Aircraft Oxygen Equipment — Page 269, Lesson 346

Let's pick up with the oxygen mask controls, because that's where the crew oxygen system really comes alive in your hands. On the front of the regulator you'll find the control for normal or 100% oxygen flow. It's marked with an "N" and "100% PUSH." So in normal operation, you get the diluter-demand mixture. But if you push in on the end of that control, the one marked "100% push," you get 100% oxygen. That's your high-oxygen setting for when you need maximum protection. Right next to that is the EMERGENCY control knob. Here's the key distinction: rotating that knob to the emergency setting changes the flow from diluter demand to steady flow. So instead of the regulator mixing your exhaled breath with fresh oxygen on demand, you get a continuous, steady stream of oxygen. That's your emergency override. Now, let's talk about donning the mask. You withdraw it by grasping the red release grips between your thumb and forefinger. That action initiates inflation of the harness. The inflated harness helps you get the mask on quickly—it holds its shape so you can slip it over your head. Then, when you release the grips, the pressure bleeds out of the harness, and it form-fits to your head. So it goes from rigid to snug. The masks themselves include R/T communication facilities—that's radio telephony, so you can talk to ATC and the crew. They can also be modified to include a mask ventilation feature. When you select that, it provides ventilation to the smoke goggles to overcome misting problems. So if you're in a smoky cabin, you can keep your goggles clear. Now for testing. The emergency knob is also marked "PRESS TO TEST." When you press it together with the RESET-TEST lever, it allows oxygen to flow into the mask. You check that flow on the flow indicator. That's your pre-flight check that the system is actually delivering. Let me show you the mask itself. Now, moving on to the passenger oxygen system. This is a completely different setup. It provides an emergency oxygen supply to the passengers and cabin attendants, and it's of the continuous flow type. It's supplied either by a high-pressure gaseous system or a chemical generator system. The masks are stowed in the passenger service units—the PSUs. The doors of those units open automatically by a barometrically controlled release mechanism if the cabin altitude reaches 14,000 feet. Or, the crew can open them manually from the flight deck at any altitude below that. So you have two triggers: automatic at 14,000 feet cabin altitude, or manual selection by the crew. Here's an important distinction: the release mechanism is actuated electrically for the chemical generator system, and pneumatically for the gaseous system. So the two systems use different actuation methods. When the PSU doors open, the masks drop to the "half-hung" position. They hang down, ready for the passenger to grab. Pulling the mask towards the face initiates the oxygen flow. For the gaseous system, that pull opens a check valve. For the chemical generator, it operates the electrical or percussion cap firing mechanism. Either way, the masks are now ready for use. Now let's look at the chemical oxygen generators themselves. These are relatively light, self-contained devices. They're located in each passenger, cabin attendant, and lavatory service unit. The oxygen is generated by the chemical reaction of sodium chlorate—that's NaClO₃—and iron, which is Fe. The complete reaction is NaClO₃ + Fe = (NaCl + FeO) + O₂. So you start with sodium chlorate and iron, and you end up with sodium chloride, iron oxide, and oxygen. The sodium chlorate and iron core is shaped to provide maximum oxygen flow at starting—so it's designed to give you a strong initial burst. Inside the generator, a filter removes any contaminants and cools the oxygen to a temperature not exceeding 10°C above cabin ambient temperature. So the oxygen you breathe is clean and not too hot. There's also a relief valve that prevents the internal pressure in the generator from exceeding 50 psi. The normal flow pressure is 10 psi. So you have a working pressure of 10 psi, with a safety relief at 50 psi. The generator supplies enough oxygen to meet the requirements of descent in emergency conditions—a minimum of 15 minutes. And there's now been developed a chemical generator that lasts for a period of 22 minutes. So you have two durations: the standard 15-minute minimum, and a newer 22-minute version. That's the full picture of the oxygen equipment—the crew mask controls, the passenger system, and the chemical generators.