TheAvatar:
There are many reasons why an aircraft might depressurize. I'm sure mempilot can tell us how a airliner may be intentionally depressurized to help stifle an inflight fire.
Modern pressurized passengerliners have two redundant systems and are composed of controllers, packs (air conditioning units), and outflow valves. Aircraft are not airtight vessels by design, but do contain rubber gasket seals around doors, cockpit escape windows/hatches, and any other through hull doors or hatches to limit the transfer of pressure from the inside to the outside. Pressure can leak from these areas at an acceptable rate well less than the ability of the packs to provide positive pressure from the inside.
The controller takes readings of outside ambiant pressure and inside ambiant pressure to calculate differential, and monitors the rate of ascent and descent via dynamic pressure differential to determine when to open and close the outflow valves. The OFV's open and close to maintain a predermined differential schedule in the climb, level, and descent phases of flight. Any one of each of the paired components can fail, and the system maintains integrity. There is also manual reversion, if the auto control system fails, and the pilots can control the outflow valves from the cockpit. Outflow valves are spring loaded and fail closed, so if the pilots do not respond immediately, the aircraft will try to maintain the inner pressure.
A plane depressurizes for the following reasons:
1. normal depressurization schedule during a normal descent
2. unscheduled slow depressurization due to loss of positive pressure differential
3. rapid depressurization due to loss of control of the outflow valves
4. explosive depressurization due to hull breach
5. intentional depressurization to clear smoke (below 10,000 feet)
With regards to number
(1) if the a/c has an operable controller (manual or auto), it is barely noticeable (situation normal)
(2) can happen when there is a partial loss of positive airflow into the system which is insufficient to maintain a positive differential. The aircraft is descended normally to a safe altitude where the system can keep up. The passengers are typically unaffected or unaware that this has happened. (very rare)
(3) if a single controller fails, the second will control the OFVs. If both fail, the OFVs fail closed. If for some strange reason, an OFV fails to close (shouldn't happen due to design) then air can leak out (I've never heard of this actually happening) Maybe due to foreign object debris blockage?
(4) is bad. Really bad and you won't care whether you are bent or not.
In regards to (5), the PPO2 is kept within life-sustaining limits by maintaining this positive pressure through the climb and level phases of flight. Fires, in most cases, need O2 to burn; however, people need O2 to live. Depressurizing an aircraft in the upper flight levels to starve a fire could have devastating impact on the people aboard. Therefore, at the very first indication of fire or smoke anywhere on the aircraft, an emergency descent is iniated until the aircraft is at or below 10k feet and the plane can be safely depressurized.
Note on 5: The crew of the Swiss Air flight that went down off Nova Scotia due to fire turned off the packs which actually sucked the fire into the cockpit through the vents. By degrading the pressurization system, they allowed the fire to enter the cockpit. They were at an altitude where even without pressurization, there was enough O2 for fire to breath. This information was released recently as a result of the pursing investigation.
So, do we depressurize on purpose? A. Very very rarely and not at cruise altitude. We typically try to get below the dehibilitating hypoxic altitudes above 10000 feet.
We actually just had an aircraft with smoke in the cabin and cockpit do an emergency descent and landing in Talledaga several weeks ago. Both the captain and first officer are friends of mine. The aircraft held normal pressure until they got below 10k at which time they flushed the smoke on the way to the runway by forcing the OFV's open and allowing ram air to sweep through the aircraft. The passengers felt only a small spike in cabin pressure when the plane went through 10000 feet, at which point they were descending at 6000 ft per minute. There was a 'burp' in the cabin pressure as it went from 8k to about 8500 as the plane caught the cabin pressure on the way down. - Almost seamless.