“The negative pressure relief valve is a mechanical device and operates independently.It does not interface with other airplane
pressurization systems and requires no crew action.
The negative pressure relief valve is a flapper type valve. The valve hinges on its top edge and opens inward. A spring on its hinge pin holds the valve closed.
Negative differential cabin-to-ambient pressure opens the valve. The valve opens when pressure outside of the airplane is 1.0 psi more than the pressure inside of the airplane (-1.0 psid)
The negative pressure relief valve is on the lower aft fuselage, on the right side, near the aft service door.
Access to the valve is from the aft cargo compartment.
There are two positive pressure relief valves. They are on the lower, aft airplane fuselage. One valve is on each side of the aft outflow valve.
General Description
The pressure relief valves are fail safe devices that bleed fuselage pressure overboard if the aft outflow valve fails closed.
The positive pressure relief valves are mechanical devices and operate independently. They do not interface with other airplane pressurization systems and no crew action is necessary.
The positive pressure relief valves are pneumatically operated by cabin-to-ambient pressure differential. They control pressure to a nominal 8.95 +/- 0.15 psi more than ambient.
When the differential pressure is too high, the valve opens. The open valve lets air out of the airplane. This relieves the cabin pressure. When the cabin-to-ambient pressure is safe, the valve closes.
The positive pressure relief valves have filters. The filters clean the air used in the internal servo and actuator mechanisms.”
I personally seen the positive pressure valves not operate in an old clapped out 200. The differential gauge have swung around past the max to almost the 12 o’clock position. We had had to use manual pressurization mode, but the FO had fallen asleep and had not periodically opened the valve. We were in an intense thunderstorm activity and we had both agreed that I do the radio transmissions for the multiple and complex heading changes in the midst of several lines of thunderstorms. I could not conceive of anyone falling asleep in such a demanding environment, but he did. And I was too busy running the range and angle of the radar so much that I hadn’t noticed his “checking out.” After getting through the thunderstorms, I did a quick scan and saw the pressurization gauge looking like I had never seen it in flight. It looked like it would on the ground. Thankfully the aircraft was able to take an extreme overpressure.
Last edited by 70 Mustang; 15th Apr 2023 at 08:39.