Pressurization Systems — Page 227, Lesson 283

I want to walk you through the pressurization system's control logic now — specifically how the crew and the automatic controllers manage cabin pressure through the different phases of flight. First, let's set the scene. The cabin altitude control panel is the crew's interface, and it's remote from the controller itself. That means the panel you see on the flight deck is separate from the actual electronic brain that does the work. In practice, you'll find this panel fitted to the overhead panel on the flight deck. Now, the key concept here is that there are two modes of operation: auto and manual. And within auto, there are two channels — auto 1 and auto 2. The outflow valves — those are the valves that let air escape the cabin to control pressure — are electrically operated. Under automatic control, they're driven by either of two AC motors, and each AC motor is under the control of one of the automatic controllers. For manual or emergency operation, there's a separate DC motor that drives the outflow valves instead. Here's the redundancy logic: only one controller is in use at any one time. The other sits on standby. If the active controller fails, the standby controller automatically takes over. That's your fail-safe — you always have a backup brain ready to go. If the crew selects manual, that locks out all normal automatic functions. Now the outflow valves are positioned by the manual control switch, which drives the DC motor. The pilot sets the controller to produce the required flight profile. Now let's walk through the flight phases, because the system behaves differently in each one. Taxi. When the aircraft begins to taxi, the pressurization GROUND/FLIGHT switch is selected to FLIGHT, and the aircraft is pre-pressurized to a differential pressure of 0.1 psi. Differential pressure — that's the difference between cabin pressure and the outside ambient pressure. Pre-pressurizing to 0.1 psi ensures the transition to pressurized flight will be gradual. It prevents surges of pressure on rotation — that's the moment the nose wheel lifts off — and it also prevents ingress of fumes from the engines and other sources. Take off and climb. As the aircraft takes off, the ground/air logic system signals the controller to switch to proportional control. Now the controller senses both ambient pressure and cabin pressure, and it positions the outflow valves to control the rate of change of cabin altitude in proportion to the rate of climb of the aircraft. The target rate of change is between 300 and 500 feet per minute. So the cabin climbs at a controlled, comfortable rate that tracks the aircraft's climb. Cruise. When cruise altitude is reached, the controller switches to isobaric control — and that's where the excerpt cuts off, but I'll pause here because that's the natural break in the material. Let me make sure you've got the key terms locked in: differential pressure, outflow valves, proportional control, isobaric control, the auto 1/auto 2 standby logic, and the manual DC motor override. Those are the building blocks for everything that follows in this chapter.