421B Pressurization Issues

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    Hello. I have most of my C421 experience in C model aircraft. I have been asked to test fly a 421B model that is pressurizing on the ground. The C421C has the squat switch wired to the dump valve so that it will not pressurize on the ground. I am being told this is not the case with the C421B (and I don’t have access to the manual at this point). Can anyone shed some light on this?


    That seems strange to me but I don’t have a 421B manual or experience. What is the nature of the test flight? Could you just dump pressurization and fly with ram air at lower altitudes? How pressurized is it getting? (PSI differential)

    Regardless of how it is done, there should be an obvious way of keeping the cabin from pressurizing on the ground. Do you have access to the plane POH? When has it last flown? Does the squat switch work?

    You could also take off with the cabin dumped, then once at a safe but low altitude close the ram valve and open one of the pressurization lines. This way you could observe if it is controlling the cabin properly. If it is, open the 2nd engine valve to see if it stays stable. if it is not, return it to unpressurized flight.

    What else have they told you about how this 421B differs from the 421C? 🙂


    Thanks for the information.

    I have flown this aircraft unpressurized. We have been having pressurization issues with the aircraft.

    The POH is clear. Take off and land unpressurized. I always confirm that I am unpressurized before landing a pressurized aircraft, but I have rarely (unless there was a maintenance issue) had to depressurize the aircraft manually prior to landing.

    I am in the process of getting a complete set of manuals for this aircraft. But in the meantime, all I have is my 421C manuals.

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