I'm confused. The AC units on all 737s are internal recycle units. Can someone who actually knows, tell me, how this could have caused depressurization. I am presently in QLD with a commercial pilot who is also asking the same question.
As jb747 mentioned previously, an aircraft is not a completely sealed unit. Air leaks out and is not completely contained within the cabin. Pressurisation is maintained when the air leaking out is replaced by new air coming in. The new air coming in is "pumped" in by the air conditioning "packs". While its true that on aircraft like the 737 that the AC system does recirculate cabin air, it is also adding new air from the engine bleed system.
oz_mark said:
Simple. You pressurise with conditioned air (which includes bleed air to balance what you are losing, but bleed air is rather warm, so you need to cool it down)
The reason the new air is "hot" and needs to be cooled is not because its been heated by the "fire" in the engine, but due to the diabatic process of pressurisation. Bleed air is taken from the low or intermediate stage compressor of the jet turbine and is compressed to a very high pressure that results in it being very hot (up to 200 degrees C). Its then expanded and cooled for delivery to the cabin. Bleed air is used by more than just the cabin anvironmental control system.
So the air being fed into the cabin by the AC packs is a combination of recirculated cabin air and "fresh" compressed air from outside. The cabin pressure is maintained by controlling amount of "fresh" air being added against the air leaving the aircraft through leaks and the out-flow valve. Even with the outflow valve(s) closed, without the AC packs operating its only a matter of time for the cabin pressure to drop (also referred to as cabin altitude increasing) as air leaks from the cabin.
Cabin pressure is generally measured/reported as an equivalent altitude, such that the air pressure inside the cabin is the equivalent of being at a certain altitude. Cabin altitude is generally maintained around 6,000 to 8,000 feet. The lower the cabin altitude the more comfortable it is for passengers but the bigger the pressure differential between inside and outside. Note that 8000 feet above sea level is about the same as being at the north rim of the Grand Canyon - lots of people live at such altitudes every day, with Mexico City being around 7,400 feet above sea level. Most people can breath quite happily up to 10,000 feet or more which is why an aircraft with a pressurisation problem will descend to around 10,000 feet. Oxygen masks will automatically drop when the cabin altitude reaches a point somewhat above 10,000 feet (around 14,000 feet - jb747 may be able to tell us the exact threshold).
So for the oxygen masks to have dropped in this incident, the outflow of air from the cabin obviously caused the cabin altitude to increase above the threshold before the aircraft had descended below the threshold.
The outflow value is very important to ensure the cabin is fully depressurised before the doors are opened on the ground.