[fingpilot]

The 'altitude' inside the cabin is controlled by a system that controls the flow of air OUT of the cabin. In a perfect world, the cabin of an airplane would be an airtight chamber. It's actually pretty leaky, but nonetheless it works most of the time.

On a flight, say, taking off from a sea level airport and landing at a sea level airport, the plane, if the trip is long enough, will climb to it most efficient altitude, say, on a x-country flight, 38000 feet. Obviously, we humans cannot survive the lack of oxygen (or the temp) at that altitude, so the plane is 'pressurized', pumped full of air that has been heated, or sometimes cooled to some shirtsleeve temp and altitude. The system is setup ahead of time, programmed for the flight. Normally, fuselages these days are designed for a working pressure of 8 to 15 PSI. Limits can and are higher, but these numbers are average. What that means is that the DIFFERENCE in inside and outside pressure is 8 to 15 PSI. That usually translates to a cabin ALTITUDE of 8000-ish feet above sea level. The fuselage is stressed to always have some positive pressure in it, negative pressure is bad.

The system does this by taking compressed air from the engines (simple explanation here) and pumping it into the cabin. The pressure (altitude) in the cabin is regulated by the system opening and closing, regulating, actually the leakage (outflow) of air from the (mostly) airtight cabin. A lot of other things are going on as well, but the simple answer is that the flow of air out of the cabin is what is controlled.

What is ASSuMEd is the the flow of air into the cabin is relatively constant, AND more importantly, greater volume than is being let out by the cabin controller. There are times that things can be in flux, and you feel a 'bump' in your ears.

Worst case normal-normal scenario, high differential pressure (high plane altitude), top of descent, throttles are brought to idle, or lower power, reducing the air coming in, and the outflow valves, even closed, cannot maintain the cabin pressure.

Back to MH370. Cruise altitude set for 10700 meters (you fly metric altitudes over there), or approx. 35000 feet. Cabin controller sets cabin altitude based on that, and allows cabin to go maximum differential pressure, lowest cabin altitude for pax comfort. Someone orders the plane to climb, in this case above max ceiling, and the best the cabin controller can do is to maintain this max differential as the plane climbs, The cabin alt climbs, uncomfortably fast, because the max dif pressure is absolute. the altitude alerter in this cockpit cannot be set above maximum, so the climb above service ceiling has to be done on a vertical rate, or hand flown. At the weight they were at one hour into this flight, the performance ceiling was probably 38000 or so. Hand flying was probably a workout. My bet is that the airplane, if it really made it to 45000, stalled, and fell off on a wing, diving, and a good guess is that the next altitude of 23000 feet was a recovery from the stall. Could that episode have been done on autopilot? Maybe, but still incredibly lucky to have escaped tearing something off of the airplane.

At the top of this climb, once started down, the speed grows scary VERY quickly, and the engines were probably idled. No air coming in, the cabin altitude climbed VERY quickly. My bet is that the outflow valve was intentionally manually opened prior or during this climb, after the pilot/s were on 100% oxygen, and pressure breathing. Anyone never having experienced this, it is something you had better have mastered prior to having to do it for real. The mask pressure flows oxygen, Aviators' Breathing Oxygen, into the mask, and this pressure inflates your lungs. you have to exhale hard to get it to stop. Most people puke when you have to actually do it unprepared.

Again, the pax had to be 'neutralized' prior to getting back in cell range. Sounds brutal, sorry, but the bad guys learned from 9-11 too.