pressure inside the aircraft. you have a constant flow of pressurization coming from the engines into cabin on this aircraft. it's just regulated as far as what altitude. now, remember altitude and pressure are what maintain the breathability of the air. so this outflow valve then controls how much pressure is inside the cabin. you're maintaining that cabin pressure at a normal 8,000 feet or 10,000 feet at the most. so that would make it regular air. now, when this outflow valve fails, let's say it gets plugged up it can overpressurize the cabin, so there's another valve called the safety valve. although it's normally closed in this situation where the joet flow valve fails, the safety valve would open and release that excess pressure. what's happened in the stewart situation, when of course everyone was incapacitated, is the safety valve failed. when the outflow valve fail and the safety valve didn't do its