10.2 Fluid Lines, Flexible Hose & Minimum Bend Radius Standards

10.2 Fluid Lines, Flexible Hose & Minimum Bend Radius Standards

[!NOTE] Where this lives: AC 43.13-1B, Chapter 9, Aircraft Systems and Components. Section 2 covers hydraulic lines and fittings, and carries Table 9-2 (Tube data with wrench torque values), Table 9-3 (Aircraft hose specifications), Figure 9-9 (Proper hose installations), Figure 9-10 (Minimum bend radii), Table 9-4 (Ball diameters for testing hose restrictions or kinking), and Figure 9-12 (Minimum bend radii for Teflon hose).

Fluid lines are where the IAR test hides its chart-reading questions. The material is not conceptually hard; the difficulty is entirely in reading the right row of the right table and noticing which variable the question is changing.

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Rigid Tubing: Sizing, Bending, and Torque

Tube size is stated as a dash number equal to the outside diameter in sixteenths of an inch. A -6 line is 6/16 inch, or 3/8 inch, OD. Wall thickness is stated separately, so a complete specification reads like "1/2 inch × 0.042 inch."

Table 9-2 gives, for each dash size, the wrench torque range for tightening the AN-818 nut on aluminum-alloy tubing and on steel tubing, plus a separate column for aluminum-alloy tubing with the MS33583 flare used on oxygen lines only. Two inspection points follow:

• Torque is material-dependent. Steel tubing takes substantially higher torque than aluminum of the same size. A joint torqued to the steel value on aluminum tubing will crack the flare.
• Overtightening a flare fitting does not cure a leak. It thins and cracks the flare. A leaking flare is disassembled and inspected, not tightened harder.

Minimum bend radii for rigid tube are measured to the tubing centerline, and the AC tabulates them separately for aluminum alloy and steel. The same OD gives different minimum radii in the two materials.

Flares: the single 37-degree AN flare is standard in aircraft fluid systems. Double flaring is used on soft aluminum tubing in smaller sizes to resist cracking. Automotive 45-degree flare fittings are not interchangeable with AN 37-degree fittings, and a mixed joint will leak or fail.

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Flexible Hose: Identification

Table 9-3 identifies aircraft hose by military specification, listing tube size, hose ID and OD, recommended operating pressure, minimum burst pressure, minimum proof pressure, and minimum bend radius. Reading a hose specification such as MIL-H-8788-6 decomposes as:

• MIL-H-8788 — the specification family (high-pressure hydraulic, pneumatic, coolant, fuel, and oil use)
• -6 — the dash size, 6/16 inch = 3/8 inch tube size
• Construction: seamless synthetic rubber inner tube, one fabric braid, two or more steel wire braids, synthetic rubber cover
• Operating temperature range and identification data are printed on the hose along with the quarter and year of manufacture

That last item is why hose is an age-limited item in practice: the layline stamp lets an inspector determine the hose's age even when no record exists.

Teflon (PTFE) hose is treated separately. It resists the fluids used in aircraft, operates over roughly −65 °F to 450 °F, and has its own minimum bend radius figure (Figure 9-12) — do not use the rubber-hose radii for Teflon.

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The Two Variables That Set Minimum Bend Radius

This is where the IAR test earns its reputation. Minimum bend radius for a given hose is not a single number: it varies with the hose specification and size and with whether the installation flexes.

The reasoning is mechanical. A hose that flexes in service works its reinforcement braid back and forth at the bend; the tighter the bend, the higher the strain per cycle, so a moving installation is given a more generous radius to keep braid strain within limits. A question that changes only "flexing" to "non-flexing" and asks how the minimum bend radius compares is asking whether you understand that: the flexing installation has the greater minimum bend radius.

A related question form fixes the pressure and asks how the minimum allowable bend radius changes as the installation angle increases in a non-flexing constant-pressure system. Reading Figure 9-10, the required radius grows with the angle through which the hose must turn.

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Installation Rules the Inspector Enforces

AC 43.13-1B Chapter 9 is unusually specific, and each rule is a checkable inspection item:

1. Install hose assemblies without twisting. A twisted hose is pre-loaded in torsion, and pressurization amplifies the twist. The layline printed along the hose exists so an inspector can see twist at a glance — if the layline spirals, the hose is twisted.
2. Provide about 5 to 8 percent slack. A hose stretched tight between two fittings is overstressed and will eventually fail. Hose contracts in length when pressurized, which is precisely why slack is required rather than optional.
3. Never exceed the minimum bend radii of Figure 9-10. The AC directs keeping the bend radius as large as possible at all times to avoid tube collapse.
4. End fittings are not part of the flexible portion. Bend radius is measured over the flexible hose only; you cannot count fitting length toward the bend.
5. Use elbows and adapters where they ease installation and remove strain from the hose — the AC specifically recommends this to increase service life.
6. Replace like with like. When a flexible line must be replaced, use the same type, size, part number, and length of hose as the line being replaced, and check TSO requirements. Fabricating a "close enough" replacement from bulk hose of a different specification is not an acceptable practice.
7. Support and route so hose cannot chafe on structure, contact hot sections, or bear against moving parts through the full travel of whatever it is attached to.

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Inspecting Aging Hose

Table 9-4 supports an internal check: dropping a specified ball diameter through the hose confirms that the bore is not restricted or kinked. The ball size scales with hose size, from 5/64 inch for a −4 hose upward.

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High-Yield Exam Traps

• A flexing installation requires a larger minimum bend radius than the same hose in a non-flexing installation.
• Slack is 5 to 8 percent — a taut hose is a defect, not a tidy installation.
• Minimum tube bend radii are measured to the centerline, and differ between aluminum alloy and steel.
• Teflon hose has its own bend radius figure. Do not apply rubber-hose values to it.
• Replace with the same type, size, part number, and length, and check TSO requirements.
• Torque values in Table 9-2 are material-specific; steel values applied to aluminum flares crack them.