Oxygen and Respiration — Page 55, Lesson 84

I want to walk you through cabin pressurization and decompression — two critical topics for any professional pilot operating pressurized aircraft. Let's start with cabin pressurization systems. Their purpose is to ensure that the effective altitude the occupants are actually exposed to is much lower than the altitude at which the aircraft is flying. In an ideal world, we'd keep the cabin at sea level pressure all the time, but that's impractical because of two limitations: aircraft weight and fuselage strength. The structure simply can't handle the enormous pressure differential that would require at high altitude. So what do we actually get in practice? For a commercial airliner flying at 30,000 feet, the pressurization system produces an internal cabin pressure equivalent to about 6,000 feet, with a maximum allowable cabin altitude of 8,000 feet. That's the regulatory limit you'll see in operations. Now, the pressure differential — that's the difference between the pressure inside the cabin and the pressure outside the aircraft. The aircraft skin is normally designed not to exceed a differential of 8 to 9 psi. That's a structural limit you must know. There's also a limit on how fast the cabin pressure changes. The rate of change of cabin pressure is restricted to 500 feet per minute during the ascent and 300 feet per minute during the descent. Why? To minimize passenger discomfort due to the pressure equalization limitations of the middle ear. If you change cabin pressure too quickly, people can't equalize the pressure in their ears fast enough, and that causes pain. Let's move to cabin decompression — loss of cabin pressurization in flight. There are two types to understand. First, a slow decompression. The rate of loss is gradual. The crew recognizes the problem and makes appropriate height reductions before the passengers are even aware anything is wrong. That's the ideal scenario — you catch it early and descend. Second, a rapid decompression. This happens very occasionally, perhaps due to loss of a window or door, or a failure in the fuselage structure. In this case, occupants — both crew and passengers — will rapidly be exposed to the full rigours of high altitude. And what are those rigours? Three things: hypoxia (oxygen deficiency), cold (the extreme low temperatures at altitude), and decompression sickness (the same condition divers can get, caused by nitrogen coming out of solution in the blood and tissues). Oxygen can be supplied to all occupants, but only for a limited period. That's why the immediate action is critical. And here's the rule you must memorize: THE AIRCRAFT MUST RAPIDLY DESCEND TO 10,000 FEET OR MSA, WHICHEVER IS THE HIGHER. MSA stands for Minimum Safe Altitude. You descend to 10,000 feet unless the terrain below you requires you to stay higher — in which case you descend to the minimum safe altitude for that area. There's one more important effect in rapid decompression. The altitude of the cabin may actually rise to above the pressure altitude of the aircraft. That sounds counterintuitive, but here's why: the Venturi effect of air passing over the fuselage can actually suck air out of the cabin. This can make up to a 5,000-foot difference in pressure terms — meaning the cabin altitude could be 5,000 feet higher than the aircraft's actual altitude. So you can't assume the cabin is at the same pressure as the outside air; it can be worse. That's the foundation of cabin pressurization and decompression. Let me know when you're ready to move on.