Let’s pick this up right where the pressurization controller is managing the… — Page 227, Lesson 284

Let’s pick this up right where the pressurization controller is managing the flight profile. We’ve already covered the climb phase, so now I want to walk you through what happens in the cruise, the descent, and then the instrumentation you’ll see on the panel. Cruise. Once the aircraft reaches its cruise altitude, the controller switches from proportional control to isobaric control. Isobaric means "constant pressure" — so the controller now maintains a constant differential pressure, which is the difference between the pressure inside the cabin and the pressure outside. Small altitude changes of plus or minus 500 to 1000 feet will be accommodated automatically without any change in cabin pressure. But if you have to increase the cruise altitude significantly, you’ll need to reset the flight altitude selection on the controller. Now, there’s a limit here. If the maximum differential pressure has been reached, the controller will not allow any further increase in differential pressure. At that point, the system is in what we call Max. Diff. Control — maximum differential control. The cabin pressure stays fixed relative to the outside, and the cabin altitude will rise with the aircraft. Descent and landing. At the start of the descent, the controller switches back to proportional control. It gives a cabin rate of descent of 300 feet per minute, and it does this to produce a differential pressure of 0.1 psi on touchdown, at an airfield altitude of minus 200 feet. That’s a small positive pressure remaining in the cabin at landing. Then the ground/air logic system switches to ground mode. When you change the cabin pressure controller’s GROUND/FLIGHT switch to GROUND, the outflow valves are driven fully open to equalize cabin and ambient pressures. On older aircraft, the controller will reduce the differential pressure to zero on touchdown. Let me summarize the valve logic for you, because this is the heart of the system: if the differential pressure is increasing, the discharge valves are closing. If the differential pressure is decreasing, the discharge valves are opening. And if the differential pressure is constant, then since the mass flow in is constant, the discharge valve will not move. Now let’s talk about manual control. With the system in manual control, you can vary the outflow valve position using the main outflow valve control, and you do this with reference to the cabin altitude gauge and the valve position indicator. The maximum permissible rate of change of cabin pressure is 0.16 psi per minute, which is approximately a rate of climb or descent of 1500 feet per minute. But here’s the operational limit you need to respect for passenger comfort: cabin rates of climb and descent should be carefully monitored and should not normally exceed 500 feet per minute during the climb, or 300 feet per minute in the descent. This is to avoid causing too much discomfort for passengers, particularly those with colds, and to reduce the effect of rapid pressure changes in the ears. Finally, let’s look at the system instrumentation. The minimum indications required for a pressurization system are three instruments. First, the Cabin Altimeter — this gauge reads cabin pressure but is calibrated to read it in terms of the equivalent altitude of the cabin. Second, the Cabin Vertical Speed Indicator — this indicates the rate at which the aircraft cabin is climbing or descending. And third, the Cabin Differential Pressure Gauge — this indicates the difference in absolute pressure between the inside and outside of the aircraft cabin, and it’s generally calibrated in psi. In the event of a malfunction of the pressure controller or outflow valve, this instrument would indicate that the safety valves were controlling the cabin pressure at the structural, or emergency, maximum pressure differential. So to tie it all together: the controller manages the flight profile automatically, but you have manual override capability, and the three instruments give you the information you need to monitor and verify the system is working correctly.