Monthly Archives: April 2013

King Air Passenger Oxygen Masks Drop

The Beechcraft King Air 350 has a pressurized cabin that can maintain a cabin pressure altitude of 10,200 feet at an aircraft altitude of 35,000 feet. The air used for cabin pressurization is high pressure hot air supplied by each of the Pratt and Whitney PT6A turbo prop engines. This compressed air is conditioned to a comfortable temperature before entering the cabin and excess air is vented from the cabin by a cabin pressure outflow valve on the aft pressure vessel bulkhead. The outflow valve will vary the amount of air escaping through the valve to maintain a set cabin pressure/altitude set by the pilot. If this valve fails the cabin pressure safety valve will limit the pressure differential of the cabin to 6.6 psi thus preventing cabin over pressurization. If an engine were to fail the remaining engine can supply enough bleed air to pressurize the cabin. If the cabin were to lose pressure and the passenger oxygen auto deployment system is working properly then as the cabin altitude climbs to 10,000 feet the CABIN ALTITUDE warning light in the cockpit illuminates, at 12,000 feet the CABIN ALT HI warning illuminates red on the master warning panel, at 12,500 feet a barometric pressure switch energizes a solenoid which opens the passenger oxygen system shutoff valve allowing pressurized oxygen to flow into the oxygen supply line. This pressure opens the passenger masks doors and the masks drop out. At the same time the cabin lights come on full bright, the no smoking fasten seat belt light illuminates and a chime sounds. In order for oxygen to flow into the passenger masks the lanyard valve pin must be pulled out by the user. At 13,500 feet the cabin dump switch limiter closes both outflow valves and attempts to stop the rise in cabin altitude. The passenger oxygen autodeplyment system is required to be operationally checked and this is done by flying the airplane with the cabin unpressurized to an altitude that would activate the system and drop the mask as shown in the video.