Aircraft Fluid Lines, Fittings, and Hardware

1100-1/2 hard or 3003-1/2 hard aluminum alloy tubing.

By its outside diameter.

Aircraft fittings have a 37° flare angle and automotive fittings use 45°.

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

By its inside diameter.

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

Teflon hose retains its high strength under conditions of high temperature.

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.

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

5052-0 and 6061-T aluminum alloy tubing in sizes from 1/8-inch to 3/8-inch OD.

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

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

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

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.

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

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

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

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.

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

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.

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.

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.

2024-T

7075

Graphite (carbon).

Kevlar®.

When steel is hardened, it becomes brittle. Tempering removes some of this brittleness.

It is heated to just above its upper critical temperature until it reaches a uniform temperature throughout, then it is allowed to cool very slowly in the oven.

It is heated to its critical temperature and quenched in water, brine, or oil.

The steel is first hardened; then some of the hardness is removed to relieve some of the internal stresses and brittleness.

The surface of the metal is hardened by the infusion of carbon or aluminum nitride. The interior of the metal remains strong and tough.

Carburizing and nitriding.

The steel part is heated in a retort in which there is an atmosphere of ammonia (a compound of nitrogen and hydrogen). Aluminum, an alloying element in the steel, combines with the nitrogen to form an extremely hard aluminum nitride on the surface of the steel.

The metal is hardened by heating it in a furnace to a specified temperature and immediately quenching it in water. It is soft when it is removed from the quench, but as it ages it regains its hardness.

The metal is heated and quenched, then it is returned to the oven and heated to a lower temperature. It is held at this temperature for a specified time, then removed from the oven and allowed to cool in still air. This increases the strength and hardness of the metal. Precipitation heat treating is also called artificial aging.

Artificial aging.

Heat-treated aluminum alloy designations are normally followed with a -T, followed by a number (1-10) that designates the type of heat treat. A common aircraft aluminum alloy is 2024-T3. Heat-treated aluminum is stronger than non-heat-treated aluminum.

Any delay in quenching aluminum alloy after it is removed from the oven will allow the grain structure to grow enough that intergranular corrosion is likely to form in the metal.

Tension-A force tending to stretch or elongate the material. Compression—A force tending to compress or shrink. Torsion—A twisting force on an object. Bending—A perpendicular force on the longitudinal axis of an object, causing it to flex or bend. The inner surface of the bend will be in compression and the outer surface of the bend will be in tension. Strain-The stress on an object which can cause the object to change shape or to become distorted Shear-A force that tries to cut or slice through an object. The force is parallel to the material cross section.

A self-locking nut should not be used for any application where there are any rotational forces applied to the nut or to the bolt.

The grip length of the bolt should be the same as the combined thicknesses of the materials being held by the bolt.

The nut on a clevis bolt should not be tight enough to prevent the clevis bolt from turning in the cable fitting and the horn.

AN6 = Hex head bolt, 6/16 (3/8) inch diameter. 14 = Length = 1-4/8 (1-1/2) inch long. A = The shank is not drilled for a cotter pin.

This is a close tolerance bolt.

They may be used to hold nonstructural components onto the aircraft.

When you can screw the nut onto a bolt without having to use a wrench.

A series of nuts mounted loosely in a channel that is riveted to the aircraft structure. You can install screws in a channel nut without having to hold the nut with a wrench.

Low-carbon steel and corrosion-resistant steel.

1/8-inch diameter.

Extra-flexible cable (7 x 19).

To prevent a nut, bolt, or other component from loosening.

No. Never over-torque or loosen a torqued nut to align safety wire or cotter pin holes. The nut should be torqued to the minimum torque, and then tightened to align with the safety hole, without exceeding the maximum torque.

A dial indicator.

The outside diameter of the shaft is measured with a micrometer caliper. The inside of the bushing is measured with a telescoping gauge and the same micrometer caliper.

One ten thousandth (0.0001) inch.

The range of a vernier caliper is far greater than that of a micrometer caliper.

Thickness gauge.

A telescoping gauge and a micrometer caliper.

A small hole gauge and a micrometer caliper.

60/40 resin-core solder.

Flux covers the cleaned and heated metal to keep oxygen away from it so oxides cannot form. Oxides keep the solder from adhering to the surface of the metal.

Deflecting beam, dial indicating, and micrometer setting types.

In the aircraft manufacturer's illustrated parts catalog (IPC).

In the aircraft manufacturer's structural repair manual (SRI).

The pure aluminum used for the cladding is noncorrosive, but the aluminum alloy below the cladding is susceptible to corrosion. If the cladding is scratched through, corrosion could form.

Preload is the force or torque required to overcome the friction between the threads on the nut and the threads of the bolt prior to the nut contacting the mating surface.

The preload torque is added to the specified torque to obtain the required setting on the torque wrench.

By manufacturer markings that are on the bolt head.

It should be removed by scrubbing it with hot water and a bristle brush.

A good weld is uniform in width; the ripples are even and well feathered into the base metal and show no burn due to overheating. The weld has good penetration and is free of gas pockets, porosity, or inclusions.

A bad weld has irregular edges and considerable variation in the depth of penetration. It often has the appearance of a cold weld.

All traces of the old weld must be removed so the new weld will penetrate the base metal.

Normalizing removes stresses that are locked into the material by welding or machining.

Heat the steel structure to a temperature above its critical temperature and allow it to cool in still air.

The thickness of the material being welded. The size of the tip orifice determines the amount of flame produced and thus the amount of heat put into the metal.

Hearing protection, eye protection, gloves, aprons, dust masks, and respirators.

Too low of torque could cause components to loosen and come apart. Too high of torque could cause failure of the bolt or component or cause a rotating component to seize and not function as designed. All of these failures could cause loss of a system, or even loss of an aircraft and potential loss of life.

In addition to creating an unairworthy condition, the risk of failure increases, which could cause damage to a system or the aircraft.

In the aircraft manufacturer's maintenance manual.

Pull the wrench assembly with a smooth, steady motion until the proper torque is reached. A fast or jerky motion will result in an improper torque