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Oxygen Systems - Overview

This section provides an overview of oxygen systems installed in aircraft. An oxygen system provides oxygen to flight crew and passengers. A standard oxygen system is shown in Figure 1. The system shown in Figure 1 would be representative of a small commercial or business airplane oxygen system.

Figure 1 Standard Oxygen System

The system in Figure 1 shows a single bottle as the oxygen source. This bottle will be filled with aviator’s oxygen (clean with minimal impurities). Bottle pressure will be around 2000 psig at 70°F when full. Attached to the bottle is a regulator assembly. The regulator contains a pressure regulator, fill valve and overpressure burst disk. Pressure regulation reduces the bottle pressure to approximately 70 psig. The overpressure disk will rupture around 2500 psig to protect the bottle from rupturing. When the disk ruptures, oxygen in the bottle is vented overboard. There is usually a bottle pressure transducer for providing bottle pressure indication to the flight crew and a bottle pressure gauge used when filling the tank.

Oxygen lines route oxygen directly to the crew masks and to a shut off valve that isolates the passenger masks. The oxygen lines will typically be aluminum. If the pressure applied to the passenger shut off valve is not regulated (i.e., full bottle pressure is applied to the shut off valve), than a pressure regulator will be downstream of the shut off valve to limit passenger mask pressure to approximately 70 psig. Additional oxygen lines connect the passenger masks to the pressure regulator and shut off valve. The reason for the shut off valve is to preserve oxygen to the flight crew should oxygen quantities run low. Also, flight requirements require at least one flight crew member to be on oxygen above certain altitudes or if the other flight crew member leaves the flight deck. The shut off valve also protects against loss of oxygen in event a leak would develop in the passenger distribution system. More information on typical oxygen system components can be found in other modules on oxygen systems.

Figure 2 shows another example of an oxygen system. This system contains many of the same components as the system in Figure 1, but is a little simpler. This system is representative of very small airplanes, such as a fighter jet or trainer.

Figure 2 Oxygen System – 2nd Example

The oxygen source shown in Figures 1 and 2 is a charged oxygen bottle. Alternative sources of oxygen would be a liquid oxygen container/system or an onboard oxygen generator. A liquid oxygen system stores liquid oxygen in a tank, which is allowed to warm and expand to become gaseous before being sent into the distribution system. An oxygen generator extracts oxygen from an onboard chemical source. For passenger systems, stand-alone chemical oxygen generators may be used.

Oxygen systems can be continuous flow or demand type systems. A continuous flow system is designed to flow a preset amount of oxygen at all times, regardless of a person’s breathing rate. A pressure regulator and orifice control the flow rate. The pressure regulator sets the upstream orifice pressure and downstream pressure would be ambient pressure. With the pressure drop across the orifice and orifice area known, the flow rate can be computed. A demand system can be either a full demand system or a diluter demand system. In a full demand system, the entire flow provided to a person is from the oxygen source. The demand flow is determined by the inhalation “pull” from the user. The inhalation pull is the lower pressure created by the lungs expanding. In a diluter demand system, the inhalation pulls in a combination of oxygen and ambient air. Thus the oxygen flow requirements in a diluter demand system are less than a full demand system. Since oxygen flow is reduced, the oxygen supply lasts longer for a diluter demand system. Most oxygen systems contain a combination of continuous flow, diluter demand and demand systems. The type of system is determined by the crew and passenger oxygen mask design.

The oxygen needs of the flight crew and passengers are different – this difference is controlled by differences in the design of crew and passenger oxygen masks. Should a depressurization occur, it is imperative that the flight crew have sufficient oxygen to be able to perform their flight duties. Therefore, crew oxygen needs are greater. Oxygen masks for flight crew will typically have 2 modes of operation: diluter demand and full demand. On diluter demand, oxygen flow to the mask is reduced and is simply used to supplement existing cabin air. In this case, the air breathed by the flight crewmember will be a combination of cabin air and oxygen flow from the bottle. Diluter demand mode is convenient way to minimize oxygen flow when oxygen needs are not great. Diluter demand mode would be used when a flight crew is required to have an oxygen mask on above certain altitudes or one flight crewmember has left the flight compartment. Should a cabin depressurization occur, the flight crew would use full demand mode. In full demand mode, the flight crew would receive 100% oxygen to perform their duties up to cabin altitudes in the 40,000 feet range. When a crew mask is in demand mode, upon inhalation the person will inhale full oxygen as the lungs are expanded. When breathing out in either diluter demand or demand modes, a valve in the crew mask will open and let the exhaling air out of the mask. This valve acts like a check valve since flow only goes in one direction. Crew masks will fit tightly over a person’s nose and mouth, may contain smoke goggles and microphone, and can be quickly donned using one hand (usually within 5 seconds). A typical crew mask is shown in Figure 3.

Figure 3 Crew Oxygen Mask

Passenger masks are simpler than crew masks. Passenger masks in commercial airplane applications contain a “cup” that fits loosely over the nose and mouth. Figure 4 shows a typical passenger mask and bag. The masks provide a single flow rate that will vary with cabin altitude. A passenger mask will contain two valves. Each valve acts like a check valve. One valve opens upon inhalation while the other remains closed. This valve is designed to pull in a certain amount of ambient air along with oxygen as the person inhales. The other valve opens upon exhalation while the inlet valve remains closed. This valve is designed to allow full exhalation. Passenger mask flow is normally continuous and higher than crew mask flow. Passenger masks also have some leakage hence they waste some oxygen. When passenger masks are flowing, oxygen quantity will start to drop faster – partially because there are more passenger masks than crew masks and partially because they are inefficient in delivering oxygen. However, they are small, light weight and store easily in a small container. Another type of oxygen mask or oxygen breathing device is a cannulus mask. These are not generally used in aerospace, except possibly low altitude airplanes or for therapeutic purposes.

Figure 4 Passenger Oxygen Mask

To analyze oxygen requirements for a given oxygen system, there are 3 basic types of calculations required. These are

The equations required to perform this calculations along with examples are provided in Pneumatic Systems – Equations and Oxygen Systems – Calculations.

Oxygen system design is governed by government standards and other specifications. FAA requirements specified in 14CFR 25.1443 provide required crew and passenger oxygen flow requirements. These requirements are applied almost universally in commercial aerospace. Commercial specifications are used to define the requirements for crew and passenger oxygen masks. In military or space applications, requirements were historically provided by military specifications. However, many of these specifications are no longer current and commercial specifications are used. Other standard specifications are used to cover other components used in oxygen systems. More details on design requirements and standards are provided in Oxygen Systems – Design Requirements and Oxygen Systems – Standards.

Installation characteristics of oxygen systems are very important. Oxygen leakage can lead to flammable mixtures when around fuel or hydraulic fluid. In addition, any electronic spark in the vicinity of large concentrations of oxygen can cause ignition. Sources of oxygen, such as the bottle, need to be located where a leak will not cause ignition of fire or be susceptible to damage from incidents such as birdstrike or wheels up landing. Oxygen lines need to be installed with large clearances to electrical wires, cables, moving parts and other hardware. Further information on installation design requirements for oxygen systems can be found in Oxygen Systems – Installation Requirements.

Sources of oxygen in aerospace applications are pressurized bottles (as shown in Figure 1), liquid oxygen sources and chemical oxygen sources. Sources may be fixed or portable. For liquid and chemical generation oxygen sources special precautions are required. The special precautions are necessary because certain failures in these systems can result in a complete dump of all oxygen into cabins, flight compartments or other compartments. High concentrations of oxygen are generally considered hazardous and should be avoided. Compartments with good ventilation are good locations to locate any oxygen source.