C 130 cockpit air conditioning11/16/2023 Of course when we descend to land at DEN we are going to have to land unpressurized so the system will allow the outflow valve to release some more pressure, say down to the vicinity of 12.0 PSI. It will do this automatically during the approximate time it takes the airplane to climb to cruise altitude. The pressure controller will close some initially so that we take off with slight positive pressure, then it will modulate to release some of the trapped internal pressure down to about 12.4 PSI. So that is the 'cabin altitude' for our flight. Well this is approximately the pressure you'd find at about 4500 feet above sea level. You remember that our plane is capable of maintaining 8.6 PSID, so the 3.8 ambient, plus 8.6 differential equals a cabin internal pressure of 12.4 PSI. We are going to climb to 33000 feet today where the actual air pressure is about 3.8 PSI. It is more economical to re-condition this air than to use raw bleed air.Īll the air that does not get recirculated goes to the outflow valve(s) where the release of it is regulated by the cabin pressure controller.īack to our flight, we shut in sea level pressure of 14.7 PSI. Some of the air goes to a 'RECIRC' intake and gets back into the air supply system. Some of the air will go from the passenger cabin to at least one of the bag pits where it will pressurize and heat that space. Passenger cabin air may circulate through the sidewalls to insulate you from the minus-40 degree outside air, and be blown in your face by eyeball vents. The air that enters here also cools the circuit breaker panels behind them, and is vented directly to the E&E compartment to cool the radios and computers. The pilots are sitting under very large, heated windows, one of them in almost direct sunlight and they are surrounded by hundreds of little lightbulbs and other heat-producing items. The bigger share of the air goes to the cockpit for good reason. In this ducting, there is a water separator and then the ductwork that distributes the air. Hotter air is added from upstream of the packs to maintain the temperature called for by the pilots. Then it is ducted toward the passenger spaces. A big bunch of this air gets precooled in the engine pylons, then goes to the 'packs.' The packs have additional heat exchangers and an air-cycle machine that drops this air to well below freezing. Some of the bleed air is just released into the fan discharge air but some is used for various utilities aboard the plane, including, possibly, flushing the toilets. Jet engines take in more air than they need to produce their thrust, even on takeoff, so some of this is bled off at the compressor section. When we close the doors at SFO we trap 14.7 PSI in the cabin. We are going to make a flight from San Francisco (sea level) to Denver (let's call it 5400' I don't have my Jepps handy) and barometric pressure is 'standard' over the whole route. So if a typical airliner has the capability to maintain, say 8.6 PSI of 'differential' pressure then the following would be true. Let me explain a few things just so we are talking the same language.Ītmospheric pressure: Nominally about 14.7 pounds per square inch at sea level, about 10.1 PSI at ten thousand feet and only about 3.5 PSI at 35000'. It is maintained at a lower 'altitude' than that at which the plane is flying but it is still somewhat higher than most people are used to breathing. The cabin altitude is hardly 'constant' during a flight. I'd like to see some evidence before I believe anything quoted in that article.ĭoes this make any sense? Why would the air be thinner when cabin altitude is constant in all the cabin. I've never heard of an airline passenger getting hypoxia under normal conditions. I suspect that the difference in flow rate would be pretty hard to detect. Air pressure remains constant, typically 8 psi diff or about 8,000 foot cabin altitude. The air pressure isn't reduced, the flow rate is. What makes matters worse for those in the economy section is that pilots commonly turn off one or more of the air conditioning "packs" as a fuel-saving measure, says Fairechild, which further lessens the air pressure."Īn uninformed statement. So it's not like the aircraft is dumping "first class air" before it can get to the rabble in the back. Essentially all air exits the fuselage through the outflow valve, at the aft end. There is no difference in how the air is piped into the cabin - it enters through ducts spread through the cabin. There are a few more seats jammed into the same amount of space in economy. I'm sure the difference in flow rates has more to do with the amount of space allocated to a passenger.
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