Aircraft Tubing and Hose Maintenance Standards

Tubing and Hose Types

Rigid tubing: Stationary, straight runs.
Flexible hose: Used for movement and vibration.

Examples: Fuel, oil, hydraulic, oxygen, and instrument lines.

Tubing vs. Hose Measurement

Tubing: Measured by Outside Diameter (OD).
Hose: Measured by Inside Diameter (ID).

Size units: 1/16 inch increments.

• #6 = 6/16 = 3/8"
• #8 = 8/16 = 1/2"

Materials

Aluminum

• Low/medium pressure
• ~1000–1500 psi

Steel (CRES)

• High pressure
• 3000+ psi

Titanium

• Stronger and lighter

❌ Never use in oxygen systems due to combustion risk.

Fabricating Tubing

1. Cut
2. Deburr
3. Bend
4. Flare
5. Bead

Cutting Procedures

Use a tubing cutter.

• Tighten lightly
• Rotate
• Tighten gradually

❌ Warning: Too tight will deform the tube.

Deburring

Removes internal burrs to:

• Prevent flow restriction
• Prevent flare cracking

Use a deburring tool and finish with a flat sandpaper surface.

Bending

Use a tube bender. Hand bending is only allowed for 1/8" and 3/16" sizes. Tools are required for sizes ≥ 1/4".

Avoid: Kinks, wrinkles, and flattening. Maximum flattening allowed is 75% of the original OD.

Tube Damage Limits

Maximum allowable damage is 10% of wall thickness. If damage exceeds this, replace the tube.

Flaring

Aircraft flare: 37°
Automotive flare: 45° (Do not use)

Protrusion: Extend tube above the die by the thickness of a dime.

Assembly Parts

• Tube
• Sleeve (ferrule)
• B-nut
• Fitting

Single vs. Double Flare

Single flare: Common in aviation.
Double flare: Stronger; used for small tubing.

Torque Specifications

Example: 1/4" aluminum fitting ≈ 50 in-lb. Steel fittings allow higher torque.

Warning: Too tight crushes the flare; too loose causes leaks.

Torque Tools

Use a torque wrench with a crowfoot adapter. Position the crowfoot at 90° to the wrench.

Hose Installation

Never twist, kink, or stretch tight. Always leave 5–8% slack.

Beading

Used with hose clamps to prevent the hose from sliding off.

Common Failures

Most failures result from improper installation, such as bad flares, wrong torque, rubbing, or improper bends.

Fitting Colors

• Aluminum: Blue
• Steel: Black
• Stainless: Silver

General Rules

• Replace tubing with the same material and size.
• Tubing must withstand several times the operating pressure.

Instructor Notes

• Do not scratch aluminum; it creates stress risers.
• Always inspect the flare before installation.
• Always deburr before flaring.