The valve is equipped with a fusible metal safety plug and a safety disc to release the contents of the cylinder if the pressure becomes excessive because of high temperatures. The safety plug is filled with a fusible metal designed to melt at temperatures ranging from 208° to 220°F (97.8° to 104.5°C). The cylinder and valve assembly is connected to the oxygen tubing by silver soldering the tubing to a coupling nose and securing the nose to the valve outlet with a coupling nut. The self-opening (automatic) oxygen cylinder valve is automatically opened when it is connected to the oxygen line. The use of this type of valve permits remote location of the oxygen cylinder to places less vulnerable during combat and more readily accessible for servicing. Regulators The success or failure of high-altitude flight depends primarily on the proper functioning of the oxygen breathing regulator. Acting as a meter- ing device, the regulator is the heart of the oxygen system. To perform successfully in an air- craft system, a regulator must deliver the life- supporting oxygen in the quantities demanded throughout its entire range of operation. Although personnel of the PR rating are primarily responsible for maintenance of regulators, the AME is responsible for per- forming operational checks in the aircraft and for removal and installation. In other words, the AME removes a malfunctioning regulator from the aircraft and delivers it to the shop where the PR determines the trouble and makes the necessary repairs. When the trouble is corrected, the AME reinstalls the regulator in the aircraft. Tubing Two types of tubing are used in aircraft oxygen systems. Low-pressure aluminum alloy tubing is used in lines carrying pressures up to 450 psi. High-pressure copper tubing is used in lines carrying pressure above 450 psi. NOTE: Some of the newer naval aircraft are equipped with high-pressure oxygen lines made of aluminum alloy. Lines running from the filler valve to each of the cylinders are called filler lines. Those running from the cylinders to the regulators are called distribution or supply lines. Oxygen lines, like all other lines in the aircraft, are identified by strips of colored tape. The strips of tape are wrapped around each line near each fitting and at least once in each compartment through which the line runs. The color code for oxygen lines is green and white with the words Breathing Oxygen printed in the green portion, while black outlines of rectangles appear in the white portion. Resistance to fatigue failure is an important factor in oxygen line design because the line pressure in a high-pressure system will at times exceed 1,800 psi, and at other times be as low as 300 psi. Because of these varying pressures and temperatures, expansion and contraction occur all the time. These fluctuations cause “metal fatigue,” which must be guarded against in both the design and the construction specifications for tubing. Steps are taken during installation to prevent fatigue failure of the tubing. Tubing is bent in smooth coils wherever it is connected to an inflexible object, like a cylinder or a regulator. Every precaution is taken to prevent the accidental discharge of compressed oxygen because of faulty tubing or installation. Although simple in con- struction and purpose, tubing is the primary means by which oxygen is routed from the cylinders to the regulator stations. High-pressure tubing is usually seamless copper tubing, and is manufactured in accordance with strict specifications. It has an outside diameter of 3/16 inch and a wall thickness of 0.035 inch. For application in high-pressure oxygen installations, copper tubing is type N (soft annealed), and is pressure tested at not less than 3,000 psi. High-pressure tubing is used between the oxygen cylinder valve and the filler connection in all systems, between the cylinder valve and the regulator inlet in high-pressure systems, and between the cylinder valve and pressure reducer in reduced high-pressure systems. To connect high-pressure copper tubing, adapters and fittings are silver soldered to the tubing ends. Due to the high pressures involved, the security (leak tightness) of all high-pressure lines relies primarily on a metal-to-metal contact of all its fittings and connections. A fitting properly silver soldered to the end of a length of copper tubing will not come loose or leak. Some of the later models of naval aircraft use aluminum alloy or stainless steel tubing in high- -pressure oxygen system installations. Replacement tubing should be manufactured of the same type 4-6