replace the damaged line and correct the routing or clamping issue that caused the chafing.
• Over-tightening Clamps: Clamps should be tightened only enough to hold the line securely without causing deformation. Over-tightening creates stress concentrations and can damage the line. Visible compression marks on a tube indicate over-tightening.

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Summary of Key Limits and Standards