Masks

  Masks

(a) Continuous Flow Masks

Almost all of the masks used with a continuous flow oxygen system are of the rebreather type and vary from the simple bag-type disposable mask used with some of the portable systems to the rubber bag-type mask used for some of the flight crew systems.

Oxygen enters a rebreather mask, shown in Figure 9.6, at the bottom of the bag, and the mask fits the face of the user very loosely so air can escape around it. If the rebreather bag is full of oxygen when the user inhales, the lungs fill with oxygen. Oxygen continues to flow into the bag and fill it from the bottom at the same time the user exhales used air into the bag at the top. When the bag fills, the air that was in the lungs longest will spill out of the bag into the outside air, and when the user inhales, the first air to enter the lungs is that which was first exhaled and still has some oxygen in it. This air is mixed with pure oxygen, and so oxygen rich air is always breathed with this type of mask. More elaborate rebreather-type masks have a close-fitting cup over the nose and mouth with a built-in check valve which allows the air to escape, but prevents the user breathing air from the cabin.

The oxygen masks that automatically drop from the overhead compartment of a jet transport aircraft in the event of cabin depressurization are of the rebreather type. The plastic cup that fits over the mouth and nose has a check valve in it, and the plastic bag attached to the cup is the rebreather bag. 

Figure 9.6: Rebreather-type mask used with con­tinuous flow oxygen regulators.

Demand-type Masks

All demand-type masks must fit tightly to the face so no outside air can enter to disturb the metering of the regulator, as illustrated in Figure 9.7. Demand masks all connect to the regulator with a large diameter corrugated hose, whereas the continuous flow masks all use a small diameter tube to carry the oxygen to the mask.

A full-face mask is available for use in case the cockpit should ever be filled with smoke. These masks cover the eyes as well as the mouth and nose, and the positive pressure inside the mask prevents any smoke entering.

Plumbing And Valves

Most of the rigid plumbing lines that carry high­-pressure oxygen are made of stainless steel, with the end fittings silver soldered to the tubing. Lines that carry low-pressure oxygen are made of aluminium alloy and are terminated with the same type fittings used for any other fluid-carrying line in the aircraft. The fittings may be of either the flared or flare less type. It is essential in any form of aircraft maintenance that only approved com­ponents be used. This is especially true of oxygen system components. Only valves carrying the cor­rect part number should be used to replace any valve in an oxygen system.

Many of the valves used in oxygen systems are of the slow-opening type to prevent a rapid in-rush of oxygen that could cause excessive heat and become a fire hazard. Other valves have restrictors in them to limit the flow rate through a fully open valve.