Did the article’s title cause you a moment of uncertainty? That was my intent: To change the name of a well-known King Air system component so as to turn it into a description of its function. As most of you have likely figured out by now, this article is about the Pressurization Controller … which has one purpose – to govern cabin altitude.
In “The King Air Book,” there is a section comparing two governors: the less-familiar PT6’s FCU (Fuel Control Unit) to the more familiar PPG (Primary Propeller Governor). The section tries to make the FCU’s operation easier to understand by comparing it to the good ol’ prop governor. One of the discussion points was that any governor is attempting to govern – or maintain constant – some parameter. In the case of the PPG it is Np; in the case of the FCU it is N1. However, no governor is perfect and neither can govern the parameter 100 percent of the time. For example, when we reduce power and airspeed for our landing flare, the propeller blade angle becomes fixed at its lowest setting (the Low Pitch Stop); when that takes place the governor is no longer capable of maintaining the selected propeller speed and RPM starts to decrease.
In a similar fashion, if a Power Lever gets positioned to Idle at a higher altitude – say, FL200 – with the Condition Lever at Low Idle, N1 will not decrease to the Low Idle speed expected. Instead, the Minimum Fuel Flow Stop prevents the fuel flow from going low enough to compensate for the reduced compressor drag of the thin air it is now experiencing.
Allow me to take this governor comparison one additional step further. The Pressurization Controller is nothing more than a governor of cabin altitude. These altitudes, by the way, are always pressure altitudes, always referenced to an altimeter setting of 29.92 in Hg at sea level.
Here is perhaps a new way of defining the Controller: To the best of its ability, the Pressurization Controller will climb or descend the cabin altitude, at the rate set by the Rate knob, to the altitude dialed into its face and then will maintain that cabin altitude to the best of its abilities. It is nothing more nor less than a governor of cabin altitude.
I should probably specify here that I am talking about normal in-flight operation: Normal power settings on the engines, proper air inflow from two good Flow Packs or a Supercharger, an airplane without excessive air leaks, and the control switch not selected to Dump. The automatic dumping action that occurs based on a weight-on-wheels (WOW) switch, nullifies the Controller’s action totally when we are on the ground.
The range of cabin altitudes that may be selected on the Controller go from negative (minus) 1,000 feet to a positive 10,000 or 15,000 feet MSL, depending on your King Air model and the Controller it contains. The 10,000 feet covers all airports in the United States, but enough complaints came in from operators in places like La Paz, Bolivia, that Beech switched to the 15,000-foot controllers for all 300- and later 200-series.
Here’s a good operational challenge for you to consider: How do you land at 14,000 feet in your model 200, without popping your passengers’ ears, when flying with a controller that only goes to 10,000 feet?
It is rare to find a controller that will yield identical rates of cabin climb and descent when the Rate knob remains in one position. Almost always, a slightly higher setting – like the one o’clock position – yields a 500 fpm climb while a lower setting – more like 11 o’clock – yields a 500 fpm descent. Also, the results you get in one airplane with its controller rarely will be the same as what you find in another airplane with its controller. Make sure to monitor cabin rates of climb and descent on the Cabin’s VVI and adjust the Rate knob to what you desire.
There are two situations that prevent this governor from governing. As mentioned before, when the propeller slows down in the landing flare or Low Idle speeds are observed to increase with altitude, nothing is wrong. Instead, the Prop Governor or the FCU has merely reached a limit of its ability to govern.
Before I present the answers, give yourself a quiz: What are the two situations, both perfectly normal, when the Controller cannot maintain the selected cabin altitude?
Tick-tock-tick-tock-tick-tock … got your answers yet?
The first situation in which the governor cannot govern is when the airplane descends below the selected cabin altitude. This should happen once on every flight as the airplane, on final approach, descends below the cabin. If this did not take place, then the outside pressure would be higher than the interior cabin pressure. The airplane is not designed to handle these types of compression forces, not to mention that it would be difficult to open the cabin door on the ramp if the higher pressure outside were still pushing in!
From your training, you may recall that the technical description of this first inability to govern situation is due to the fact that both the Outflow and the Safety valves contain a Negative Differential Pressure Relief function. When the outside pressure tries to exceed cabin pressure – because the airplane is descending below the cabin altitude – both valves allow themselves to be pushed open, permitting outside air to freely flow into the cabin, equalizing the pressures.
The other situation in which the controller fails to maintain the selected cabin altitude, even though it is working perfectly? That comes into play when the airplane climbs high enough that the Maximum Differential Pressure Relief function of the Outflow or Safety valve is reached. If the cabin were to remain low while the airplane climbs too high, the excessive expansion forces acting on the airframe could eventually lead to damage. Hence, at the normal Maximum Differential Pressure value, the valve opens automatically to expel enough air to cause sufficient cabin climb so that the maximum design Differential Pressure, ∆P, is never exceeded.
Every cabin pressure corresponds to a different cabin altitude, so it follows that governing cabin altitude is the same as governing cabin pressure. The pressure inside any fixed-volume container depends on (1) how much air is in the container, and (2) the temperature of that air. Since cabin temperature is held fairly constant (we hope!) most of the time, cabin pressure depends on how much air is in the cabin. More air, higher pressure, lower altitude. Less air, less pressure, higher altitude.
As in most other aircraft pressurization systems, the inflow of air into the pressure vessel – the fancy term for the cabin and other pressurized parts of the fuselage – is held relatively constant so the regulation of total air mass in the vessel depends on how much is flowing out. The outflow is controlled by the position of the Outflow Valve … duh! It follows that the Controller’s job gets accomplished by its management of Outflow Valve position.
Left alone, the Outflow Valve is closed with springs applying the force to close the moving poppet against its seat. It is suction applied to the valve that overcomes the spring force and causes the valve to open … the greater the suction, the larger the opening.
The Controller has three lines connected to it. The first is filtered cabin air; the second is instrument suction; the third is a line going to the Outflow Valve. A reference chamber in the Controller has air being sucked out by the suction line and air flowing in from the cabin. Based on the setting of the Altitude knob, the suction this chamber feels can be stronger or weaker. The stronger the suction, the lower the reference chamber pressure. It is this reference chamber pressure that is felt by the Outflow Valve through the line that connects the two.
The overall result is that every cabin altitude dialed into the Controller equates to a different reference pressure for the Outflow Valve and that valve then modulates cabin air outflow so as to maintain a constant cabin pressure, cabin altitude.
“Delta P” (∆P), Differential Pressure, as the name indicates, is simply the difference between inside and outside pressures. The inside pressure is cabin pressure; the outside is ambient. The Controller, as we have presented, determines only cabin pressure. It is the airplane’s altitude that determines ambient pressure. The Controller, therefore, does not determine the amount of ∆P, the amount of pressurization, taking place. No! A Controller can be working perfectly while ∆P can be anything from zero to maximum!
I would make an educated guess that for every Controller that was sent in for exchange or overhaul, only a third or less were actually found to be defective. Instead, the problems that the crew was observing had to do with lack of inflow or too much outflow – too many cabin leaks. Either of these abnormalities can cause the inability to achieve maximum ∆P.
So before you or your mechanic concludes the Controller is at fault, perform this simple test. Set the controller for, say 5,000 feet, and then fly to 9,000 feet. This will yield a ∆P of about two psid, such a low amount that even the leakiest of airplanes can probably maintain it. Dial in a cabin altitude that’s about 1,000 feet lower. Set the Rate knob at Minimum and see if the cabin descends very slowly, less than 200 fpm or so. Now spin the Rate knob fully clockwise to Maximum and see if the descent increases to well over 1,000 fpm. Dial the cabin back up to 5,000 feet and repeat the Rate control checks for a climbing cabin. So far, so good?
Next, do some airplane maneuvering: Climb 500 to 1,000 feet, then descend an equal or greater amount, while maintaining a reasonably high and constant power setting. Did the cabin obediently maintain 5,000 feet while you did this? If so, then your Controller is fine, doing exactly what it is supposed to do … acting as the governor for cabin altitude. So look for excessive leaks or weak inflow, don’t waste money on exchanging the Controller.
In closing, what, to me, shows that a King Air pilot is really on top of his pressurization system? First, that he is diligent in monitoring the cabin rate of climb and descent and tweaking the Rate knob as needed to get the desired amount … almost always 400 to 500 fpm. Second, that he observes if and when the maximum attainable ∆P is reached … when the cabin starts climbing above the selected value. Third, that before landing, he verifies that ∆P is at zero and the cabin is descending with the airplane.
By the way, did you formulate a plan for landing at the 14,000-foot elevation airport with a 10,000-foot controller? Here’s one method: In the descent, when at about 15,000 feet above the airport, turn off one Bleed Air switch. There should be a momentary cabin climb followed by a recovery back to 10,000 feet. Next, reduce the other side’s Power Lever while monitoring the cabin VVI. As the inflow of bleed air is reduced due to the slower compressor speed, the cabin will start to climb. You can regulate the climb rate – keeping it in the 500 fpm vicinity – by regulating Power Lever position. If you have planned carefully and have a little luck on your side, about the time the cabin reaches 14,500 feet or so, the airplane will be at traffic pattern altitude. Now turn off the remaining Bleed Air switch, use both Power Levers normally, and complete the landing. One other thing! Before the cabin exceeds 12,500 feet, pull the Oxygen Control circuit breaker to prevent the cabin’s oxygen masks from deploying!
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.
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