Pressurization Systems — Page 218, Lesson 278

Let's start with the very reason we pressurize an aircraft at all. Modern aircraft are built to operate efficiently at high altitudes, and they climb and descend at high rates. But the human body isn't built for that. So the interior of the aircraft is pressurized to let passengers and crew function normally without needing extra oxygen. The danger up high is hypoxia — insufficient oxygen. It causes a reduction in the ability to concentrate, then loss of consciousness, and finally death. The full effects are covered in your Human Performance notes, but the key point here is the altitude thresholds. Up to 10,000 feet — that's 3.3 km — the air pressure and the amount of oxygen are sufficient for humans to operate without too many problems. Above that, lack of oxygen becomes apparent. So the pressurization system is designed to produce conditions equivalent to approximately 8,000 feet, or 2.6 km, or less. That means no oxygen equipment is needed except for emergency use by crew or passengers, and the effect of low atmospheric pressure on passengers is negligible. Now, two critical crew and passenger limits. Once the cabin altitude reaches 10,000 feet, the crew must be on oxygen. And at 15,000 feet cabin altitude, the passengers must be on emergency oxygen. Note the term cabin altitude — that's the pressure altitude corresponding to the pressure inside the cabin. It's a way of expressing the cabin's internal pressure in terms of an equivalent altitude. Because the cabin is held at a lower altitude than the aircraft's actual flight altitude, the aircraft can achieve high rates of climb and descent while making correspondingly small rates of change of cabin pressure. That's a big operational advantage. Now let's look at the structure. The airframe must be strong enough to withstand the differential pressures generated — the difference in pressure between the inside and outside of the pressurized areas — without being too heavy and therefore uneconomic in operation. That differential pressure produces hoop stresses. These are applied cyclically every time the aircraft is pressurized and de-pressurized, causing fatigue which can ultimately lead to structural failure. So keeping the maximum differential pressure to its lowest practical value reduces the hoop stress. Pressurizing the cabin to the 8,000-foot level reduces the stresses and therefore the fatigue on the airframe, as well as reducing the required structural strength and keeping the weight down — which increases the economy of operation and reduces the initial costs of the aircraft. Typical maximum differential pressures for large jet transport aircraft are between 8 and 9 psi — that's 552 to 621 hPa. And here's the layout: the passenger cabin, flight deck, and cargo compartments are normally pressurized, while the undercarriage bays, tail, and nose cones are unpressurized. Let me show you the pressurized and unpressurized areas on the aircraft. So to tie it together: pressurization exists to keep the cabin at or below 8,000 feet equivalent, the crew oxygen limit is 10,000 feet cabin altitude, the passenger emergency oxygen limit is 15,000 feet, and the structural design is driven by managing differential pressure and hoop stress fatigue.