Let me define the key terms right at the beginning — Page 173, Lesson 264

I want to walk you through the start of Chapter 12, which is all about pressurization failure. This is a critical safety topic because the failure of an aeroplane's pressurization system is potentially life threatening. The reason is that at high altitude, the outside air pressure — specifically the partial pressure of oxygen — is inadequate to preserve life. So if the cabin loses its pressurization, the occupants cannot get enough oxygen to stay conscious or survive. Let me define the key terms right at the beginning. Decompression is defined as either slow, or rapid (or explosive). So we have two main categories: slow decompression, and rapid or explosive decompression. Rapid or explosive decompression is the result of a failure of the airframe to contain the cabin pressure. In other words, the structure itself — the pressure hull, a door, a window — has failed, and the pressurized air inside the cabin escapes suddenly. A slow decompression, by contrast, is the failure of the pressurization system to maintain the cabin pressure where there has not been a failure of the airframe. So the system itself — the valves, controllers, or compressors — stops working properly, but the airframe remains intact. Now, let's look more closely at rapid or explosive decompression. This results in the cabin altitude quickly — or virtually instantaneously — decreasing to the ambient, or outside, pressure. That means the pressure inside the cabin drops to match the pressure outside the aeroplane almost immediately. This will only occur due to a catastrophic failure of the pressure hull or the loss of a major door or hatch. In the case of an explosive decompression, major damage will have occurred — for example, a bomb exploding within the pressure hull, or a major fatigue failure of the structure. If the flying integrity of the airframe is preserved — meaning the aeroplane can still be controlled and the wings and tail are still structurally sound — then the aeroplane may be landed safely. Rapid decompression, on the other hand, results from a relatively minor rupture of the pressure hull or the loss of a small hatch — such as an emergency escape hatch or a window. The key difference is the size of the rupture. The text says 'in essence, if the size of the rupture' — and it cuts off there, but the idea is that the severity of the decompression depends on how large the opening is. Regardless of the type of decompression, the immediate action is the same. Decompression of a pressurized cabin under any circumstances requires that the aeroplane is descended to a minimum of 10,000 feet, or the lowest safe flight level, whichever is the highest. So you descend to at least 10,000 feet, but if there are terrain or obstacles that require you to stay higher, you go to the lowest safe altitude that still keeps you clear of those hazards. During the descent, the aeroplane should be flown with regard to maintaining the flying integrity of the airframe — you don't want to overspeed the structure or pull excessive G-forces that could cause further damage. At the lower altitude, sufficient oxygen should be present in the atmosphere to sustain life without supplemental oxygen. During the descent itself, supplemental oxygen is required for crew and passengers in accordance with a table — that table is referenced as Figure 12.1, which shows the oxygen requirement. So to summarize: pressurization failure is life-threatening because of low partial pressure of oxygen at altitude. Decompression can be slow (system failure, airframe intact) or rapid/explosive (structural failure). The immediate response is to descend to at least 10,000 feet or the lowest safe flight level, while protecting the airframe's structural integrity, and using supplemental oxygen as required during the descent.