Aircraft plumbing systems are the lifeblood of any aircraft, transporting essential fluids like hydraulic fluid, oil, and fuel. Understanding the components, installation, and maintenance of both rigid tubing and flexible hoses is critical for aviation students and professionals. This article will provide a comprehensive overview, drawing directly from industry best practices.
Understanding Aircraft Plumbing Systems and Flexible Hoses
Aircraft plumbing involves a network of lines designed to withstand rigorous operational conditions. These systems are typically comprised of metal tubing and fittings, or flexible hose, each with specific applications and installation requirements.
The Backbone: Rigid Aircraft Plumbing Lines
Rigid tubing is essential for many fixed-route fluid transfers in aircraft. Proper handling and installation are key to its integrity.
Materials and Sizing of Rigid Lines
Aircraft plumbing lines are frequently made of metal tubing. For aluminum alloy tubing, it's crucial to use corresponding aluminum alloy fittings to ensure compatibility and prevent corrosion.
The size of a rigid fluid line is always determined by its outside diameter. This standardization helps in selecting the correct components and fittings.
Installation and Maintenance Best Practices for Tubing
When tubing passes through a hole in the aircraft structure, grommets must always be used. This prevents chafing and potential damage to the tubing.
For minor damage, scratches and nicks may be repaired if they are less than 10 percent of the wall thickness. Deeper damage typically requires replacement.
During deburring of a cut tube end, great care must be taken. The process should not crack the tubing or reduce its wall thickness, which could weaken the line.
If flaring aluminum alloy tubing, the end must be polished beforehand. This removes any sharp edges that could cause the tubing to crack during the flaring process.
Flaring and Flare Angles in Aviation Tubing
Aviation tubing uses a specific flare angle of 37 degrees. Both single flares and double flares are used in aircraft plumbing systems.
When making a flared tube connection, a sleeve is crucial. The nut fits over the sleeve and, when tightened, draws the sleeve and tubing flare tightly against the male fitting to form a secure seal.
Routing Rigid Lines and Electrical Wiring
When routing fluid lines adjacent to electrical wiring, strict precautions are necessary. Fluid lines should be routed below electrical wiring whenever possible, maintaining a clearance of at least six inches.
In no instance should fluid lines and electrical wiring be closer than one-half inch. Both the wiring and fluid lines must be fastened to the aircraft structure using clamps or other methods to maintain this separation.
Flexible Hoses in Aircraft Systems
Flexible hoses are vital for connections that require movement, vibration absorption, or frequent disconnections. They offer flexibility where rigid lines cannot.
Materials and Construction of Flexible Hoses
Several synthetic materials are commonly used in the manufacture of flexible hoses due to their durability and resistance to various fluids and temperatures. These include:
- Buna-N
- Neoprene
- Butyl
- Ethylene Propylene Diene Rubber (EPDM)
- Teflon
Flexible Teflon hoses are particularly advantageous in aircraft hydraulic systems. They retain their high strength under conditions of high temperature, making them ideal for demanding applications.
Identification and Sizing of Flexible Hoses
Flexible rubber hoses typically have several identification markings. These include the hose size, manufacturer, date of manufacture, and pressure and temperature limits.
These required markings must be repeated at intervals not exceeding nine inches along the length of the hose. This ensures easy identification even after installation.
The size of a flexible hose is determined by its inside diameter, distinguishing it from rigid tubing.
Installation Considerations for Flexible Hoses
During installation, a specific amount of slack is required in a flexible hose. Five to eight percent of the total hose length must be allowed for freedom of movement under pressure. This prevents stress on the hose and fittings.
The lay line (identification stripe) running along the length of a flexible hose serves a critical purpose. It is used to determine if the hose has twisted during installation. After installation, the stripe should not spiral around the hose, indicating a correct, untwisted setup.
Flexible hose assemblies undergo a proof test to ensure their integrity. The pressure for this test varies by hose type but is generally about two times the recommended operating pressure.
Ensuring System Integrity: Fittings and Fasteners
Correct fittings and secure fastening are paramount for preventing leaks and maintaining system performance.
AN Fluid Line Fittings
Standard AN-type fluid line fittings are easily identified by their color, being either blue or black. A key identifying feature of an AN fitting is a shoulder between the end of the flare cone and the first thread. This shoulder helps differentiate them from other fitting types.
Military Standard (MS) Flareless-Tube Fittings
MS flareless-tube fittings consist of three main parts: the body, the sleeve, and a nut. They provide a secure connection without flaring the tube.
When tightening an MS flareless fitting, it should be tightened by hand until snug. Then, use a wrench for no more than 1/3-turn further. Overtightening can be detrimental, as the sleeve's cutting edge will cut too deeply into the tube, weakening it.
Support Clamps and Torque Seal
Bonded support clamps are specifically used to secure metal hydraulic, oil, and fuel lines. The bonding material prevents galvanic corrosion between the clamp and the metal tubing.
Torque seal is applied to critical fittings after installation as a visual indicator. If a visual inspection reveals that the torque seal has broken, it indicates that the fitting may have loosened, requiring immediate attention.
Quick Disconnect Couplings
Quick disconnect couplings are specialized fittings installed where frequent uncoupling of lines is necessary for inspection or maintenance. They allow for quick disconnection without significant fluid loss or air entering the system.
Each half of a quick disconnect coupling features a valve that is open when coupled together and spring-loaded closed when disconnected. This design minimizes spills and system contamination.
FAQ: Common Questions About Aircraft Plumbing Systems
How is the size of a flexible hose determined?
The size of a flexible hose is determined by its inside diameter, unlike rigid tubing which is sized by its outside diameter.
What is the purpose of the lay line on a flexible hose?
The lay line, or identification stripe, on a flexible hose is used to check if the hose has twisted during installation. A properly installed hose will have a straight, untwisted lay line.
What are the dangers of over-tightening an MS flareless tube fitting?
Over-tightening an MS flareless tube fitting can cause the sleeve's cutting edge to cut too deeply into the tube, which weakens the tube and can lead to failure.
What precautions should be taken when deburring the cut end of a tube?
When deburring, care should be taken to ensure that the tubing is not cracked and that the wall thickness is not reduced by the deburring process. Both issues can compromise the tube's integrity.
What synthetic materials are commonly used for flexible hoses in aircraft?
Common synthetic materials include Buna-N, Neoprene, Butyl, Ethylene Propylene Diene Rubber (EPDM), and Teflon. Teflon is particularly valued for its high strength retention at high temperatures.