So the decompression is rapid, but it is not explosive — Page 173, Lesson 266

I want to walk you through the different types of decompression and what they mean for you as a pilot. Let's start with the smallest category. A very minor rupture of the pressure hull, or the loss of a small hatch like an emergency escape hatch or a window, causes what we call a rapid decompression. The key point here is that the size of the rupture is such that the cabin pressure and the outside air pressure do not equalize immediately. So the decompression is rapid, but it is not explosive. In practice, the distinction between rapid and explosive is somewhat academic, as long as the flying integrity of the aeroplane is maintained. Now, let's contrast that with a slow decompression. This happens when the pressurization system simply cannot overcome the loss of pressure caused by a normally controllable vent or opening in the pressure hull. Examples include a leaking pressure seal, a pressure relief valve that isn't fully closed, or an outright failure in the pressurization system itself. In a normal, functioning system, once the cabin pressure reaches 10,000 feet, which corresponds to about 700 millibars, the altitude warning horn will sound. But before that horn goes off, the crew should notice the loss on their gauging systems, if fitted. You'd see the cabin altimeter showing an increase, or the cabin differential pressure gauge showing a reduction. However, the first indication of a problem is often physiological changes—what you feel in your body. During a slow decompression, passengers and crew will be aware of barometric pressure changes on the ears. Other body cavities—teeth, sinuses, and the gut—may give rise to discomfort. If it's not possible to equalize the differential pressure by natural venting, serious damage may result. And at night, night vision will be seriously impaired at relatively low cabin altitudes. In extreme cases—rapid and explosive decompression—the effects are much more severe. Sinuses and teeth may explode, eardrums can rupture, and severe abdominal distension may occur, potentially leading to rupturing of internal organs. These effects, especially in the head, are more pronounced if the person is suffering from a vent blockage due to a build-up of mucus from a cold. During prolonged periods of reduced oxygen, you may experience tunnel vision and sensorial depletion. The most obvious indication of a rapid or explosive decompression is white-out—where the moisture in the atmosphere vaporizes, causing instantaneous fog. Now, let's move to the oxygen supply requirements. The text gives us several specific minimum supply quantities. First, the required minimum supply is that quantity of oxygen necessary for a constant rate of descent from the aeroplane's maximum certificated operating altitude down to 10,000 feet in 10 minutes, followed by 20 minutes at 10,000 feet. Second, another requirement is the same constant rate of descent from maximum certificated operating altitude to 10,000 feet in 10 minutes, but then followed by 110 minutes at 10,000 feet. The oxygen required under CS-OPS 1.780(a)(1) may be included in determining this supply. Third, there is a requirement for a constant rate of descent from maximum certificated operating altitude to 15,000 feet in 10 minutes. Finally, for the purpose of these tables, the term 'passengers' means passengers actually carried and includes infants. So when you're calculating oxygen supply, infants count as passengers. Let me show you a diagram that illustrates these oxygen requirements visually. That covers the types of decompression and the oxygen supply rules you need to know.