9.1 Rigid Metal Tubing Fabrication, Bending & 37° AN Flaring

9.1 Rigid Metal Tubing Fabrication, Bending & 37° AN Flaring

Aircraft fluid lines and fittings constitute the vital circulatory network of modern aircraft, transporting hydraulic fluid under extreme pressures, aviation fuel, engine lubricating oil, compressed breathing oxygen, fire extinguishing agents, deicing fluids, and pitot-static pressures. According to FAA-H-8083-30B (Aviation Maintenance Technician Handbook — General) and FAA AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair), fluid lines are divided into two fundamental engineering categories: rigid metal tubing and flexible hose assemblies.

Rigid tubing is installed in stationary runs throughout the airframe, engine nacelles, and landing gear bays where relative motion between connected components does not occur. An Aviation Maintenance Technician (AMT) must master the material characteristics, sizing conventions, precision bending mechanics, flaring standards, and flareless connection techniques required to fabricate and maintain airworthy rigid fluid lines.

1. Rigid Tubing Materials & Metallurgical Applications

Selecting the correct metal alloy for an aircraft fluid line depends on operating pressure, fluid chemical compatibility, environmental operating temperatures, structural vibration exposure, and weight constraints.

                    AIRCRAFT RIGID TUBING ALLOY SPECTRUM
  ┌─────────────────────────────────┼─────────────────────────────────┐
  │                                 │                                 │
  ▼                                 ▼                                 ▼
ALUMINUM ALLOYS                 STAINLESS STEELS (CRES)           TITANIUM ALLOYS
• 5052-O (Low/Med Pressure)     • 304 / 321 CRES (3,000+ psi)    • 3Al-2.5V (High Pressure)
• 6061-T6 (Medium Pressure)     • Fire Zones & Landing Gear      • Transport Category Jets
• 1100 / 3003 (Vent Lines)      • High Vibration & Abrasion      • 40% Lighter than CRES

Primary Aviation Tubing Alloys

1. 5052-O Aluminum Alloy (Magnesium Primary Alloy):
   • Characteristics: Fully annealed (-O temper) aluminum alloy containing $2.5%$ magnesium and $0.25%$ chromium. Delivers moderate tensile strength ($28\text{ ksi}$), exceptional ductility, outstanding corrosion resistance (especially in marine/saltwater atmospheres), and superior cold-working formability.
   • Aviation Applications: The standard general-purpose tubing material for low-to-medium pressure aviation plumbing ($<1,500\text{ psi}$), including fuel feed lines, engine oil lines, instrument air systems, vacuum lines, and low-pressure hydraulic return circuits. It is easily hand-bent and flared without cracking.
2. 6061-T6 Aluminum Alloy (Magnesium-Silicon Alloy):
   • Characteristics: Solution heat-treated and artificially aged alloy with substantially higher tensile strength ($45\text{ ksi}$) than 5052-O. However, in the -T6 temper it exhibits reduced ductility and work-hardens rapidly.
   • Aviation Applications: Used in medium-pressure hydraulic lines and structural fluid runs where line stiffness and rigidity are necessary. Bending and flaring require specialized mechanical tooling to prevent cracking.
3. Corrosion-Resistant Steel (CRES 304 / 321 / 347):
   • Characteristics: Austenitic 18-8 stainless steel ($18%$ chromium, $8%$ nickel). Type 321 is stabilized with titanium, and Type 347 is stabilized with columbium to prevent intergranular carbide precipitation at elevated temperatures ($800^\circ\text{F}$ to $1500^\circ\text{F}$).
   • Aviation Applications: Mandatory for high-pressure hydraulic systems operating at $3,000\text{ to }5,000\text{ psi}$, lines traversing engine fire zones and exhaust nacelles, high-pressure gaseous oxygen systems, and landing gear wheel well lines exposed to physical impact from thrown runway gravel, ice, and tire tread debris.
4. Titanium 3Al-2.5V (AMS 4943 / AMS 4944 / AMS 4945):
   • Characteristics: An alpha-beta titanium alloy containing $3%$ aluminum and $2.5%$ vanadium. Possesses an extraordinary strength-to-weight ratio, delivering equivalent tensile strength to high-grade CRES while achieving a $40%$ weight reduction (density $\approx 0.162\text{ lb/in}^3$ vs $0.283\text{ lb/in}^3$ for steel).
   • Aviation Applications: Standard for high-pressure ($3,000 - 5,000\text{ psi}$) hydraulic flight control and landing gear systems in modern commercial transport jets (Boeing 777, 787; Airbus A350) and military aircraft. Due to its high yield strength, it cannot be standard hand-flared and requires swaged fittings or specialized orbital flaring equipment.
5. Copper Tubing (Historical Context & Obsoletion):
   • In early aviation, annealed copper tubing was widely used for fuel and oil lines. However, copper work-hardens rapidly under engine vibration, becoming brittle and susceptible to catastrophic crystallization and fatigue fracturing. Modern FAA regulations prohibit copper for structural fluid lines, replacing it entirely with 5052-O aluminum alloy or CRES.

Rigid Tubing Material Comparison Matrix

2. Rigid Tubing Sizing, Wall Thickness & Internal Flow Calculations

Standard aircraft metallic tubing is manufactured and sized in accordance with strict aerospace standards governed by the Society of Automotive Engineers (SAE) and military specifications.

The Outside Diameter (OD) Sizing Convention

Rigid tubing is always sized by its Outside Diameter (OD) in increments of $1/16\text{ inch}$, designated by standard dash numbers:

$\text{Outside Diameter (OD)} = \frac{\text{Dash Number}}{16}\text{ inch}$

• A -4 tube has an $\text{OD} = \frac{4}{16}" = \frac{1}{4}" = 0.250"$
• A -6 tube has an $\text{OD} = \frac{6}{16}" = \frac{3}{8}" = 0.375"$
• A -8 tube has an $\text{OD} = \frac{8}{16}" = \frac{1}{2}" = 0.500"$
• A -12 tube has an $\text{OD} = \frac{12}{16}" = \frac{3}{4}" = 0.750"$
• A -16 tube has an $\text{OD} = \frac{16}{16}" = 1.000"$

Rigid Tubing Cross-Sectional Geometry:
  ┌─────────────────────────────────────────────────────────────┐
  │                                                             │
  │      ◄────────────────── Outside Diameter (OD) ───────────────►
  │     ┌─────────┬───────────────────────────────────┬─────────┐
  │     │  Wall   │                                   │  Wall   │
  │     │Thickness│   ◄────── Inside Diameter (ID) ──►│Thickness│
  │     │   (t)   │                                   │   (t)   │
  │     │         │                                   │         │
  │     └─────────┴───────────────────────────────────┴─────────┘
  │                                                             │
  └─────────────────────────────────────────────────────────────┘

Wall Thickness & Inside Diameter (ID) Formula

While the dash number dictates the outside diameter, the wall thickness ($t$) is measured and specified in thousandths of an inch (e.g., $0.020"$, $0.035"$, $0.049"$, $0.065"$, $0.083"$).

Because the tube wall exists on both sides of the circular cross-section, the Inside Diameter (ID) that governs fluid flow velocity and volume is calculated as:

$\text{ID} = \text{OD} - 2t = \text{OD} - 2 \times (\text{Wall Thickness})$

$\text{Flow Area } (A) = \frac{\pi \times \text{ID}^2}{4} = 0.7854 \times \text{ID}^2$

Worked Example — Inside Diameter & Flow Area: Calculate the inside diameter and cross-sectional flow area of a -8 5052-O aluminum tube having a nominal wall thickness of $0.035\text{ inch}$.

1. Determine Outside Diameter: $\text{OD} = \frac{8}{16}" = 0.500\text{ inch}$.
2. Calculate Inside Diameter: $\text{ID} = 0.500" - 2(0.035") = 0.500" - 0.070" = 0.430\text{ inch}$.
3. Calculate Flow Area: $A = 0.7854 \times (0.430)^2 = 0.1452\text{ in}^2$.

Rigid Tubing Sizing Reference Table

Pressure Design Factors (Barlow's Formula)

The theoretical burst pressure of rigid tubing is calculated using Barlow's formula relating internal pressure ($P$), ultimate tensile strength ($\sigma_t$), wall thickness ($t$), and outside diameter ($D$):

$P_{\text{burst}} = \frac{2 \times \sigma_t \times t}{D}$

Per 14 CFR Part 23 and Part 25 certification standards, critical hydraulic fluid lines must maintain a minimum factor of safety of 4:1 on burst pressure ($P_{\text{burst}} \ge 4 \times P_{\text{operating}}$) and 2:1 on proof pressure without permanent plastic deformation.

3. Tubing Bending Mechanics, Radius Rules & Tooling

When routing rigid lines through an aircraft structure, straight linear runs between two rigid endpoints are strictly prohibited. A straight rigid tube expands and contracts with ambient temperature shifts and flexes under structural wing/fuselage aerodynamic loading. Without bends, thermal expansion and structural flexure exert massive cyclic axial tension and compression loads directly into the end fittings, causing metal fatigue, fitting pullout, and seal failure.

Core Installation Rule: Every rigid tubing run MUST incorporate at least one bend (or offset expansion loop) to absorb vibration, accommodate thermal expansion/contraction, and provide mechanical compliance during structural deflection.

                      RIGID TUBE BEND GEOMETRY & LIMITS
  ┌─────────────────────────────────────────────────────────────────────┐
  │                                                                     │
  │                     Outside Radius (Extrados)                       │
  │              ───► Tension: Wall Stretches and Thins ◄───            │
  │            ╭─────────────────────────────────────────╮              │
  │           │   ▲ Minor Diameter (D_min >= 0.75 OD)     │             │
  │           │   │                                       │             │
  │           │   ▼ (Max Allowable Flattening = 25%)      │             │
  │            ╰─────────────────────────────────────────╯              │
  │              ───► Compression: Wall Thickens ◄───                   │
  │                      Inside Radius (Intrados)                       │
  │                     (Must be free of wrinkles)                      │
  │                                                                     │
  │                 ◄────── Bend Radius (R) ──────►                     │
  │                 (Measured to Centerline of Tube)                    │
  │                 (Minimum R = 3x to 6x Tube OD)                      │
  └─────────────────────────────────────────────────────────────────────┘

Minimum Bend Radius Rules (AC 43.13-1B)

The bend radius ($R$) is measured from the center of curvature to the centerline of the tubing. To prevent excessive tensile thinning on the outside radius (extrados) and structural wrinkling or buckling on the inside radius (intrados), AC 43.13-1B Table 9-1 establishes minimum bend radius limits:

• Standard Aluminum Tubing (5052-O): Minimum bend radius is $3\times$ the tube outside diameter ($R_{\text{min}} = 3 \times \text{OD}$).
• Hard Aluminum Alloys (6061-T6) & CRES Stainless Steel: Minimum bend radius is $5\times\text{ to }6\times$ the tube outside diameter ($R_{\text{min}} = 5\text{ to }6 \times \text{OD}$).
• Titanium 3Al-2.5V: Minimum bend radius is $3.5\times\text{ to }5\times$ the tube OD depending on wall thickness.

Tube Flattening Limits

During bending, the cross-section of the tube deforms from a perfect circle into an ellipse. AC 43.13-1B defines strict flattening tolerances:

1. Maximum Allowable Flattening: The tube flattening in a bend must NOT exceed 25% of the original outside diameter.
2. Minimum Minor Diameter: The measured diameter across the flattened portion of the bend ($D_{\text{minor}}$) must not be less than 75% of the original nominal outside diameter ($D_{\text{minor}} \ge 0.75 \times \text{OD}$). Formula:

$\text{Flattening Percentage } (\%) = \frac{\text{OD}_{\text{original}} - D_{\text{minor}}}{\text{OD}_{\text{original}}} \times 100\% \le 25\%$

Worked Example — Bend Flattening Inspection: An AMT inspects a bend on a -8 ($0.500\text{ inch}$ OD) 5052-O hydraulic return tube using a vernier caliper. The minor diameter across the bend measures $0.390\text{ inch}$. Is this bend airworthy?

1. Calculate flattening percentage: $\text{Flattening } \% = \frac{0.500" - 0.390"}{0.500"} \times 100\% = \frac{0.110"}{0.500"} \times 100\% = 22.0\%$
2. Verify minimum diameter: $0.75 \times 0.500" = 0.375"$. Since $0.390" > 0.375"$ and $22.0% \le 25.0%$, the bend is AIRWORTHY.

Bending Tools & Techniques

• Hand Lever Tube Benders: Consist of a fixed radius block marked in degrees ($0^\circ$ to $180^\circ$), a tube clamping latch, and a movable lever arm carrying a forming slide shoe. Used for tubing sizes from $1/8"$ to $1/2"$ OD.
• Mechanical Mandrel Benders: Used for production lines and larger tubing ($>1/2"$ OD). An internal steel mandrel supports the inside diameter of the tube precisely at the tangent point of the bend, preventing ovalization, wrinkling, and wall collapse.
• Filler Material Bending (Field Emergency Method): When mechanical benders are unavailable for thin-wall tubing, the tube is packed with fine dry silica sand or filled with a low-melting bismuth alloy known as Cerrobend (Wood's metal), which melts at $158^\circ\text{F}$ ($70^\circ\text{C}$) in hot water. Once solidified, the tube is bent smoothly without collapsing. The Cerrobend is completely evacuated by flushing the completed tube with boiling water and cleaning thoroughly.

4. Tubing Flaring: 37° AN Aviation Standard vs 45° Automotive Flares

Flaring is the mechanical cold-forming process that expands the open end of a rigid tube into a conical funnel shape to mate with the matching nose of an aviation fitting.

The 37° Aviation (AN/MS) Standard vs 45° Automotive Standard

[!WARNING] STRICT SAFETY PROHIBITION — NEVER MIX 37° AND 45° HARDWARE: Aviation $37^\circ$ fittings and automotive $45^\circ$ fittings are NEVER INTERCHANGEABLE. Attempting to mate a $37^\circ$ flared tube to a $45^\circ$ fitting creates an acute line-contact interface instead of a full conical surface seal. When torqued, the mismatched angles crush the soft tube flare, strip fitting threads, leak immediately under hydraulic pressure, and fail catastrophically under flight vibration.

              37° AVIATION FLARE vs 45° AUTOMOTIVE FLARE
    37° AN Aviation Flare                 45° Automotive Flare
   (74° Total Included Angle)           (90° Total Included Angle)
          /  37°  \                            /   45°  \
         /       \                          /        \
        /         \                        /          \
             │ │                                  │  │
             │ │                                  │  │
       AN / MS Standard                      SAE Automotive
  (MANDATORY FOR AIRCRAFT)               (PROHIBITED IN AIRCRAFT)

Single Flare vs Double Flare Protocols

1. Single Flare:
   • Formed by expanding the tube wall outward in a single mechanical stroke against a $37^\circ$ die block.
   • Applicability: Standard for all CRES stainless steel lines, titanium lines, 6061-T6 aluminum lines, and all aluminum alloy tubing LARGER than $3/8\text{ inch}$ OD (e.g., -8, -10, -12, -16).
2. Double Flare:
   • Formed in a two-stage cold-forming operation: the tube end is first upset and bulged outward using a specialized adapter button, then folded back inside itself to form a double-thickness conical flare wall.
   • MANDATORY FAA REQUIREMENT: Double flaring is strictly mandatory on 5052-O aluminum alloy tubing of $3/8\text{ inch}$ OD (-6) and smaller (including $1/8"$, $3/16"$, $1/4"$, $5/16"$, and $3/8"$).
   • Engineering Rationale: Thin-wall aluminum tubing ($0.028"$ to $0.035"$ wall thickness) subjected to a single flare undergoes severe stretching and thinning at the flare lip. Under high engine vibration and repeated coupling nut torquing, a thin single flare shears, cracks at the sleeve shoulder, or pulls out of the fitting. A double flare doubles the metal thickness at the sealing seat, providing extreme resistance to thinning, cutting, shearing, and fatigue cracking.

Single Flare vs Double Flare Cross-Section:
      SINGLE FLARE                            DOUBLE FLARE
   (Single Wall Thickness)              (Folded Double Wall Thickness)
         ▲ 37°                                     ▲ 37°
        ╱                                         ╱ ╱
       ╱                                         ╱ ╱  ◄── Folded Back
      │  ◄── Single Wall Thickness              │ │   Inside Itself
      │                                         │ │
  (Mandatory for >3/8" OD)                (MANDATORY for 5052-O <=3/8" OD)

Precision Step-by-Step Flaring Procedure

1. Square Cut: Cut the rigid tubing using a rotary wheeled tube cutter. Turn the cutter feed knob slowly ($1/4$ turn per revolution) to avoid work-hardening the metal or swaging the tube ID inward.
2. Deburring & Cleaning: Lightly remove internal and external burrs using a deburring reamer and 400-grit abrasive cloth. Do not create a deep chamfer, which would reduce the effective flare wall thickness.
3. Hardware Loading: Slide the AN818 coupling nut and AN819 sleeve onto the tube in their correct orientations BEFORE flaring:
   • Nut threads must face outward toward the tube end.
   • Sleeve bevel must face outward toward the tube end to support the back of the $37^\circ$ flare.
4. Die Clamping & Protrusion: Place the tube in the correct matching size hole of a $37^\circ$ flaring block. The tube end must extend slightly above the die face (typically the thickness of a dime or per tool gauge stop).
5. Cone Operation: Lubricate the flaring cone with a drop of light hydraulic fluid or engine oil. Rotate the flaring spindle smoothly to form the flare. Back the cone out, rotate the tube $90^\circ$, and re-compress lightly to eliminate any die seam flash.
6. Inspection: The flare must be concentric, free of radial cracks, chatter marks, scores, or metal slivers. The outer diameter of the flare must extend to the outer edge of the AN819 sleeve shoulder without overhanging into the coupling nut threads.

5. MS Flareless (Bite-Type) Fittings & Ferrule Presetting Procedures

In high-pressure hydraulic systems (3,000 to 5,000 psi) utilizing heavy-wall stainless steel, titanium, or 6061-T6 aluminum tubing, standard flaring is difficult and prone to micro-cracking. For these applications, MS flareless fittings (MS51500 / MS21900 series) are utilized.

                MS FLARELESS (BITE-TYPE) FITTING ANATOMY
  ┌─────────────────────────────────────────────────────────────────────┐
  │                                                                     │
  │     MS21921 Nut          MS21922 Ferrule/Sleeve      Fitting Body   │
  │    ┌────────────┐            ┌─────────┐            ┌───────────┐   │
  │    │ ────────── │            │ ─────── │            │           │   │
  │────┼────────────┼────────────┼─┐     ┌─┼────────────┼─────┐     │   │
  │    │  Internal  │            │ │Bite │ │            │20°  │Tube │   │
  │    │  Threads   │            │ │Edge │ │            │Ramp │Stop │   │
  │────┼────────────┼────────────┼─┘     └─┼────────────┼─────┘     │   │
  │    │            │            │         │            │           │   │
  │    └────────────┘            └─────────┘            └───────────┘   │
  │                                                                     │
  │    ◄────────────── Tube Bottoms Against Internal Stop ──────────►   │
  └─────────────────────────────────────────────────────────────────────┘

Working Principle & Mechanical Bite

An MS flareless fitting assembly consists of three components:

1. Fitting Body: Features an internal 20° lead-in ramp and a square shoulder tube stop.
2. MS21922 Ferrule (Sleeve): A hardened metallic sleeve with a sharp, internal cutting wedge.
3. MS21921 Coupling Nut: Compresses the ferrule axially into the fitting body.

When the coupling nut is tightened during the presetting operation, the 20° body ramp forces the cutting edge of the ferrule inward. The ferrule bites into the outer circumference of the tubing to a depth of 0.003 to 0.008 inch, raising a slight, visible ridge of displaced metal (metal wash/dam) ahead of the cutting edge. This creates a permanent, leak-proof metal-to-metal mechanical interlock capable of holding 3,000+ psi without flaring.

Ferrule Presetting Procedure

To ensure consistent ferrule bite without damaging operational aircraft fittings, ferrules must be preset using a hardened steel presetting tool (or a dedicated spare presetting fitting body):

1. Preparation: Cut the tube square, deburr internally and externally, and ensure the outer tube wall is perfectly smooth and free of scratches.
2. Assembly: Slide the MS21921 nut and MS21922 ferrule onto the tube with the ferrule cutting edge facing the tube end.
3. Lubrication: Lubricate the threads and ferrule ramp with clean hydraulic fluid or petroleum jelly.
4. Bottoming: Insert the tube into the presetting tool until it bottoms firmly against the internal shoulder stop. Maintain firm axial forward pressure on the tube to prevent it from backing out during tightening.
5. Hand Tightening: Screw the nut onto the tool finger-tight until bottomed.
6. Wrench Turn (Biting): Hold the tool in a vise and tighten the nut with a wrench the exact number of turns specified in AC 43.13-1B or the manufacturer's manual (typically 1/6 to 1/3 turn / 1 to 2 hex flats after finger tightness for aluminum, or up to 1/2 turn for stainless steel).

Mandatory Post-Presetting Inspection Criteria

After presetting, the technician must unscrew the nut and slide it back to inspect the ferrule before final installation:

• Uniform Bite Depth: The ferrule cutting edge must have produced a continuous, uniform 360° indentation (0.003" - 0.008" depth) into the tube wall with a visible raised metal ridge ahead of the bite edge.
• Axial Play (CRITICAL CRITERION): The preset ferrule may rotate freely around the tube circumference by hand, but it MUST EXHIBIT ZERO AXIAL (LONGITUDINAL) PLAY (it must not slide forward or backward along the tube length).
• Pilot Length ($L_{\text{pilot}}$): The bare tube end extending forward from the ferrule cutting edge must meet the minimum pilot length specified in the MS drawing to ensure proper seating against the fitting body shoulder.
• Tube Bowing: The tube must not be bowed or deformed inside the ferrule.

6. Realistic Exam Scenarios & Case Studies

Scenario 1: Engine Fuel Primer Line Failure on a Continental O-470

During a 100-hour annual inspection on a single-engine aircraft, an AMT discovers raw aviation gasoline leaking behind the engine cylinders. Inspection reveals that the -4 ($1/4\text{ inch}$ OD) 5052-O aluminum fuel primer line has fractured circumferentially directly behind the AN819 sleeve shoulder.

• Diagnostic Root Cause: The technician who previously fabricated the line formed a single flare instead of a double flare. Engine vibration caused the thinned single flare wall to work-harden and shear under the clamping pressure of the sleeve.
• Airworthy Corrective Action: The AMT must fabricate a brand-new 5052-O aluminum tube assembly. Per FAA regulations, because the tube is 5052-O and 3/8 inch OD or smaller, it must be double-flared using the correct two-stage flaring tool adapter button.

Scenario 2: Main Landing Gear Brake Line Restriction

Following replacement of a rigid brake line on a multi-engine aircraft, flight crew reports that the right main gear brake drags and overheats during taxi. The AMT inspects the newly fabricated -6 ($0.375\text{ inch}$ OD) 5052-O aluminum line where it navigates a tight $90^\circ$ turn around the landing gear trunnion. Measuring the minor diameter across the bend with a micrometer yields $0.255\text{ inch}$.

• Flattening Calculation: $\text{Flattening } \% = \frac{0.375" - 0.255"}{0.375"} \times 100\% = \frac{0.120"}{0.375"} \times 100\% = 32.0\%$
• Diagnosis: The bend exhibits $32%$ flattening, drastically exceeding the FAA maximum allowable limit of $25%$ ($D_{\text{min}} < 0.75 \times \text{OD} = 0.281"$). The severely restricted flow area impedes hydraulic fluid return, preventing the brake caliper pistons from releasing.
• Corrective Action: Reject and scrap the defective line. Fabricate a replacement line using a calibrated hand tube bender that enforces the mandatory minimum bend radius ($R \ge 3 \times \text{OD} = 1.125"$) and maintains flattening below $25%$.