Pressurized aircraft enable pilots to fly faster at higher, more fuel-efficient altitudes where human physiology would suffer without some help. By pressurizing the inside of the aircraft's cabin, or pressure vessel, passengers feel as if they are still comfortably on the earth's surface, instead of in a cold, hypoxic, high-altitude environment. The difference between pressure inside the cabin and outside the aircraft is called cabin differential pressure, and it has an engineered limitation to avoid overstressing the cabin, which is much like overinflating a balloon. Maintaining a proper pressure differential is therefore crucial to maintaining safety.
Set the pressure-sensitive altimeter to read pressure altitude by adjust the Kollsman window to 29.92 inches mercury. Find the pressure altitude outside the aircraft by reading the barometric pressure altimeter. As an example, let's use 18,000 feet.
Find the cabin pressure altitude by reading the cabin altimeter. The cabin pressure altitude will always be less than 8000 feet, since people can live comfortably up to that altitude without significant loss of performance or stress on the respiratory system. For our example, let us keep the cabin altitude constant at 6,000 feet.
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Find the cabin altitude differential by subtracting cabin altitude from pressure altitude. Our example yields an altitude differential of 12,000 feet.
Convert from altitude differential to pressure differential. Pilots use common units like inches of mercury (inHg) or pounds per square inch (psi). Earth's atmospheric pressure decreases by one inch of mercury or 0.49 psi for each thousand feet in altitude, so first divide the altitude differential by 1,000. Simply read the answer for pressure differential in inches of mercury or multiply by 0.49 to obtain the pressure in pounds per square inch. Our example would be 12 inches mercury (inHg) or 5.9 psi.