Page 21 - August 2015 Volume 9, Number 8

the Preset Solenoid preventing suction from getting to the Controller until after we lift off, it cannot actually start sending the cabin to the desired altitude until we leave the ground.
For example, let’s say we are departing from a sea level airport and have the desired cabin altitude for cruise set for 7,000 feet. After a couple of initial surges, we should observe the cabin climbing obediently toward 7,000 when we check the gauges after takeoff. If the rate of climb is too fast or too slow, we adjust the Controller’s Rate knob appropriately.
If the Preset Solenoid had a loose connection and did not energize closed on the ground as it should, the result after takeoff would be the cabin climbing right along with the airplane: Same climb rate; zero ∆P. Oops. After consulting the checklist and verifying that the Bleed Air Valve switches are on and that the Pressurization Control switch is not in the Dump position, we have just about decided that we’ll need to return to the departure airport to have our mechanic find the problem when – Voila! – the cabin stops climbing, ∆P starts increasing, and all is well once again.
What happened here is that the Controller began trying to climb the cabin to the selected altitude as soon as we set it during our Before Takeoff procedure. Since the actual cabin altitude may never be above the
airplane’s altitude – that would represent a negative ∆P value, which is impossible to have – what I call a “Phantom Cabin” exists somewhere above us, that the Controller is trying to reach. Only when we climb above this Phantom Cabin, do things start working 100% normally. Unless you took off quite quickly following your setting of the Controller, in most cases you will fly all the way up to the selected cabin altitude – 7,000 feet, in our previous example – before the cabin stops its unpressurized climb and levels off.
Until the loose connection or bad solenoid valve gets repaired, this problem is relatively easy to abide: Simply leave the Controller where it had been for your landing at this airport and don’t dial in the new cabin altitude for cruise until after departure. This is exactly what the 90, A90, and B90 pilots must do routinely, since those models have no Preset Solenoid.
If the Preset Solenoid valve can fail open as we have just discussed – the more likely scenario since that is its de-energized state – then it can also fail the other way, stuck in the closed position, failing to open after liftoff. Now what?
The closed valve prevents suction from reaching the Controller so it, in turn, cannot regulate that suction and modulate the Outflow Valve with it. Hence, the Outflow Valve remains in its normally-closed position
AUGUST 2015
KING AIR MAGAZINE • 19