AIR 1120 Rigid Lines

Metal tubing fluid lines are sized by wall thickness and

• outside diameter in 1/16 inch increments

• inside diameter in 1/16 inch increments.

• outside diameter in 1/32 inch increments.

• inside diameter in 1/32 inch increments.

Answer explanation

Metal tubing used in aircraft fluid power installations is sized by its outside diameter, which is measured fractionally in sixteenths of an inch

What is an advantage of a double flare on aluminum tubing?

• Ease of construction.

• More resistant to damage when the joint is tightened.

• Can be applied to any size and wall-thickness of tubing.

• Makes for easy identification of tubing material

Answer explanation

Tubing made of 5052-O and 6061-T aluminum alloy in sizes from 1/8- to 3/8-inch OD should be flared with a double flare. Double flares are smoother than single flares and are more concentric. Also, the extra metal makes the flare more resistant to the shearing effect when the fittings are torqued.

What is the color of an AN steel flared-tube fitting?

• Black

• Blue

• Red

• Yellow

Answer explanation

Steel AN flared tube fittings are colored black. Aluminum alloy AN fittings are colored blue.

Which of the following statements is correct in reference to flare fittings?

• AN fittings have an identifying shoulder between the end of the threads and the flare cone

• AC and AN fittings are considered identical except for material composition and identifying colors.

• AN fittings are generally interchangeable with AC fittings of compatible material composition.

Answer explanation

AN flare fittings have a shoulder between the end of the threads and the flare cone. In AC fittings the threads go all the way to the cone. AN fittings are dyed blue or black and AC fittings are gray or yellow. The threads on an AN fitting are courser than those on an AC fitting. AN and AC fittings are not interchangeable.

Excessive stress on fluid or pneumatic metal tubing caused by expansion and contraction

due to temperature changes can best be avoided by

• using short, straight sections of tubing between fixed parts of the aircraft

• using tubing of the same material as the majority of the adjoining structure

• providing bends in the tubing

• installing a shroud around the tubing

Answer explanation

Never select a path for a rigid fluid line that does not require bends in the tubing. Bends are necessary to permit the tubing to expand and contract under temperature changes and to absorb vibration.

The material specifications for a certain aircraft require that a replacement oil line be fabricated from 3/4-inch 0.072 5052-0 aluminum alloy tubing. What is the inside dimension of this tubing?

• 3/4 inch

• 3/4 inch minus 0.072"

• 3/4 inch minus 2 times 0.072"

• 0.072"

Which statement about Military Standard (MS) flareless fittings is correct?

• During installation, MS flareless fittings are normally tightened by turning the nut a specified amount, rather than being torqued

• New MS flareless tubing/fittings should be assembled clean and dry without lubrication

• During installation, MS flareless fittings are usually tightened by applying a specified torque to the nut

Answer explanation

MS flareless fittings are attached to the end of a metal tube by presetting the fitting on the tube. Presetting consists of putting enough pressure on the fitting to deform the ferrule and cause it to cut into the outside of the tube. Presetting is done by lubricating the threads of the presetting tool and the nut with hydraulic fluid, assembling the nut and ferrule on the tube, putting it in the presetting tool, and tightening the nut by hand until resistance is felt, then turning it with a wrench from 1 to 1-1/4 turns. When installing the fitting in an aircraft hydraulic system, tighten the nut by hand until resistance is felt, then turn it 1/6 to1/3 of a turn (one hex to two hexes) with a wrench

Scratches or nicks on the straight portion of aluminum alloy tubing may be repaired if they are no deeper than

• 20 percent of the wall thickness

• 1/32 inch or 20 percent of wall thickness, whichever is less

• 10 percent of the wall thickness

• 75 percent of the wall thickness

Answer explanation

Scratches or nicks are allowed in a piece of aluminum alloy tubing provided it is no deeper than 10 percent of the wall thickness of the tube, and it is not in the heel of a bend. Scratches and nicks should be burnished out of the tube, to prevent stress concentrations

A scratch or nick in aluminum tubing can be repaired provided it does not

• appear in the heel of a bend

• appear on the inside of a bend

• exceed 10 percent of the tube OD on a straight section

• exceed 75 percent of the tube wall thickness on a straight section.

Answer explanation

Scratches or nicks not deeper than 10 percent of the wall thickness in aluminum alloy tubing, that are not in the heel of a bend, may be repaired by burnishing (forcing the displaced metal back into the nick or scratch) with a polished steel burnishing hand tool.

In a metal tubing installation,

• rigid straight line runs are preferable.

• tension is undesirable because pressurization will cause it to expand and shift

• a tube may be pulled in line if the nut will start on the threaded coupling

• up to 5 degree misalignment is acceptable

Answer explanation

When making an installation of rigid metal tubing, each run of the tubing must have at least one bend in it to allow for the shifting of the line as it is pressurized. There must be no tension on the line (the flare in both ends of the tube should rest squarely on the flare cones of the fittings and not have to be pulled into place with the nut).