Liquid oxygen systems are designed for the rapid removal of the LOX converter for ease of servicing and maintenance. This is accomplished by the use of supply and vent quick-disconnect couplings, a single point converter retainer wing nut hold down, and quick-disconnect quantity indicator lead disconnects (fig. 4-9). The vent and supply quick- disconnect couplings are of two-piece construction. The male half is mounted on the LOX converter, and the female half is attached to the flexible oxygen supply and vent lines. The coupling for the supply line contains a spring-loaded check valve, which closes auto-matically when the supply line is uncoupled from the converter. This prevents contaminating the air-craft oxygen system when the converter is removed for servicing. The vent coupling has no check valve; however, it forms a positive seal between the vent port of the converter and over-board vent line.

Heat Exchanger

The lungs would be damaged if gaseous oxygen were breathed at the temperature at which it exits the LOX converter. The purpose of the air-to-oxygen heat exchanger is to increase the temperature of the gaseous oxygen after it leaves the LOX converter. The heat exchanger is located in the cockpit area of the aircraft in order to

Figure 4-9.- LOX converter installation.

expose it to a temperature capable of warming the gaseous oxygen regardless of the altitude of the aircraft. The heat exchanger is constructed of aluminum and has a large interior surface area (fig. 4-10).

Low-Pressure Switch

The low-pressure switch is located in the oxygen system supply line (fig. 4-7). It indicates to the flight crew, through a caution light in the aircraft cabin, when system pressure falls below minimum operating pressure of the system. This alerts and allows the pilot to descend to a safe altitude.

Quantity Indicating System

The quantity indicating system consists of a quantity gauge and a warning light. These are located in the cockpit of the aircraft. A quantity probe is also a part of the liquid oxygen converter. This probe senses the amount (quantity) of liquid contained in the converter. This informa-tion is transmitted to the quantity gauge by an

Figure 4-10.- Aircraft air-to-oxygen heat exchanger.

electrical coaxial cable. The quantity gauge is marked in liters from zero to the systems maximum storage capacity (i. e., 0 to 10). The gauge constantly shows the remaining liquid in the converter. The low quantity warning light is also connected to the coaxial cable and illuminates when the quantity of liquid in the converter falls below 1 liter. The maintenance of the quantity indicating and warning light system is the responsibility of the AE rating. You, as an AME, must understand the operation of the electrical portion of the LOX system in order to troubleshoot the indicator and warning systems.

Oxygen Shutoff Valve The oxygen shutoff valve is installed in the system to control the flow of oxygen to the pilot or flight crew, as required. Figure 4-11 illustrates a typical manually operated two-position valve. This valve has an inlet port, outlet port, and a relief port. The pressure-relief valve is located in the inlet chamber to protect the oxygen regulator and crew member from excessive system pressure if there is a malfunction of the liquid oxygen converter. Also, excessive pressure due to thermal expansion of gaseous oxygen trapped within the system when not in use is relieved by this valve. If the oxygen system incorporates a console-mounted regulator, the shutoff valve is a part of the regulator.

Composite Quick-Disconnect Coupling The purpose of the composite quick-dis-connect coupling is to provide a single-point connection for quickly connecting and dis-connecting the pilot with aircraft oxygen, anti-g, communications, and ventilation air services.

Oxygen Lines LOX systems are classed as low-pressure systems. As such, low-pressure tubing is used in the manufacture and repair of LOX lines. All low-pressure tubing used in LOX systems are aluminum alloy 5052 tubing and is nonheat-treatable. It is manufactured in seamless, round lengths, and is annealed to provide greater flexibility. Aircraft oxygen systems are fitted with 5/ 16-, 3/ 8-, and 1/ 2-inch sizes. Low-pressure tubing is also installed from the pressure reducer to the outlets in reduced high-pressure oxygen systems.

Oxygen Regulators Regulators used with LOX systems are either console-mounted or miniature mask-mounted.

Figure 4-11.- Oxygen shutoff valve.

The miniature mask-mounted regulator was especially designed for use with aircraft that have ejection seats. The console-mounted regulator is normally used in large nonelection seat equipped multiplace aircraft such as the E-2 and the P-3.