I recently received an email from Jay Vierling of 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. What is electric power used for in the pressurization 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, only getting power when the squat switch activates or 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 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.
Now I want to add a few additional 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.
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 supercharger air overboard, not 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. The logical improvement took place in 1973 when Beech moved the 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.
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 problem persists. Before committing yourself to a low-altitude cruise up the peninsula, try one more thing: pulling the Pressure Control CB. Maybe a rock whacked your squat switch during the takeoff roll, making it act as if you are still on the ground. As you have reviewed in this article, pulling the CB gives you a neat work-around to allow pressurization even with a bad squat (or WOW, weight-on-wheels) switch. Cool!
Thanks again to Jay for the interesting question!
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