Chapter 10: Fluid Lines and Fittings

Of what materials are most low-pressure rigid fluid lines made?

1100-1/2 hard or 3003-1/2 hard aluminum allow tubing

Is the size of a rigid fluid line determined by its inside or its outside diameter?

By its outside diameter

What is the difference between the flare angle for the aircraft and automotive fittings?

Aircraft fittings have a 37 degree flare angle and automotive fittings use a 45 degree angle

Where are quick-disconnect fluid line couplings normally used in an aircraft hydraulic system?

In the lines that connect the engine-driven pump into the hydraulic system

Is the size of a flexible hose determined by its inside or its outside diameter?

By its inside diameter

Of what material should rigid fluid lines be made that carry high-pressure (3,000psi or greater) hydraulic fluid?

Annealed or 1/4-hard corrosion-resistant steel

What is the principal advantage of Teflon hose for use in an aircraft hydraulic system?

Teflon hose retains its high strength under conditions of high temperature

How tight should an MS flareless fitting be tightened?

Tighten the fitting by hand until it is snug, and then turn it with a wrench for 1/6-turn to 1/3-turn. Never turn it more than 1/3-turn with a wrench

What damage can be caused by overtightening an MS flareless fitting?

Overtightening drives the cutting edge of the sleeve deep into the tube and weakens it

What kind of rigid tubing can be flared with a double flare?

5052-O and 6061-T aluminum alloy tubing in sizes from 1/8" to 3/8" OD

What should be done to the end of a tube that is to be flared?

The cut end should be polished to remove any sharp edges that could cause the tubing to crack

What is the minimum amount of slack that must be left when a flexible hose is installed in an aircraft hydraulic syste?

The hose should be at least 5% longer than the distance between the fittings. This extra length provides the needed slack

How much pressure is used to proof-test a flexible hose assembly?

This varies with the hose, but it is generally about two times the recommended operating pressure for the hose

Why is it important to use a torque wrench when securing fluid hose and line fittings?

If the connection is too loose, it may leak. If the connection is too tight, it can cause the fitting to become damaged and fail

What is the purpose of using torque seal on a critical fitting after installation?

If a visual inspection reveals that the torque seal has broken, it is an indication that the fitting may have loosened

What are the two types of fluid lines commonly found in aircraft?

Rigid metal lines and flexible hoses

Describe the method of classifying metal tube according to size

Metal tubing is sized according to wall thickness and outside diameter. Outside diameter is measured in 1/16th" increments

When installing stainless steel tubing, what type of fittings should be used?

Stainless steel fittings

What is the function of the sleeve on a flared-tube fitting?

The nut fits over the sleeve and draws the sleeve and the tubing flare tightly against the male fitting to form the seal

Describe the two types of flares commonly used on aircraft tubing

The single flare and the double flare

How can you determine if a fitting is an AN type rather than an AC type?

AN fittings have a shoulder between the flare cone and the end of the threads. AC fittings do not

What are the most significant differences between AN and AC fittings?

Sleeve length, thread pitch, and the shoulder between the threads and the flare cone on AN fittings

What are the names of the parts of a flareless tube fitting?

The nut and the ferrule or sleeve

How do you tell a flareless fitting from a flare-type fitting?

Flareless fittings don't have a flare cone and there is no space between the threads and the end of the fitting

What is the effect of over-tightening a flare type fitting?

The sealing surface may be damaged or the flare cut off

In addition to being securely clamped, what is an additional requirement for installing metal fuel, oil, and hydraulic lines?

The lines must be electrically bonded to the structure

What are some of the important advantages of Teflon tubing?

It is compatible with nearly every liquid, has a broad operating temperature range, low resistance to fluid flow and has a very long shelf and service life

Describe the identification markings commonly found on flexible hoses

A lay line, identification such as a Mil Spec number, the manufacturer's name or symbol, the hose size and a date code

How can you determine if a flexible hose has been correctly or incorrectly installed?

The lay line will be straight if the hose is properly installed. A twisted lay line indicates an incorrect installation

How are flexible hose sizes designated?

By the inside diameter, measured in 1/16th" increments

What precautions must be observed when deburring the end of a tube after it is cut?

The wall thickness must not be reduced in size or fractured

Describe the operation of quick disconnect couplings

Each half has a valve that is held open when coupled and is spring-loaded closed when disconnected

If you fabricate a replacement for a flexible hose, what percentage of the total length must be added to allow for movement under pressure?

5-8&

What happens to the tube when a flareless fitting is overtightened?

The tube is weakened when the nut drives the cutting edge of the sleeve too deeply into the tube

Why are quick disconnect fittings used?

To provide a quick means to connect or disconnect a fluid line without loss of fluid or entrance of air into the system

In the early days of aviation, what tubing material was used extensively in aviation fluid applications?

Copper tubing

What percentage of flattening is allowed when bending metal tubing?

25% Maximum (Must maintain 75% of the original tube diameter)

What precautions should be taken when routing fluid lines adjacent to electrical wiring?

Route the fluid lines below the electrical wiring and maintain a clearance of at least six inches whenever possible. In no case should fluid lines and electrical wiring be closer than 1/2". Ensure that both the wiring and the fluid lines are fastened to the aircraft structure by clamps or other methods that will maintain the separation

What are some synthetic materials commonly used to manufacture flexible hoses?

Buna-N, Butyl, Ethylene propylene diene rubber, Neoprene, and Teflon

How are standard AN type and AC type fluid fittings identified?

AN fluid line fittings are colored either blue or black while AC type fittings are usually gray or yellow. AN fittings have coarser threads and they have a small shoulder between the threads and the 37 degree flare cone. The AC fittings are completely threaded all the way to the 35 degree flare cone

How often should flexible hoses be clamped?

Every 24 inches

Name considerations when installing flexible hose assemblies

Slack, flexing, twisting, bending, clearance, and clamps

What pressure is used to proof-test a flexible hose assembly?

Typically, 1.5 times the recommended operating pressure of the hose

What is the advantage of flexible Teflon hose over rubber hose?

Teflon hoses have better resistance to chemicals and a higher strength property at higher temperatures

What type of rubber is used in flexible lines that carry phosphate-ester hydraulic fluid (Skydrol)

Butyl

How can you tell that a flexible hose has been installed in a twisted position?

By the identification stripe running along the hose length

How is flared tubing affected by overtightening of the fittings?

The flare is likely to be damaged

What is the distance between hose identification markings?

9 inches

What would cause a flare to be crooked?

Improperly cutting the tube

Risk management: Why is it important to understand system configurations during aircraft maintenance?

A lack of understanding of a system could lead to fluid lines being reversed or incorrectly installed, leading to a potentially hazardous situation

Risk management: What safety equipment should be considered when working on fluid lines?

Depending on the types of fluid, safety equipment may include gloves and clothing protection, eye protection, spill containment materials, respirator, and fire extinguisher

Risk management: How can an aviation mechanic determine what fluids are contained in the hoses and fluid lines that are encountered when working on an aircraft?

Fluid lines and hoses are labeled at regular intervals to identify the type of fluid. Additionally, the aircraft maintenance manual provides information on each aircraft system

Risk management: What risk mitigation action should occur prior to working on any pressurized fluid system?

The system must be depressurized prior to beginning any work on the system

Risk management: What is the function of the lay line (the identification stripe) that runs the length of a flexible hose?

To show the mechanic whether or not the line was twisted when it was installed. The line should be straight, not spiraled. A twisted hose may move from its intended position and chafe or interfere with other systems

Risk management: What is the danger of a loosened fitting or a hose that has moved out of position?

A loosened fitting may leak or allow a hose to move out of position. A hose that has moved out of position may chafe on adjacent structures and begin to leak, or it may cause interference and damage to other systems

Risk management: What precaution must be used when applying torque to a fluid line?

The fitting that the line is being torqued to must be held in position with a second wrench to prevent twisting or damage to the secondary fitting or line