Aircraft Oxygen Equipment — Page 269, Lesson 344

Let’s pick up with the normal operation of the narrow panel system. I want you to picture the crew member’s oxygen control panel. For normal operation, the supply lever is set to “on,” the oxygen selection lever is set to “normal,” and the emergency lever is in the “off” position. Those three positions are the starting point for everything that follows. Now, here’s the key mechanism. When the user inhales, a differential pressure is created across the demand diaphragm. That means the pressure on one side of the diaphragm is different from the pressure on the other side, and that difference is what moves the diaphragm. This pressure differential causes the demand valve to open, and that valve supplies oxygen to the mask. The important detail is that this pressure differential exists only during the user’s inhalation cycle. So the valve opens when you breathe in, and it closes when you stop inhaling. That’s why it’s called a demand system — it delivers oxygen on demand, only when you inhale. After the oxygen passes through the demand valve, it is mixed with air that enters through the air inlet port. So you have two flows coming together: the oxygen from the demand valve, and the air coming in through the inlet port. The mixture ratio — that is, how much air versus how much oxygen — is determined by an aneroid controlled air metering valve. Let me unpack that. An aneroid is a sealed capsule that expands or contracts with changes in ambient pressure. As altitude changes, the ambient pressure changes, and the aneroid moves. That movement controls the air metering valve, which adjusts the mixture. The result is that at low altitudes, you get a high air ratio — mostly air with a little oxygen. At high altitudes, you get a high oxygen ratio — mostly oxygen with little air. So the system automatically enriches the mixture as you climb. And note this detail: airflow begins at the same time as oxygen flow, through the air inlet valve. They start together. Now, what if you want pure oxygen? You move the oxygen selector lever to “100%.” That cuts off the air supply through the inlet port from the flight compartment. The reason for this is important: it prevents fumes and other contaminants from entering the mask. So in a smoke or fume event, you select 100% to isolate yourself from the cabin air entirely. Then there’s the emergency lever. Selecting the emergency lever to the “on” position mechanically loads the demand diaphragm to provide positive pressure. Let me explain what that means. Normally, the diaphragm moves because of the pressure differential created by your inhalation. But in an emergency, you don’t want to rely on your own breathing effort. So the emergency lever mechanically pushes on the diaphragm, forcing the demand valve open and delivering oxygen under positive pressure — that is, the oxygen is forced into your lungs even if you’re not inhaling strongly. This is used in situations like rapid decompression or smoke, where you need a guaranteed supply. Now let’s move to the Emergency Regulating Oxygen System, abbreviated EROS. These are the crew oxygen masks. They are combined masks and regulators, fitted at each crew station, to provide the flight crew with either diluted or 100% oxygen. So each crew member has their own unit, and the regulator is built right into the mask. Here’s how they’re stowed. They are kept in a panel mounted box, and they’re arranged so that the regulator controls and the feed hose protrude through apertures in the stowage doors. That means the controls and the hose stick out through openings in the doors, so you can grab them quickly. When the mask and regulator are stowed and the box doors are closed, oxygen flow to the mask is prevented by a shut-off valve inside the box. That valve is held closed by the Reset-Test Lever on the left door. So the lever physically keeps the valve shut when the mask is stowed. There’s a flow indicator, and it’s visible whether the doors are open or closed. So you can check that oxygen is flowing without opening the box. The mask is held to the face by a pneumatic harness. Pneumatic means it uses air pressure. When stowed, that harness is deflated — it’s flat and loose. When you don the mask, the harness inflates to tighten around your head. The harness fits all head sizes, so one size fits everyone on the flight deck. Finally, there’s a regulatory requirement. Under JAR-OPS 1, Subpart K, these quick donning masks must be provided for the flight deck crew on all aircraft that have a maximum operating altitude above 25,000 feet. So if the aircraft can fly above 25,000 feet, the flight deck crew must have these quick donning masks. “Quick donning” means they can be put on rapidly in an emergency. Let me tie this together. The narrow panel system gives you three modes: normal, which mixes air and oxygen based on altitude; 100%, which cuts off cabin air to keep fumes out; and emergency, which forces positive pressure oxygen. The EROS masks are the combined mask-and-regulator units that provide diluted or 100% oxygen, stowed with a shut-off valve held by the Reset-Test Lever, a visible flow indicator, and a deflated pneumatic harness that fits all head sizes — all mandated above 25,000 feet by JAR-OPS 1 Subpart K.