Page 18 - Volume 10 Number 12

Ask the Expert
How Does Electric Power
Affect Pressurization
by Tom Clements
Irecently received an email from Jay Vierling of only getting power when the squat switch activates or
Cincinnati, Ohio, seeking my opinion on some
pressurization anomalies and what the mechanics had found. Here’s what he wrote:
I have a question and haven’t been able to find any good answers. I own and fly a King Air C90B. I’ve been noticing erratic pressurization during climb. Many times, the cabin climbs at the aircraft climb rate and then around 7,000 to 8,000 feet, it starts to build differential pressure and seems to work fine. When I say fine, the rate control seems to function on the descent. On one low altitude (8,000-foot cruise) short flight, the cabin vertical speed was jumping all around and I could feel it in my ears. Luckily, no passengers were on that leg.
I took the aircraft in for service. Friday, they called and said they think they found the problem ... that the pressurization control circuit breaker was bad. I said you mean intermittent? They said no, no power to the controller.
So, that leads me to a bunch of questions:
1. Whatiselectricpowerusedforinthepressurization controller besides lighting?
2. Can the controller partially function with no power?
3. What exactly is in that little box?
4. Is it reasonable to think a bad breaker is the problem?
I’d be very interested in any insight you might have.
Here is the reply I sent, with some additions and modifications for the purpose of the article:
“Hi Jay,
Yes, I bet your shop is 100 percent correct.
The Pressure Control Circuit Breaker (CB) sends power to (1) the Dump Solenoid, (2) the Door Seal Solenoid, (3) the Preset Solenoid, and (4) the Ram Air Door electromagnet. Except for the Ram Air Door’s magnet, all of these things are de-energized in flight,
16 • KING AIR MAGAZINE
when the Cabin Pressure Control switch is moved to “Dump.” The three solenoids connect to and operate valves, with the combination known as a “solenoid valve.” Two of the solenoid valves, the Door Seal and the Preset, are of the N.O. type. That means they are “Normally Open,” and that the valve closes only when electricity activates the solenoid. The Dump Solenoid valve is of the N.C. type, “Normally Closed,” only opening when the solenoid is energized.
On the ground, activated, the Dump Solenoid valve opens to allow a vacuum to flow to the safety valve, sucking it wide open. The Preset Solenoid valve closes, shutting off the vacuum line going to the controller, preventing the controller from working. This allows you to preset your final cabin altitude for cruise while still on the ground and yet the controller does not begin to operate the Outflow valve until after liftoff. The Door Seal Solenoid valve closes to prevent the door seal from inflating, making it easier to open and close the door with an engine running.
In your case, with the bad CB, the Preset Solenoid getting no power meant that when you dialed in, say, a 7,000-foot cabin altitude before departure, the controller immediately began trying to climb the cabin to that altitude by opening the Outflow Valve. By the time you lifted off, the controller – depending on how long you spent on the ground – would “think” the cabin should be above you and therefore it would not do anything until you climbed above this “phantom” cabin, and then all would be just fine. This explains the lack of rate control
DECEMBER 2016