Aircraft Oxygen Equipment — Page 269, Lesson 349

Let’s pick up right where the chemical generator story leaves off, because there’s a critical safety warning that goes with it. Once that chemical reaction in an oxygen generator starts, you cannot stop it. That’s a hard fact. The surface temperature of the generator can reach 232°C, which is 450°F. To give you a visual check, there’s a strip of heat-sensitive tape or paint on the generator. When the generator has been used, that strip changes colour, usually to black, and that tells you at a glance that the generator is expended. And one more number to file away: chemical generators have a shelf life, or installed life, of ten years. After that, they’re no longer considered serviceable. Now, let’s move to portable oxygen systems. These are the first-aid and sustaining oxygen cylinders installed at suitable locations in the passenger cabin. They come in a carrying bag with straps. A typical cylinder holds 120 litres of oxygen at a pressure of 1800 psi. You can usually set one of two flow rates, depending on the requirement. Those are Normal and High, which correspond to 2 litres per minute and 4 litres per minute. So at those rates, a 120-litre bottle lasts 60 minutes on Normal, or 30 minutes on High. There are also larger 310-litre bottles available, fitted with four-way manifolds so you can supply multiple outlets at once, and they offer high or medium rates as I just described. Then we have crew portable oxygen systems and smoke hoods. Standard portable oxygen bottles can be used by the crew to move about the cabin during reduced cabin pressure situations. But when harsh environmental conditions exist, you use portable sets with a full face smoke mask. These may be standard cylinders, or they may be special smoke sets with built-in generators that can produce oxygen for 15 minutes once initiated. Special training is required before you use these, and they are not suitable for passengers. So the key contrasts here: 120-litre versus 310-litre bottles, Normal versus High flow, standard cylinders versus built-in generator smoke sets, and crew-only versus passenger use. That’s the portable oxygen picture.