Aircraft hydraulic, fuel, oil, oxygen, and pneumatic systems depend on precisely sized fluid lines to deliver the right flow at the right pressure. Pick the wrong line and you risk leaks, pressure loss, or catastrophic failure. Yet the sizing system used in aviation can trip up even experienced technicians: some lines are specified by their outside diameter (OD), others by their inside diameter (ID), and both systems use a shorthand called the dash number. Mastering the distinction is a core competency for every Aviation Maintenance Technician (AMT) and a reliable topic on the FAA General knowledge test.
This article walks through how fluid lines are classified, what the dash number actually represents for each line type, how to convert between dash numbers and real dimensions, and how to apply that knowledge correctly during inspection, repair, and replacement.
Types of Aircraft Fluid Lines
Aircraft use two broad categories of fluid lines: rigid tubing and flexible hose. Rigid tubing is typically aluminum alloy, corrosion-resistant steel, or titanium, bent and flared to fit a fixed routing. Flexible hose — constructed of an inner synthetic rubber or polytetrafluoroethylene (PTFE) tube, wire braid reinforcement, and an outer protective jacket — accommodates vibration and movement between components. Both categories use dash numbers, but the dash number references a different dimension depending on which type you are working with.
Outside Diameter (OD) and the Rigid Tubing Dash Number
For rigid metal tubing, the dash number directly encodes the outside diameter in sixteenths of an inch. The convention is simple: the dash number equals the OD expressed as a numerator over 16. So a -4 tube has an OD of 4/16 inch, which reduces to 1/4 inch. A -6 tube has an OD of 6/16 inch (3/8 inch). A -8 tube has an OD of 8/16 inch (1/2 inch), and so on.
Why does the specification reference the outside diameter rather than the inside diameter? Because the OD is the controlled, measurable dimension in tubing manufacture. The wall thickness varies by material and pressure rating, which means the ID changes with wall thickness even if the OD stays constant. Fittings — AN flareless fittings, AN flared fittings, MS fittings — are designed to mate with a specific OD, so the OD is the critical mating dimension for assembly and leak-free sealing.
Quick OD Conversion Reference
- Dash -2: OD = 2/16 in = 1/8 in
- Dash -3: OD = 3/16 in
- Dash -4: OD = 4/16 in = 1/4 in
- Dash -5: OD = 5/16 in
- Dash -6: OD = 6/16 in = 3/8 in
- Dash -8: OD = 8/16 in = 1/2 in
- Dash -10: OD = 10/16 in = 5/8 in
- Dash -12: OD = 12/16 in = 3/4 in
- Dash -16: OD = 16/16 in = 1 in
To find the OD of any rigid tube dash number, simply place the dash number over 16 and reduce the fraction. To find the dash number from a known OD, multiply the OD (in inches) by 16.
Inside Diameter (ID) and the Flexible Hose Dash Number
For flexible hose, the dash number encodes the inside diameter in sixteenths of an inch — not the outside diameter. This is the single most important distinction to memorize for the knowledge test. A -4 hose has an ID of 4/16 inch (1/4 inch). A -6 hose has an ID of 6/16 inch (3/8 inch).
The reason flexible hose is specified by ID is rooted in its function: a hose carries fluid, so the flow capacity — governed by the internal cross-sectional area — is the primary engineering concern. Because flexible hose is built up from multiple layers of varying thickness (liner, braid, jacket), the OD varies widely with construction and pressure rating, making it an unreliable reference for ordering or interchangeability. The ID, by contrast, directly determines flow characteristics and remains the standard reference across AN, MS, and MIL-spec hose standards.
It is also worth noting that for a given dash number, a flexible hose and a rigid tube will have different actual dimensions. A -6 rigid tube has an OD of 3/8 inch, while a -6 flexible hose has an ID of 3/8 inch. The OD of that same -6 hose will be noticeably larger, depending on its construction and pressure rating. Never assume a dash number means the same physical size regardless of line type.
AN and MS Specification Numbers
Dash numbers appear within larger part number strings governed by AN (Army-Navy Aeronautical Standard) and MS (Military Standard) specifications. For rigid tubing you will see designations such as AN818 (tube nut) and AN819 (sleeve) with a trailing dash number indicating size. For flexible hose, common standards include MIL-DTL-6000 (rubber hose) and MIL-DTL-8794 (PTFE hose), each with their own pressure ratings and temperature limits. The AMT Handbook — General (FAA-H-8083-30) details these standards and emphasizes that replacement lines must match the original specification number, dash number, and end fitting type. Substituting a lower-pressure-rated hose with the correct dash number but a different specification is an improper and potentially dangerous repair.
Wall Thickness, Pressure Rating, and Material
Once you know the OD (rigid tube) or ID (flexible hose), you still need the correct wall thickness and material to complete the line identification. Wall thickness, combined with material strength, determines the burst pressure and working pressure of the line. Aluminum alloy tubing (such as 2024-T3 or 5052-O) is used for lower-pressure systems — typically fuel and oil lines under 1,500 psi. Corrosion-resistant steel (CRES) is required for high-pressure hydraulic lines, often rated at 3,000 psi or above. Titanium tubing finds use where weight savings and high strength are both required.
For flexible hose, the number of wire braid layers determines pressure rating. A single-braid hose handles low to medium pressures; a double-braid or spiral-wound hose is used in high-pressure hydraulic applications. Always verify both the dash number and the full specification number when sourcing replacement hose.
Why It Matters: Safety and Airworthiness
Fluid line sizing errors are not merely procedural — they directly affect aircraft airworthiness. An undersized line restricts flow, starving actuators or engine fuel systems and causing erratic operation. An oversized line may not seal correctly with its fittings, producing leaks that create fire hazards (fuel or hydraulic fluid near ignition sources) or loss of system pressure. An incorrect pressure rating — especially a lower-rated hose installed in a high-pressure system — can result in hose rupture in service. The FAA and aircraft manufacturers specify exact replacement requirements, and 14 CFR Part 43 requires that maintenance return an aircraft to its original or properly altered condition using acceptable data, which includes matching line specifications exactly.
Key Numbers and Rules
- Rigid tube dash number = OD in sixteenths of an inch.
- Flexible hose dash number = ID in sixteenths of an inch.
- To convert any dash number to inches: divide by 16.
- To find a dash number from a measured OD or ID (in inches): multiply by 16.
- A -4 rigid tube and a -4 flexible hose are NOT the same physical size — only the dash numbering convention is the same.
- Replacement lines must match specification number, dash number, material, and end fitting type — not just the dash number alone.
- Minimum bend radius, end fitting compatibility, and pressure rating must all be verified during replacement.
Memory Aid
"Rigid is Outside, Hose is Inside" — For rigid tubing, the dash number gives you the outside diameter. For hose, the dash number gives you the inside diameter. Think of it this way: a rigid tube has a hard exterior you grip and fit — so its OD matters. A hose carries flow through its interior — so its ID matters. Linking the controlling dimension to the function of each line type makes the rule easy to recall under test pressure.
Common Test Traps
- Assuming the dash number always means OD. Many technicians learned rigid tube sizing first and automatically apply OD to hose as well. The FAA tests this confusion directly — remember that flexible hose dash numbers reference ID.
- Confusing a -6 tube with a -6 hose as the same size. They share a dash number but not a common dimension. A -6 tube has a 3/8-inch OD; a -6 hose has a 3/8-inch ID. Their ODs are completely different.
- Thinking the dash number alone is sufficient for ordering a replacement. The full specification number, material, pressure rating, and end fitting type are all required. Selecting the correct dash number but the wrong spec can result in an improper repair.
- Forgetting the sixteenths conversion. Test questions sometimes give an OD in fractions and ask for the dash number, or vice versa. Practice the multiply-by-16 / divide-by-16 conversion until it is automatic.
- Ignoring wall thickness in rigid tubing. Two tubes can share the same dash number (same OD) but have different wall thicknesses and therefore different IDs and pressure ratings. Wall thickness must match the aircraft manufacturer's specification, not just the dash number.