Pressurization Systems — Page 227, Lesson 286

Let me walk you through the final part of the pressurization systems chapter. We've already covered the control valves and the electronic controller, so now we're looking at the warning systems, the ground testing procedures, and the standard atmosphere table that underpins everything. First, the warnings. In addition to the automatic pressure control we discussed, there must be both an AURAL and a VISUAL warning when the cabin altitude exceeds 10,000 feet. The aural warning takes the form of a horn. The visual warning is a red light on the Centralized Warning Panel, or a warning caption on the appropriate EICAS or ECAM display. EICAS stands for Engine Indication and Crew Alerting System, and ECAM is Electronic Centralized Aircraft Monitor — these are the two main electronic display systems you'll see on modern aircraft. So if the cabin climbs above 10,000 feet, you get the horn and the red light or caption simultaneously. Now, ground testing and checking. Pressurization systems must be checked at periodic intervals to ensure there are no serious leaks, and that the pressure control components and safety devices are operating correctly. There are four specific occasions when these tests are carried out. First, the initial proof pressure test — that's done when the aircraft is first built or after major structural work. Second, when specified in the maintenance manual — that's the scheduled periodic check. Third, after actual or suspected system malfunction. And fourth, after repairs and modifications to the aircraft pressure hull — the pressure hull being the sealed structure of the fuselage that actually holds the pressure. The exact procedure for the functioning and leak rate tests is specific to type, and it's laid down in the aircraft maintenance manual. But the tests may be required to establish any or all of four things. First, general functioning and temperature control. Second, operation of the pressurization controller or controllers, and the normal maximum differential control — that's the valve that limits the pressure difference between cabin and outside air during normal flight. Third, the safety valve check, which tests the maximum structural differential pressure — that's the absolute limit the structure can withstand. And fourth, the leak rate check — how fast the cabin loses pressure when it shouldn't. Now, the table you see here is the ICAO Standard Atmosphere. This is the internationally agreed model of how temperature, pressure, and density change with altitude. It's the reference you use for everything — performance calculations, altimeter settings, and pressurization system design. Let me walk you through the key values. At sea level, zero feet, the temperature is plus 15 degrees Celsius, pressure is 1013.25 hectopascals, which is 14.7 pounds per square inch, density is 1.225 kilograms per cubic metre, and relative density is 100 percent. That's the baseline. At 5,000 feet, temperature drops to plus 5.1 degrees, pressure is 843.1 hectopascals or 12.22 psi, density is 1.056 kilograms per cubic metre, relative density 86.2 percent. At 10,000 feet — and this is the critical altitude for our warning system — temperature is minus 4.8 degrees, pressure 696.8 hectopascals or 10.11 psi, density 0.905, relative density 73.8 percent. At 15,000 feet, minus 14.7 degrees, 571.8 hectopascals, 8.29 psi, density 0.771, relative density 62.9 percent. At 20,000 feet, minus 24.6 degrees, 465.6 hectopascals, 6.75 psi, density 0.653, relative density 53.3 percent. At 25,000 feet, minus 34.5 degrees, 376.0 hectopascals, 5.45 psi, density 0.549, relative density 44.8 percent. At 30,000 feet, minus 44.4 degrees, 300.9 hectopascals, 4.36 psi, density 0.458, relative density 37.4 percent. At 35,000 feet, minus 54.3 degrees, 238.4 hectopascals, 3.46 psi, density 0.386, relative density 31.0 percent. Now notice what happens at 40,000 feet and above. The temperature stops dropping — it stays constant at minus 56.5 degrees. That's the tropopause, the boundary where the troposphere ends and the stratosphere begins. At 40,000 feet, pressure is 187.6 hectopascals or 2.72 psi, density 0.302, relative density 24.6 percent. At 45,000 feet, pressure 147.5 hectopascals, 2.15 psi, density 0.237, relative density 19.4 percent. And at 50,000 feet, pressure 116.0 hectopascals, 1.68 psi, density 0.186, relative density 15.2 percent. The key takeaway from this table is the relationship: as altitude increases, temperature decreases until the tropopause, and pressure and density decrease continuously. The relative density tells you how much air is available — at 50,000 feet you only have about 15 percent of the sea-level air density. That's why pressurization is essential — the cabin must maintain a much lower altitude than the aircraft's actual flight altitude, and the differential pressure limits we discussed are what make that possible. That completes the pressurization systems chapter. You now have the full picture — the control valves, the electronic controller, the warning systems, the ground testing procedures, and the standard atmosphere that ties it all together.