7.1 Aircraft Rigid Tubing Materials & Fabrication

7.1 Aircraft Rigid Tubing Materials & Fabrication

Aircraft fluid lines constitute the circulatory system of modern flight vehicles. They convey hydraulic power to primary flight controls and landing gear actuators, deliver high-pressure fuel to turbine combustors, distribute lubricating oil to engine gearboxes, supply breathing oxygen to flight crews, and transmit air data pressures to barometric flight instruments. These fluid distribution networks are constructed from two fundamental plumbing mediums: rigid metal tubing and flexible hose assemblies.

Rigid tubing is permanently installed throughout fixed airframe structures where lines do not encounter relative motion. Under EASA Part-66 Module 06 (Materials and Hardware), certifying technicians must possess comprehensive knowledge of tubing alloy metallurgy, dimensional sizing standards, precision cutting and deburring, minimum bend geometry, cold-working springback compensation, and specialized end-forming techniques including 37° flaring and beading.

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Aircraft Rigid Tubing Materials & Metallurgy

The selection of tubing material depends directly on system operating pressure, operating temperature, environmental exposure, vibrational fatigue susceptibility, and weight optimization.

Metallurgical Characteristics & System Demands

1. Aluminium Alloys: 5052-O vs. 6061-T6

• 5052-O: The "-O" designation denotes the fully annealed, ductile condition. It is alloyed primarily with 2.5% magnesium and 0.25% chromium. Because of its outstanding ductility, it can be cold-worked, bent, flared, and beaded with standard hand tools without risk of micro-cracking. However, technicians must avoid excessive re-bending or over-working, as local strain-hardening reduces ductility and promotes stress corrosion cracking.
• 6061-T6: Alloyed with magnesium and silicon (forming magnesium silicide, $\text{Mg}_2\text{Si}$ precipitates). While it offers nearly double the yield strength of 5052-O, its formability is severely restricted. Bending 6061-T6 requires larger bend radii and precision mechanical benders. Attempting to hand-bend or single-flare small-diameter 6061-T6 often results in immediate longitudinal splitting.

2. Corrosion-Resistant Steel (CRES: 304, 321, 347)

In critical airframe zones subject to intense mechanical abuse, foreign object damage (such as gravel, ice, and tire tread debris thrown up into unpressurized landing gear wheel bays), or extreme thermal environments, aluminium alloy tubing is prohibited. Stainless steels are austenitic chromium-nickel alloys:

• AISI 321 is stabilized with titanium, while AISI 347 is stabilized with columbium (niobium) and tantalum. This stabilization prevents chromium carbide precipitation along grain boundaries during welding or high-temperature engine exposure, preserving complete immunity to intergranular corrosion.
• CRES tubing is the mandated standard for all fluid lines traversing designated engine fire zones because it can withstand direct exposure to a 1,100°C (2,000°F) open hydrocarbon flame for 15 minutes without burn-through.

3. Titanium 3Al-2.5V (Grade 9)

Modern commercial and military airframes operating at 3,000 psi (20.7 MPa) and 5,000 psi (34.5 MPa) system pressures demand significant structural weight reductions. Cold-worked and stress-relieved seamless titanium tubing containing 3% aluminium and 2.5% vanadium provides an optimal balance of high tensile strength, high burst pressure resistance, and low density ($4.48\text{ g/cm}^3$ versus $7.9\text{ g/cm}^3$ for stainless steel). Titanium tubing is chemically inert to synthetic phosphate ester hydraulic fluids (Skydrol) but requires specialized power tooling and swaged or welded connections; field flaring of titanium tubing is generally prohibited due to high notch sensitivity.

4. The Fatigue Hazards of Copper Tubing

Early aviators utilized soft annealed copper for fuel lines, oil pressure gauges, and primer systems. Under continuous cyclic engine vibration, copper rapidly work-hardens at the atomic level, undergoing dislocation tangling that causes extreme embrittlement. Without warning, brittle copper tubing suffers catastrophic transverse fatigue fracture. While copper may still be encountered in vintage general aviation barometric instrument runs, it must undergo regular periodic annealing (heating to a dull cherry red at 500°C–650°C followed by water quenching) to restore ductility. It is strictly banned in all modern pressurized fuel and hydraulic architectures.

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Rigid Tubing Dimensional Sizing Conventions

Aerospace rigid tubing conforms to standardized imperial dimensional specifications. Unlike commercial iron pipe (which is sized by nominal internal diameter), aircraft rigid tubing is sized strictly by its Outside Diameter (OD) and its Wall Thickness ($t$).

                      Aircraft Rigid Tubing Cross-Section

                             |<────── Outside Diameter (OD) ──────>|
                             |                                     |
                             |   |◄── Inside Dia (ID) ──►|         |
                           ┌─┴───┴───────────────────────┴───┬─────┴─┐
                           │     │                       │   │     │
                           │ █ █ │                       │ █ │ █ █ │
                           │ █ █ │                       │ █ │ █ █ │
                           │ █ █ │      Fluid Bore       │ █ │ █ █ │
                           │ █ █ │                       │ █ │ █ █ │
                           │ █ █ │                       │ █ │ █ █ │
                           └─┬───┬───────────────────────┬───┴─────┬─┘
                             |   |◄── Wall Thickness ───►|         |
                             |         (t, thousandths)            |

1. Outside Diameter (OD) Dash Numbers

The outside diameter is specified in sixteenths of an inch (1/16") increments, represented by a dash number:

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

2. Wall Thickness ($t$)

Wall thickness is specified in thousandths of an inch (0.001"), typically ranging from $0.020"$ to $0.083"$. Standard thicknesses include $0.028"$, $0.035"$, $0.049"$, and $0.065"$.

3. Inside Diameter (ID) Calculation

The inside diameter ($ID$) governs the fluid flow velocity, hydraulic line volume, and friction head loss. Because the wall thickness surrounds the entire circumference, it occurs twice across the diameter:

$\mathbf{ID = OD - 2 \times t}$

Calculation Example: An aircraft hydraulic pressure line is fabricated from dash -6 CRES tubing with a specified wall thickness of $0.035\text{ inch}$:

1. $\text{Outside Diameter } (OD) = \frac{6}{16}" = 0.3750"$
2. $\text{Inside Diameter } (ID) = 0.3750" - 2(0.0350") = 0.3750" - 0.0700" = \mathbf{0.3050\text{ inch}}$ ($7.747\text{ mm}$)

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Tube Fabrication Operations

Fabricating replacement rigid lines requires rigorous adherence to aerospace manufacturing standards. Even microscopic scratches or excessive flattening can trigger fatigue cracking under high-frequency hydraulic pressure pulses.

1. Precision Cutting

Rigid tubing must be cut squarely to ensure proper sealing at the flare or bite sleeve.

• Hand Hacksaw: A fine-toothed hacksaw blade possessing 32 teeth per inch (TPI) must be utilized in conjunction with a specialized tubing miter guide box to guarantee a dead-square 90° cut. Fine teeth prevent snagging and tearing thin tube walls.
• Roller-Wheel Tube Cutter: A standard roller tube cutter is widely employed on copper and aluminium tubing. However, technicians must apply extremely light feed pressure per revolution.

Critical Maintenance Warning: Aggressive tightening of the cutter feed knob deforms thin-walled tubing out-of-round, strain-hardens the cut perimeter, and rolls a massive internal burr (lip) into the tube bore. On stainless steel tubing, roller cutters will rapidly work-harden the metal, causing the cutting wheel to chatter and dull; high-speed abrasive cutoff wheels or fine power saws are preferred for CRES.

2. Deburring and Reaming (ID and OD)

Every cut tube must undergo meticulous deburring prior to flaring, bending, or fitting assembly:

• Internal Deburring (ID): A reamer or deburring tool must remove the internal lip rolled in by the cutter. Leaving an internal burr creates local fluid turbulence, induces cavitation, reduces effective flow area, and can dislodge metal slivers into sensitive electrohydraulic servo valves (EHSVs).
• External Deburring (OD): The outer cut edge must be lightly chamfered to remove sharp wire burrs. A sharp external edge will act as a notch stress raiser, causing the tube to crack longitudinally during flaring or preventing the ferrule of a flareless fitting from seating squarely.

              Tube End Preparation & Deburring Geometry

     INCORRECT (As Cut with Heavy Lip):          CORRECT (Deburred & Chamfered):

        External Burr                                Light Chamfer (OD Deburred)
             │                                              │
             ▼                                              ▼
        ┌───┐ ┌───────────────                  ───────┐ ┌───────────────
        │ █ │ │                                        │ │
        │ █ └─┘ ◄── Internal Rolled Lip                │ └───────────────
        │ █     (Severely restricts flow)              │ ◄── Bore Clean & True
        │ █ ┌─┐                                        │ ┌───────────────
        │ █ │ │                                        │ │
        └───┘ └───────────────                  ───────┘ └───────────────
             ▲                                              ▲
        Rough Cut Edge                              Square 90° Cut Face

3. Tube Bending Mechanics & Rules

Bending a tube places the outer perimeter of the bend (extrados) under high tensile stretching, causing wall thinning. Simultaneously, the inner perimeter of the bend (intrados) experiences severe compressive stress, causing wall thickening and a tendency to wrinkle.

                       Bending Stress & Geometric Distortion

                                  Extrados (Tension / Wall Thinning)
                                 . - - - - - - - .
                             .-'                   '-.
                           .'                         '.
                          /   ┌─────────────────────┐   \
                         /    │   CLR               │    \
                        ;     │  (Center Line       │     ;
       Intrados (Comp)  │     │   Radius)           │     │
         • Thickens     │     │◄───────────────────►│     │
         • Wrinkles     ;     │                     │     ;
          if unsupported \    │                     │    /
                          \   └─────────────────────┘   /
                           '.                         .'
                             '-.                   .-'
                                 ' - - - - - - - '
                                  Bending Neutral Axis

A. Minimum Bend Radius Rule

The bend radius is measured from the geometric center of the bend arc to the Center Line of the Tube (Center Line Radius, CLR).

• As an absolute engineering rule, the minimum centerline bend radius must never be less than $2.5$ to $3.0$ times the outside diameter ($2.5D$ to $3.0D$) for soft ductile alloys (such as 5052-O).
• For higher-strength or strain-hardened alloys (6061-T6, CRES stainless steel, and titanium), the minimum bend radius must be increased to $3.5D$ or greater to avoid outer wall rupture.

$R_{\min} \ge 2.5 \times OD \quad \text{(Nominal: } 3.0 \times OD\text{)}$

B. Maximum Allowable Flattening Limit

During bending, the circular tube cross-section naturally deforms into an oval shape. Aerospace airworthiness standards (FAA AC 43.13-1B and EASA Part-66) strictly limit the degree of cross-sectional flattening:

• The flattened diameter (the smallest diameter measured across the oval at the bend) must not be less than 75% of the original outside diameter ($0.75 \times OD$).
• Expressed alternatively, the maximum allowable flattening (out-of-round percentage) must not exceed 25% of the original OD — retaining 75% of the OD means losing at most 25% — calculated as:

$\text{Flattening (\%)} = \frac{OD_{\max} - OD_{\min}}{OD_{\text{nominal}}} \times 100\% \le 25\%$

Bends displaying wrinkles, kinks, indentations, or flattening exceeding 25% cause localized flow restriction and high stress concentrations, requiring immediate rejection.

C. Bending Tools & Mandrels

• Mechanical Hand Bender (Lever Type): Features a precision-machined radius block grooved to match the exact tube OD and a clip/slide-bar shoe that smoothly wipes the tube around the groove, preventing wrinkling.
• Mandrel Production Benders: For thin-walled tubing or tight radii, a internal polished steel plug (mandrel) is inserted inside the tube bore at the point of bend tangent to support the tube walls from within, eliminating ovalization and wrinkles.
• Fusible Alloy Bending (Cerrobend / Wood's Metal): Thin-walled tubes can be filled with a low-melting-point bismuth-lead alloy (melts at ~70°C in hot water). The solidified alloy acts as an internal solid core during manual bending. Once formed, the tube is immersed in boiling water to melt and flush out the alloy, followed by chemical cleaning.

D. Springback Allowance

All metallic alloys undergo a degree of elastic recovery when released from forming dies. Technicians must incorporate a springback allowance by intentionally over-bending the tube by 2° to 5° beyond the target angle, allowing the tube to relax into its exact required geometric alignment.

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Tube End Forming: Flaring and Beading

Once bent to geometry, rigid tubes require specialized end-forming to interface with threaded fittings or flexible hoses.

1. The 37° Aerospace Flare vs. 45° Automotive Flare

This is one of the most critical distinctions in aviation maintenance:

• Aerospace Standard (AN / MS / AS4330): Standardized at an included half-angle of 37° relative to the tube centerline (74° total included cone angle).
• Commercial Automotive Standard (SAE J512 / J514): Standardized at a 45° angle (90° total included cone angle).

        AEROSPACE 37° FLARE                       AUTOMOTIVE 45° FLARE
    (AN / MS Military Standard)                 (Commercial Hardware ONLY)

               /                                           /
              /                                           /
             / ◄── 37° Half-Angle                        / ◄── 45° Half-Angle
            /     (74° Included)                        /     (90° Included)
     ──────┴───────                              ──────┴───────
     ══════════════                              ══════════════  Tube Wall
     ──────────────                              ──────────────
            \                                           \
             \                                           \
              \                                           \

Exam Trap & Life-Safety Warning: Never connect an aircraft 37° fitting to a 45° flared tube (or vice versa). Although the threads may occasionally engage, the seating cones contact only at a razor-thin circular line at either the inner or outer edge. Torquing the coupling nut will immediately crush, extrude, and crack the flare face, causing sudden blowout and fuel or hydraulic spraying under operational pressure.

2. Single Flare vs. Double Flare Mechanics

Flaring tools utilize a split die block and a rotating conical flaring plunger.

        SINGLE FLARE GEOMETRY                      DOUBLE FLARE GEOMETRY
       (Steel & Tubes > 3/8" OD)               (Mandatory: 5052-O Al ≤ 3/8" OD)

              /                                          //
             /                                          //
            /  Single Wall                             //  Doubled Wall
           /   Sealing Face                           //   Folded Inward
     ─────┴────────                              ────┴┴────────
     ══════════════                              ══════════════  Tube Wall
     ──────────────                              ──────────────
           \                                          \\
            \                                          \\
             \                                          \\

Single Flare

A single flare is formed by forcing the 37° cone directly into the end of the tube, expanding the metal outward in one operation. Single flares are used on:

• All stainless steel (CRES) and titanium tubing (where high tensile strength resists cracking).
• Aluminium alloy tubing with an outside diameter greater than 3/8 inch (-6).

Double Flare

A double flare is formed in a two-stage operation using specialized dies: the first stroke folds the tube rim inward upon itself; the second stroke expands the folded metal over the 37° cone. This creates a double-thickness, reinforced conical sealing face.

Mandatory Regulatory Requirement: Double flaring is strictly mandatory on soft aluminium alloy tubing (such as 5052-O) having an outside diameter of 3/8 inch (-6) and smaller (i.e. 1/8", 3/16", 1/4", 5/16", and 3/8").

Reasoning: In small-diameter, thin-walled aluminium tubing, a single flare thins excessively at the outer perimeter. When torqued, the clamping sleeve cuts into the thin flare neck. High-frequency hydraulic vibration then shears the flare cleanly off the tube. The doubled wall provides twice the structural shear area, superior resistance to over-torquing, and a reliable seal.

3. Tube Beading

Tube beading consists of forming a continuous, raised circumferential ridge (bead) near the end of a rigid tube.

• Aviation Purpose: Applied to low-pressure fluid lines (fuel tank venting, gravity returns, engine oil breather lines, and instrument cooling air lines where operating pressure is below 250 psi / 1.7 MPa).
• Operation: A flexible synthetic rubber hose slips over the beaded rigid tube end and is secured behind the bead with a worm-drive hose clamp.
• Function: The raised annular bead acts as a mechanical barrier that prevents the flexible hose from slipping off the metal tube end under vibrational surging, thermal expansion, or transient pressure pulses.
• Tooling: Fabricated using a mechanical hand beading tool equipped with matched internal forming rollers and external grooved guide dies.

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Summary of Tube Inspection and Rejection Criteria

Technicians must reject and scrap any newly fabricated or in-service rigid tube displaying:

1. Severe Nicks or Scratches: Any scratch or gouge whose depth exceeds 10% of the nominal tube wall thickness in straight sections, or any detectable scratch in the heel (extrados) of a bend.
2. Excessive Flattening: Flattening at a bend exceeding 25% of the original OD (flattened diameter less than 75% of nominal OD).
3. Wrinkling or Buckling: Any visible wave or wrinkle along the intrados of a bend.
4. Flare Imperfections: Flares that are eccentric, cracked, thinned, pitted, or out-of-square. The flare diameter must seat fully across the sleeve shoulder without binding on the coupling nut threads.