Aircraft Oxygen Equipment — Page 261, Lesson 337

We're starting a brand-new topic now: Aircraft Oxygen Equipment. This is Chapter 13, and I want to walk you through the fundamental reason we even have this equipment on board. Let's begin with the physiology, because everything else hangs off this. The body needs oxygen to function satisfactorily. It gets that oxygen from oxygenated blood, which is provided by the lungs. So the chain is: lungs take in oxygen, put it into the blood, and that oxygenated blood feeds the body. Now, when there isn't enough oxygen, that condition has a precise name: Hypoxia. And I cannot overstate how important it is for aircrew to recognize hypoxia. The whole point is early identification. If you know the signs and symptoms, and you spot the problem early, you can carry out the correct drills before anyone is put in jeopardy. But those drills have to be well learnt and easily accomplished, because you won't have time to think about them. Let me give you the two drills that overcome hypoxia. First: provide oxygen. Second: descend to a level where atmospheric oxygen is present in sufficient quantities to meet the body's needs. That's the whole response in a nutshell — supply oxygen, or get down to where there's enough of it naturally. Now, a critical operating rule. Aircrew must familiarize themselves with the appropriate oxygen drills for the specific aircraft they are flying, and they must do this before venturing above an altitude at which hypoxia can occur. And that altitude threshold is 10,000 feet. Above 10,000 ft, you're in the zone where hypoxia is possible, so you need to know your drills before you get there. Let me walk you through the symptoms of hypoxia, because you need to recognize these. They can be summarized as: apparent personality change, impaired judgement, muscular impairment, memory impairment, and sensory loss. And then, progressing further, impairment of consciousness — that means confusion, then semi-consciousness, then unconsciousness, and finally, death. So the sequence escalates from subtle changes in personality and judgement, through physical and memory problems, down to loss of consciousness and ultimately death. Now I want to introduce you to a very specific and important concept: Time of Useful Consciousness. This is the time available for a pilot or flight engineer to recognize the development of hypoxia and do something about it. And here's the key distinction — it is not the time to unconsciousness. It is the shorter time, measured from a reduction in adequate oxygen, until a specific degree of impairment. And that degree of impairment is generally taken to be the point when the individual can no longer take steps to help themselves. So it's the window in which you can still act to save yourself, not the time until you pass out. Let me give you the figures, because these are exact and they matter. At 20,000 feet, a person seated or at rest has a Time of Useful Consciousness of 30 minutes. But with moderate activity, that drops to just 5 minutes. At 30,000 feet, it's 1 to 2 minutes. At 35,000 feet, it's 30 to 90 seconds. And at 40,000 feet, it's only 15 to 20 seconds. Notice how dramatically that window shrinks as altitude increases — at 40,000 feet you have barely a quarter of a minute to recognize the problem and act. A more detailed study of hypoxia itself is covered in Book 8, Human Performance and Limitations. So for now, what I want you to hold onto is this: the body needs oxygen from the lungs, insufficient oxygen is hypoxia, you must know your drills before 10,000 feet, and the Time of Useful Consciousness is your action window — not your time to unconsciousness.