9.1 Aircraft Rigid Tubing, Bending & 37-Degree Flaring Standards

9.1 Aircraft Rigid Tubing, Bending & 37-Degree Flaring Standards

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Rigid fluid lines form the structural circulatory network of modern aircraft, transporting hydraulic fluid under immense pressures, aviation turbine fuel, gaseous breathing oxygen, lubricating oils, fire extinguishing agents, and engine bleed air. Unlike industrial or residential plumbing, aircraft tubing must withstand continuous airframe vibration, broad thermal differentials ranging from -55°C to over 200°C, cyclic pressure impulses, and severe aerodynamic g-loads while maintaining absolute zero-leakage integrity and minimal weight. Mastering rigid line materials, fabrication tolerances, bending mechanics, flaring standards, and identification protocols is a foundational competency for the EASA Part-66 maintenance engineer.

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Rigid Tubing Materials & Service Applications

Aircraft rigid tubing is manufactured from seamless drawn metal alloys specifically formulated to resist cyclic fatigue, internal bursting pressure, and environmental corrosion. Material selection depends directly on operating pressure, fluid chemistry, ambient temperature, and structural fire protection mandates.

1. Aluminium Alloys

• 5052-O Aluminium Alloy: An annealed, non-heat-treatable aluminium-magnesium alloy possessing exceptional cold-working ductility, high fatigue strength, and superior resistance to corrosion in marine atmospheres. Because of its excellent formability, 5052-O is the standard airframe alloy for low- to medium-pressure applications (up to 1,500 psi / 103 bar), including fuel lines, hydraulic return networks, oil vent lines, and cabin environmental ducting.
• 6061-T6 Aluminium Alloy: A precipitation-hardened aluminium-magnesium-silicon alloy exhibiting significantly higher tensile yield strength (approximately 275 MPa vs. 90 MPa for 5052-O). It is utilized for structural fluid lines and medium-pressure hydraulic pressure lines where rigidity is required. However, because the T6 temper reduces ductility, 6061-T6 is far more prone to cracking during tight bending or flaring operations and requires larger bend radii.

2. Corrosion-Resistant Stainless Steel (CRES)

• AISI 304, 321, and 347 Stainless Steel: Austenitic chromium-nickel stainless steels delivering exceptional tensile strength (yield strength exceeding 200–500 MPa depending on cold work) and corrosion resistance. Grades 321 (stabilized with titanium) and 347 (stabilized with columbium/tantalum) prevent chromium carbide precipitation at grain boundaries during welding or high-temperature exposure.
• Applications: CRES tubing is mandatory in high-pressure hydraulic circuits (3,000 psi to 5,000 psi / 207 to 345 bar), pneumatic bleed air lines exceeding 250°C, landing gear wheel wells (where lines are subjected to mechanical bombardment by thrown runway gravel and tyre debris), and designated engine nacelle fire zones.

3. Titanium Alloys

• Ti-3Al-2.5V (AMS 4943/4944): An alpha-beta titanium alloy cold-worked and stress-relieved to achieve extreme strength-to-weight efficiency. Operating at pressures up to 5,000 psi (345 bar) on commercial transport aircraft such as the Airbus A380 and Boeing 787, titanium tubing yields a 40% to 45% weight saving compared to corrosion-resistant steel. Titanium is completely immune to galvanic corrosion from Skydrol phosphate-ester hydraulic fluids.
• Handling Note: Titanium possesses high notch sensitivity and a pronounced tendency to gall against bending mandrels. It requires specialized rotary-draw bending tooling, synthetic dry-film lubricants, and slow, continuous bending speeds to avoid spring-back and surface scratching.

4. Copper (Historical Reference)

• Seamless copper tubing was common on early vintage aircraft. In modern aviation maintenance, copper is strictly prohibited for fluid transmission systems because it work-hardens rapidly under normal engine vibration, leading to severe embrittlement and catastrophic transverse cracking. Its use is restricted entirely to minor electrical conduit or capillary temperature-bulb instrumentation.

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Tube Sizing Standards & Dimensional Calculations

A critical distinction tested in EASA Part-66 examinations is the fundamental difference between aircraft tubing and commercial pipe:

• Aircraft Tubing: Sized strictly by its Outside Diameter (OD) measured in sixteenths of an inch (1/16"). The size is universally designated by a Dash Number. For example, a Dash 6 (-6) tube has an outside diameter of $6/16" = 3/8"$ (9.52 mm); a Dash 8 (-8) tube has an outside diameter of $8/16" = 1/2"$ (12.7 mm); and a Dash 16 (-16) tube has an outside diameter of $16/16" = 1.0"$ (25.4 mm).
• Wall Thickness: Measured and specified in thousandths of an inch (0.001"), such as 0.035", 0.049", or 0.065".
• Commercial Pipe: Sized by its nominal internal diameter (ID) up to 12 inches, with wall thickness designated by schedule numbers (e.g., Schedule 40, Schedule 80). Aircraft fluid systems never use pipe sizing conventions.

Aircraft Tube Sizing Formula:
Inside Diameter (ID) = Outside Diameter (OD) - (2 x Wall Thickness [t])

Example Calculation for a -8 CRES tube with 0.049" wall thickness:
OD = 8 / 16" = 0.500"
Wall thickness (t) = 0.049"
ID = 0.500" - (2 x 0.049") = 0.500" - 0.098" = 0.402"

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Tube Bending Principles, Radii & Permissible Flattening

When a metal tube is bent, the material on the outside of the bend is subjected to tensile elongation and thinning, while the material on the inside of the bend is subjected to compressive stress and thickening. If bent incorrectly, the tube will wrinkle on the inside radius, flatten excessively across its cross-section, or buckle.

Minimum Bend Radius ($R_{min}$)

The minimum bend radius is measured from the centerline of the tube, not the inside edge. As a general aviation rule (conforming to FAA AC 43.13-1B and standard EASA Part-66 practice), the minimum allowable centerline bend radius is:

$$R_{min} = 2.5 \times \text{Tube OD to } 3.0 \times \text{Tube OD}$$

For example, bending a -6 (3/8" OD) tube requires a minimum centerline radius of at least $2.5 \times 0.375" = 0.9375"$ (15/16"), or $3.0 \times 0.375" = 1.125"$ (1-1/8") depending on alloy hardness and wall thickness.

Permissible Flattening Allowance

During bending, the circular cross-section tends to deform into an ellipse. Under aviation standards, the maximum allowable flattening (ovality) in the bend zone is 25% reduction of the original outside diameter:

$$\text{Flattening Percentage} = \frac{D_{orig} - D_{min}}{D_{orig}} \times 100\% \le 25\%$$

Alternatively stated, the minimum diameter ($D_{min}$) across the flattened bend must never measure less than 75% of the original outside diameter ($D_{orig}$). Bends displaying wrinkles, kinks, longitudinal scoring, or flattening exceeding 25% are unairworthy and must be rejected, as cross-sectional restriction disrupts laminar fluid flow and initiates rapid fatigue cracking under cyclic hydraulic pulses.

+-------------------------------------------------------------------------+
|                        TUBE BENDING OVALITY CHECK                       |
|                                                                         |
|          Original Tube OD (D_orig)        Flattened Bend Section        |
|                  (Circle)                        (Ellipse)              |
|                   .---.                            .---.                |
|                  /     \                          /     \   D_max       |
|                 |   O   |  D_orig                (   O   )              |
|                  \     /                          \     /   D_min       |
|                   '---'                            '---'                |
|                                                                         |
|   Criterion: D_min must be >= 0.75 x D_orig  (Max flattening <= 25%)    |
|   Wrinkles, kinks, or wall thinning exceeding 10% -> MANDATORY SCRAP    |
+-------------------------------------------------------------------------+

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Tube Flaring Standards: 37° Aircraft vs 45° Automotive Trap

Flaring is the mechanical expansion of a tube end into a precise conical bell to mate with a matching fitting nose, establishing a high-pressure metal-to-metal seal.

The 37-Degree Aviation Standard (AN / MS / AS)

Standard aeronautical flared fittings—including AN (Air Force-Navy), MS (Military Standard), and AS (Aerospace Standard) series (e.g., AN818 coupling nut, AN819 sleeve, AN815 union)—are precision-machined with a 37° flare angle relative to the tube centerline (74° included angle).

The Fatal 45-Degree Automotive Trap

Commercial automotive, domestic gas, and refrigeration piping fittings utilize a 45° flare angle (90° included angle), conforming to SAE or ANSI standards. Under no circumstances may 45° automotive fittings, benders, or flaring tools be used on aircraft systems.

• The Mechanical Mismatch: If a 37° flared aircraft tube is mated to a 45° fitting nose (an 8° angular difference), the conical surfaces cannot seat flush against each other. Instead of distributing clamping force across the broad conical face, the fitting makes sharp knife-edge point contact at either the inner heel or outer lip of the flare.
• The Resulting Failure: When the technician torques the B-nut to standard values, the concentrated line load shears the flare lip, galls the seating face, or severely thins the metal. Under hydraulic pressure or flight vibration, the cracked flare separates catastrophically, spraying volatile hydraulic fluid or fuel into the airframe.

+-------------------------------------------------------------------------+
|                   37° AIRCRAFT VS 45° AUTOMOTIVE TRAP                   |
|                                                                         |
|       Aviation Standard (AN/MS)             Automotive / Industrial     |
|             37° Flare Angle                     45° Flare Angle         |
|            (74° Included)                      (90° Included)           |
|                                                                         |
|                 \     /                             \         /         |
|                  \ 37/                               \   45  /          |
|                   \ /                                 \     /           |
|                    |                                   |   |            |
|                    |                                   |   |            |
|                                                                         |
|   WARNING: Intermixing 37° and 45° components creates an 8° angular     |
|   mismatch. Point-contact loading shears the flare lip under torque,    |
|   guaranteeing catastrophic blowout under hydraulic operating pressure. |
+-------------------------------------------------------------------------+

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Single Flare vs Double Flare Standards

Aviation specifications recognize two distinct flare geometries depending on tubing material and wall diameter:

1. Single Flare

A single flare is formed by expanding the tube mouth outward directly over a 37° conical flaring mandrel. It is approved for:

• All corrosion-resistant stainless steel (CRES) and titanium tubing.
• Heavy-wall aluminium alloy tubing.
• Aluminium alloy tubing with an outside diameter greater than 3/8 inch (> Dash 6).

2. Double Flare

A double flare is formed using a two-stage flaring tool. The first die folds the tube mouth inward into a bell shape; the second 37° mandrel folds the metal back down against itself, producing a smooth, double-thickness conical lip.

• The Regulatory Mandate: Some approved tube and fitting processes require a double flare for specified small-diameter aluminium tubing. The actual fabrication data control material, size, and method.
• Engineering Rationale: Thin-wall, small-diameter aluminium tubing has insufficient wall mass to withstand single flaring. During single flare expansion, the extreme stretching thins the flare lip to razor thickness, creating microscopic longitudinal edge splits. When torqued by the B-nut, single-flared small aluminium lines easily shear off at the base of the flare. The double flare provides twice the wall thickness, superior resistance to B-nut shearing torque, and significantly higher fatigue resistance under cyclic vibration.

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Sleeve and B-Nut Assembly Mechanics (AN818 / AN819)

A standard flared aircraft connection consists of three precision components:

1. The Fitting Body (e.g., AN815 Union): Features external threads and a 37° male conical nose.
2. The Sleeve (AN819): An anti-friction ferrule positioned between the tube flare and the coupling nut.
3. The Coupling B-Nut (AN818): An internally threaded hex nut that draws the assembly together.

Critical Functions of the AN819 Sleeve

• Eliminating Flare Galling: When the AN818 B-nut is rotated during torquing, the nut's internal shoulder bears against the rear shoulder of the sleeve. The sleeve remains stationary relative to the tube, allowing the nut to rotate freely without wiping, dragging, or tearing the delicate aluminium flare face.
• Vibration Dampening: The extended rear collar of the AN819 sleeve supports the tubing at the critical transition zone behind the flare, absorbing high-frequency bending fatigue.
• Assembly Sequence Rule: The technician must always slide the AN818 B-nut on first, followed by the AN819 sleeve second, before flaring the tube. If flaring is performed first, the components cannot be installed, requiring the flare to be cut off and re-formed.

Proper Torquing vs. The Overtightening Hazard

Flared fittings seal purely through metal-to-metal coining pressure between the fitting nose and the inner flare face. Threads must be clean and lubricated only when specified by the Aircraft Maintenance Manual (AMM). Technicians must always use a calibrated torque wrench:

• Never overtighten a leaking flare. Overtightening crushes and deforms the flare cone, thins the metal, causes circumferential cracking, and destroys the fitting nose. If a flared connection leaks at proper torque, loosen the nut, inspect the seating faces for scratches, burrs, or contamination, and re-flare or replace the line.

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Flareless Bite-Type Fittings (MS21900 / Ermeto Series)

In high-pressure hydraulic circuits (3,000 psi to 5,000 psi) operating with thick-wall stainless steel or titanium tubing, flaring becomes impractical due to the extreme hardness of the metal and the force required. For these applications, flareless bite-type fittings (MS21900 series) are utilized.

Components and Operating Principle

A flareless fitting consists of an externally threaded body with a 20° internal seat, a heat-treated hardened steel sleeve (ferrule) with a sharp cutting edge, and a coupling nut.

• When tightened, the sleeve is forced axially into the body's internal camming taper. The front cutting wedge of the sleeve bites into the outer surface of the tubing wall, displacing a continuous 360° metallic ridge (lip) ahead of it. This bite provides an impenetrable mechanical lock and pressure-tight seal without reducing the tube bore.

The Mandatory Presetting Procedure

Flareless sleeves must never be set directly onto the tubing in final airframe installations. They must be preset using a dedicated hardened-steel presetting tool or a heavy-duty master fitting in a bench vice:

1. Cut the tube perfectly square, deburr the inner and outer diameters, and clean thoroughly.
2. Lubricate the tool threads and sleeve face with approved hydraulic fluid or anti-seize paste.
3. Hand-tighten the nut until snug, ensuring the tube bottoms solidly against the internal shoulder of the tool.
4. Rotate the nut with a wrench through the exact number of turns specified in the AMM (typically 1/6 to 1/3 turn [1 to 2 hex flats] past initial resistance, or up to 1-1/6 turns on initial bite depending on tube size and wall thickness).
5. Preset Inspection: Back off the nut and inspect the assembly. The cutting edge must have raised a uniform, unbroken 360° metallic lip pushed up ahead of the ferrule. The depth of the bite should typically be between 0.003" and 0.008" (0.08 mm to 0.20 mm). The sleeve should be firmly locked on the tube but may show slight axial bowing; the tube bore must not be collapsed or restricted.

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Fluid Line Identification Markings (MIL-STD-1247)

To prevent catastrophic servicing cross-contamination—such as introducing mineral oil into a Skydrol system or nitrogen into an oxygen bottle—all aircraft fluid lines must carry standardized identification tapes conforming to MIL-STD-1247.

Tape Structure & Application Rules

• Width: 1-inch (25.4 mm) wide pressure-sensitive adhesive tapes, featuring a solid or patterned colour band, a standardized geometric symbol, and bold capitalised text designating the fluid service, operating pressure, and hazards (such as FLAM for flammable, TOXIC, or PHDAN for physically dangerous).
• Placement Rules: Identification tapes must be applied at each end of the line, immediately adjacent to disconnect fittings, on both sides of every airframe bulkhead or floor partition, and at intervals not exceeding 24 inches (600 mm) along the entire length of the tube run.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

A licensed aircraft maintenance engineer is tasked with fabricating and installing a replacement hydraulic pressure line between a high-pressure filter module and an APU isolation valve on a commercial transport aircraft. The line operates at 3,000 psi.

1. Material and Sizing Selection: Consulting the IPC and AMM, the engineer confirms the specification calls for Dash 6 (-6) seamless corrosion-resistant stainless steel (CRES 321) with a wall thickness of 0.035". The outside diameter is $6/16" = 0.375"$, yielding an inside diameter of $0.375 - (2 \times 0.035) = 0.305"$.
2. Cutting and Deburring: The technician cuts the tubing using a fine-pitch 32 TPI hacksaw in a mitre block (avoiding rotary wheel pipe cutters that work-harden stainless steel and create heavy internal burrs). The cut is filed flat, and both inner and outer edges are deburred to a smooth 45° chamfer.
3. Bending: Using a mechanical gear-type hand bender with a 1.125" centerline radius ($3.0 \times OD$), the technician forms a 90° bend. Using a vernier micrometer, the technician measures the outside diameter at the apex of the bend: $D_{orig} = 0.375"$, $D_{min} = 0.320"$. The flattening percentage is $((0.375 - 0.320) / 0.375) \times 100\% = 14.67\%$, comfortably below the 25% regulatory ceiling.
4. Flaring: Because the material is stainless steel, the technician forms an approved single 37° flare using an aerospace flaring kit, sliding the AN818-6 coupling nut and AN819-6 sleeve onto the line prior to flaring. The flare lip is inspected under 10x magnification for concentricity and hairline cracking.
5. Installation: The line is aligned by hand so that the flare seats flush onto the fitting cone without forced prying. The B-nut is hand-threaded fully, and then torqued with a calibrated torque wrench to the exact AMM specification (135–150 in-lbs). Blue/yellow MIL-STD-1247 hydraulic identification tapes marked "3000 PSI HYDRAULIC" are affixed adjacent to both end connections.

Common Exam Traps

• Trap 1: Intermixing 37° aviation and 45° automotive fittings. Never accept 45° tools or fittings in aviation maintenance. The 8° angular difference causes knife-edge point contact, flare lip shearing, and immediate catastrophic leakage under pressure.
• Trap 2: Single flaring small-diameter aluminium lines. Some approved processes require a double flare on specified small-diameter aluminium tubing; use the actual fabrication data rather than this generic example. Single flaring small, thin-wall aluminium causes thinning, edge splitting, and circumferential flare shearing under B-nut torque.
• Trap 3: Sizing confusion between pipe and tubing. Aircraft tubing is sized by Outside Diameter in 16ths of an inch; commercial pipe is sized by nominal Inside Diameter.
• Trap 4: Forcing misaligned rigid lines into place using B-nut torque. Pulling a misaligned line into place using nut torque introduces immense residual preload bending stresses. When cyclic system pressure and airframe vibration are added, the line fractures at the flare neck within hours of operation. Rigid lines must align squarely by hand before torquing.