Summary of Aircraft Plumbing Systems and Flexible Hoses
Aircraft Plumbing Systems & Flexible Hoses Explained
Introduction
Aircraft fluid lines and hose installations are critical to safe, reliable operation of hydraulic, fuel, oil, and pneumatic systems. This guide explains how fluid lines are identified, installed, supported, and inspected. It breaks down key installation practices, materials, and precautions you must follow during maintenance.
Definition: Fluid lines are the rigid tubing and flexible hoses that carry hydraulic fluid, fuel, oil, or pneumatic pressure throughout an aircraft.
Types of Fluid Lines
Rigid tubing
- Usually made of metal (aluminum alloy, stainless steel, or copper-nickel)
- Size is specified by outside diameter (OD)
- Joined by fittings and flares
Flexible hose
- Made from synthetic rubbers or Teflon (PTFE) with reinforcement
- Size is specified by inside diameter (ID)
- Used where movement, vibration isolation, or routing flexibility is required
Definition: Flexible hose is a multi-layer assembly that includes an inner liner, reinforcement (braid or spiral), and an outer cover to resist environmental damage.
Identification and Markings
- Flexible rubber hoses are normally marked with: hose size, manufacturer, date of manufacture, pressure rating, and temperature limits.
- Required markings must be repeated along the hose length at intervals of not more than 9 inches.
Materials Commonly Used for Flexible Hoses
Compare common synthetic materials and one key property:
| Material | Typical property/use |
|---|---|
| Buna-N (Nitrile) | Good oil resistance, common in hydraulic systems |
| Neoprene | Good weather and ozone resistance |
| Butyl | Excellent air and gas impermeability |
| EPDM (Ethylene Propylene Diene) | Heat and steam resistance |
| Teflon (PTFE) | High temperature and chemical resistance, retains strength at high temperature |
Hose Sizing and Slack
- Flexible hose size is determined by the inside diameter (ID).
- Allow 5%–8% slack of total hose length to permit movement and prevent undue stress when pressurized.
Lay Line (Identification Stripe)
- The lay line or identification stripe runs along the length of a flexible hose to help identify the hose type, verify correct orientation, and show if the hose has been twisted during installation.
Proof Testing and Inspection
- Proof test pressure varies with hose type, but is generally about two times the recommended operating pressure for the hose.
- Small scratches and nicks on tubing can be repaired if they are less than 10% of the wall thickness.
Flaring and Flareless Connections
Flare types
- Two common flare styles in aircraft plumbing: single flare and double flare.
- Aviation tubing flare angle: 37 degrees (standard for many aircraft fittings).
Definition: Flaring is forming a conical enlargement at the end of a tube to create a metal-to-metal seal against a fitting.
Flareless MS fittings
- A Military Standard (MS) flareless-tube fitting contains three parts: the body, the sleeve, and the nut.
- Tightening procedure: hand-tighten until snug, then no more than 1/3 turn with a wrench.
- Over-tightening causes the sleeve's cutting edge to cut too deeply and weaken the tube.
Sleeve purpose on flared connections
- The sleeve provides a seating surface; when the nut is tightened it draws the sleeve and the tubing flare tightly against the male fitting to form the seal.
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Aircraft Fluid Lines
Klíčové pojmy: Flexible hose size is by inside diameter (ID), Rigid tubing size is by outside diameter (OD), Hose markings: size, manufacturer, manufacture date, pressure, temperature limits, Markings repeated at intervals not more than 9 inches, Allow 5%–8% slack in flexible hose length, Proof test approximately two times operating pressure (per hose spec), Repair scratches/nicks if < 10% of wall thickness, Aviation flare angle is 37 degrees, MS flareless fitting parts: body, sleeve, nut; tighten hand-snug + 1/3 turn, Over-tightening flareless sleeve weakens tube by cutting too deeply, Route fluid lines below electrical wiring, maintain 6 in clearance when possible, never less than 1/2 in, Use compatible fittings with tubing material (e.g., aluminum fittings with aluminum tubing)