Page 19 - Volume 10 Number 12

in the initial climb ... no Delta P (Differential Pressure) until you passed your phantom cabin.
The lack of the Ram Air Door magnet explains why you were seeing the cabin fluctuations during your 8,000-foot cruise: Your high IAS was intermittently blowing open the Ram Air Door. This did not happen when you climbed to normal cruise altitude because the slower climb airspeed was not enough to blow open the door. By the time you leveled off and picked up airspeed, there was enough Delta P to keep the door closed even without the magnet.
The door seal being inflated on the ground is something you’d never notice unless you operated the door, and the lack of dumping would also go unnoticed unless you dialed the cabin altitude down below field elevation ... then the airplane would have pressurized on the ground when your bleed air switches were on.
Once the airplane builds up significant Delta P, the Ram Air Door will remain closed even without the magnet.
So, cruising along normally, one can pull the Pressure Control CB and absolutely nothing changes! Except – and it can be a big deal – now you cannot dump with the Dump switch. If you needed to remove smoke quickly, you’d be out of luck. In effect, pulling that CB is the same as holding the pressurization control switch in Test. In fact, when one is doing a ground pressurization test, pulling the CB frees your hand from having to continually hold the Test switch! Many knowledgeable King Air mechanics do it that way.
To answer your question about what’s inside the controller ... it is totally mechanical, with a variable spring and diaphragm operating a bellows. It simply creates the reference vacuum that is then sent to the Outflow Valve via a plastic tube, a different amount
of vacuum for each different cabin altitude setting.
When the shop fixes the CB so that power is available to the solenoids and magnet I have mentioned, I guarantee all will be well.”
And so they did, and it was – back to normal operation.
DECEMBER 2016
KING AIR MAGAZINE • 17