Page 15 - August 2015 Volume 9, Number 8

(Notice both the Schgr Vent – Press Test switch on the top row, as well as the Press Control circuit breaker on the lower right.)
When the landing gear squat switch – or WOW, weight-on-wheels, switch – is activated, electric power is removed from the Flow Control Valve’s solenoid, causing the vacuum to be removed and hence stopping the inflow of supercharger air. Selecting the “Press Test” position of the switch in the cockpit causes the Flow Control Valve to receive electricity and vacuum, making it send its normal air supply into the airplane. At the same time, the Dump and Preset solenoids are de-energized to allow the pressurization controller to operate normally. (The Preset solenoid did not appear until the B90 model, LJ-318 and after.) With the pressurization controller set for a lower-than-field-elevation altitude, the cabin should start descending to verify a proper pressurization test.
In a similar manner, moving the cockpit switch up to Schgr Vent does half of what Press Test does. Namely, it causes the Flow Control Valve to send proper airflow into the cabin, but it has no effect on the outflow or safety valves. Thus, the airplane will be receiving air from the supercharger to ventilate, not pressurize, the cabin...explaining the name, Supercharger Vent(ilation).
Once airborne, of course the outflow and safety valves should be doing their respective things and
the supercharger should already be supplying proper inflow. That leads to the conclusion that you can select any position at all – up, center, or down – with this strange switch when you are flying and nothing changes whatsoever. It only has functionality while on the ground.
You may be wondering, “When would one use Supercharger Ventilation anyway? What purpose does it serve?” The simple answer is that it serves almost no purpose and most pilots of these airplanes have probably never used it! But its most common use in days past was smoke removal! “Huh? Smoke removal? Why would I still be taxiing out with smoke in the airplane?!” Because the boss and his friends in back are smoking cigars and cigarettes! Yes, back in the ‘60s when these airplanes were being produced, smoking was still amazingly widespread and passengers thought nothing of lighting up after they took their seats. It could get pretty unpleasant, especially for any non-smoking pilots, when all of that “polluted” air was trapped in a pressure vessel with no inflow and no outflow. By selecting Schgr Vent, bringing the outside air in and forcing flow out of the safety valve, the smoke level in the airplane could be decreased significantly. (If you are lined up behind a row of airliners on the taxiway, you have the choice of smelling the cigarette smoke or the kerosene fumes, eh?) As I said, nowadays the switch is rarely ever moved to its Up position.
OCTOBER 2015
KING AIR MAGAZINE • 13