Page 20 - Volume 10 Number 12

Now I want to add a few additional supercharger air overboard, not in 1973 when Beech moved the
comments. First, the Preset Solenoid did not appear on King Airs until the B90 model replaced the A90 in 1968. So, if and when you fly a straight 90 or A90, the correct procedure is to leave the controller set for the previous landing and don’t dial in your cruise altitude until after takeoff. The Preset Solenoid certainly simplifies this task and eliminates the chance of forgetting to crank the cabin up in the initial climb.
sending any into the cabin. So, if the same scenario – a bad Pressure Control breaker that was not allowing power to pass – befell these early models, the symptom would be total lack of any pressurization at all due to lack of inflow!
Earlier King Airs were not equipped with the Door Seal Solenoid. In the airplanes with superchargers, that same air source inflated the door seal. When the change was made to dual bleed air, for a while it was still the left side only that supplied the door seal; so if the left engine was not running when the door was opened or closed, the inflated seal provided no resistance. Then it was recognized that the advantage of dual bleed sources, left and right, was compromised to some degree since when the right side alone was operating, it would be flowing air into a rather leaky cabin since the door seal was not being inflated.
inflation source from the left side’s Environmental (“Big P3”) air to the Pneumatic Pressure manifold that was fed by the “Little P3” from both sides. Now either engine running led to an inflated seal and soon complaints started being received at the factory that it was now more difficult (sometimes impossible!) to close the door after making a quick drop-off or pick-up with only the left engine shut down. In response to this aggravation, the Door Seal Shutoff Solenoid valve made its appearance in 1978.
Next, all 90s, A90s and B90s
receive their cabin air inflow from
a supercharger driven by the left
engine, not by the dual bleed air
system we have today. (Unless
they have been extensively and
expensively modified.) In these
models, the Pressure Control CB
also is the power source for the
Flow Control Valve, the device that
regulates the flow of supercharger
air into the cabin. This Flow Control
Valve, when de-energized, defaults
to the position that dumps the The logical improvement took place problem persists. Before committing
One final thought: Suppose you have enjoyed a lovely weekend on a beach in Baja, next to the dirt and gravel strip by the private resort. Heading for home you find that you have no pressurization. After making the normal checklist steps – yes, the Control Switch is not in Dump; yes, both Bleed Air switches are On; yes, no CB is tripped – the
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18 • KING AIR MAGAZINE
DECEMBER 2016