Pressurization Failure

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    I was flying my 1982 340A at an altitude of 16000 when I heard a loud “Pop” and the cabin filled up with moisture / mist. Immediately realized the pressurization failed and probably the outflow valve had failed.??? Initiated a descent to 12000 feet. My plane had just come out of an annual inspection and one of the items worked on was the outflow valve seals being cleaned.

    I would appreciate any thoughts and comments if you have had such an experience before.


      I wouldn’t expect to have to clean the outflow valve seals unless you are a heavy smoker or you have “friend” who smoke in the plane. The outflow valve is controlled by vacuum from the engine driven vacuum pump. Maybe they left a line loose or didn’t complete the installation after seal cleaning. That would be the first place to look. Take off the rear pressure bulkhead door and look at the outflow valve which is on the right.

      Also the overpressure relief valve on the left side looks the same but operates somewhat differently, i.e., by the left hand main gear squat switch and a small solenoid mounted just above the overpressure relief valve. If the wire from the squat switch to the solenoid shorts to ground, then the solenoid opens and allows vacuum to open the valve much like what happens when you land and the squat switch closes and provides a ground for the solenoid.


      Thank you. No we do not have any smokers in the plane. I just found it odd that right after an annual service the pressurization system (seal on out flow valve) failed while it had been operating before. I will look into yor suggestions.


      I had a similar situation a while back. Thinking a seal or pump had broken or that maybe the door had popped as annunciated i made an “emergency” descent. Once levelled off at lower altitude and calmed down a bit, it turned out I had accidentally bumped the depressurize switch above the yoke while adjusting the autopilot in some turbulence 😳 Simple fix 😀

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