Fluid Lines And Fittings

Question 1

What two general types of fluid lines are commonly used in aircraft plumbing?

Answer 1

Metal tubing or flexible hose and fittings.

Question 2

How is metal tubing sized?

Answer 2

By outside diameter in 1/16-inch increments, and wall thickness.

Question 3

What material should be selected for high-pressure rigid fluid lines?

Answer 3

Annealed or 1/4-hard corrosion-resistant steel.

Question 4

What type of material should fittings be made of when used with stainless steel tubing?

Answer 4

Stainless steel fittings.

Question 5

What percentage of flattening is allowed when bending metal tubing?

Answer 5

Do not exceed 75% of original diameter.

Question 6

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

Answer 6

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 one-half inch.
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.

Question 7

What is the purpose of the identification stripe running along the length of a flexible hose?

Answer 7

indicates if the hose has twisted during installation.

Question 8

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

Answer 8

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

Question 9

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

Answer 9

AN fluid line fittings are colored either blue or black while AC type fittings are usually gray or yellow.
AN fittings have coarser (rough) 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.

Question 10

What is the purpose of using quick disconnect couplings in fluid systems?

Answer 10

They provide a means of quickly disconnecting a line without a loss of fluid or air entering the
system. Quick disconnect couplings are installed at locations where frequent uncoupling of the line
is required for inspection and maintenance. Each half has a valve that is open when coupled
together and spring loaded to close when disconnected.

Question 11

How is flexible hose sized?

Answer 11

By the inside diameter in 1/16 of an inch increments.

Question 12

How much slack must be left in a flexible hose during installation?

Answer 12

Five to eight percent of the total hose length must be allowed for freedom of movement under
pressure. 5-8% of slack of total length

Question 13

Flexible hoses should be clamped at least every how many inches.

Answer 13

Every 24 inches

Question 14

Name considerations when installing flexible hose assemblies?

Answer 14

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

Question 15

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

Answer 15

Typically 1.5 times the recommended operating pressure of the hose.

Question 16

What are the three parts of a Military Standard (MS) tube fitting?

Answer 16

The body, the sleeve, and a nut.

Question 17

What will be the result of over tightening a flareless tube fitting?

Answer 17

The compression sleeve’s cutting edge will cut too deep into the tube and cause the tube to be
weakened. Flareless fittings are made up of the tube, compression sleeve, and nut.

Question 18

What are the two kinds of flares used in aircraft plumbing systems?

Answer 18

A single flare and a double flare.

Question 19

What is the difference between automotive and aviation flares?

Answer 19

Aviation flares should be 37° and automotive are 45°.

Question 20

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

Answer 20

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

Question 21

What-precaution should be taken when deburring and inspecting the flared end of a tube?

Answer 21

Care should be taken that the tubing is not cracked or that the wall thickness is not reduced by the
deburring process.

Question 22

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

Answer 22

Synthetic.

Question 23

What type of information is on a hose lay-line?

Answer 23

Information found on the lay-line will include the type of hose, and the quarter and year of
manufacture.

Question 24

What is the purpose of a sleeve on a flared-tube connection?

Answer 24

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

Question 25

How is flared tubing affected by overtightening of the fittings?

Answer 25

The tube flare is likely to be damaged, forming a weak point which may fail when under pressure.

Question 26

What are the identification markings usually found on flexible rubber hose?

Answer 26

Hose size, manufacturer, date of manufacture, pressure, and temperature limits.

Question 27

In addition to being securely clamped in place, how should metal fuel, oil, and hydraulic lines be installed in an aircraft?

Answer 27

They should be electrically connected (grounded to the aircraft structure.

Question 28

When selecting a flexible hose for use in a phosphate ester based hydraulic fluid (Skydrol) system, the hose should be made of which synthetic rubber compound?

Answer 28

Butyl

Question 29

Scratches and nicks may be repaired if they are less than what percent of the wall thickness?

Answer 29

10 percent.

Question 30

What caution must be observed when deburring the end of tubing after cutting?

Answer 30

Extreme care that the wall thickness is not fractured or reduced in size.

Question 31

What happens to a flareless-tube fitting when it is overtightened?

Answer 31

The nut drives the cutting edge of the sleeve deeply into the tube, causing the tube to be weakened.

Question 32

What is the principle advantage of flexible Teflon hose?

Answer 32

High operating strength at elevated temperature.

Question 33

What is the maximum interval for repeating the required markings on a flexible hose?

Answer 33

Not more than nine inches along the length of the hose.