(2)
The overheat warning system has four hydraulic fluid temperature transmitters. The hydraulic control module gives a warning of a hydraulic fluid overheat condition. The temperature of each system is shown on the EICAS HYD page.
4. Hydraulic Tubing, Hoses, and Fittings
_____________________________________
A. Most hydraulic pressure lines in the hydraulic supply systems that have high pressure applied to them (more than 1500 psi) are made of titanium tubing. Most return lines or lines with medium pressure (less than 1500 psi) are made of aluminum alloy. All tubing, pressure and return, installed below the engine fire seal and in the high temperature zones is made of corrosion-resistant steel. All tubing installed below the landing gear trunnion is made of either titanium or corrosion resistant steel. Flexible hose used in the hydraulic systems are teflon high and medium pressure type. High pressure hoses are used in the pressure lines. High pressure fittings are made of steel, titanium, or corrosion-resistant steel. For repair, replacement, and approved alternatives, refer to 20-11-05.
5. Modular Units
_____________
A. Modular units are used in the hydraulic power systems to decrease the number of in-line components and tubing connections. Modular units also point hydraulic fluid and/or pneumatic flow to replaceable cartridge type components. This permits component removal without having to disconnect tubes. For a description of individual modular units, refer to the applicable hydraulic system.
6. Hydraulic System Check Valves
_____________________________
A. Check valves are installed in the hydraulic power systems to:
-prevent reverse flow of fluid
-point fluid flow
-
prevent fluid loss
-
prevent drainage during maintenance
-
isolate subsystems.
B.
Flow direction through a check valve is shown by an arrow on the valve body. Usually no maintenace other than removal and installation of the check valve is necessary.
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747-400
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7. Hydraulic Fluid
_______________
A. BMS 3-11 is the general specification for the fluid that flows through hydraulically operated systems. It is a fire-resistant base fluid. Permitted additives can be included in this fluid. All types of fluid in this specification are fully miscible with each other in any level of mixture. Separation, precipitation, or cloudiness will not occur.
B. Too much water in a hydraulic system is dangerous and can cause deteriorization of fluid, corrosion of component, and worsen system operation. Hydraulic fluid can soak up 2.5 to 3 % of water. The in-service limit is 0.8 % to make sure that the fluid is thermally stable, and to prevent the formation of acid. Corrosion and particles in hydraulic system components can occur in free water (i.e. more than 3 % of water in the fluid). Slow response and irregular flight control movements can be caused if there are particles and corrosion.
C. To repair a system contaminated with water, it may be necessary to flush the entire system and replace all of the corrosion sensitive components.
D. These are the most likely sources for water contamination:
(1)
Used containers with some remaining moisture
(2)
Drums or containers that were pressurized with shop air instead of dry nitrogen
(3)
Opened containers kept outside open to rain and other fallout
(4)
Incorrect use and cleaning of ground equipment.
E.
All servicing operations should be done with precaution to prevent contamination of fluid. Drums should be kept where rain and other fallout cannot be introduced. Ground carts should be examined at regular times.
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