Page 21 - March24

Here’s an example: Let’s say we depart from sea level and the cabin climbs to 10,000 feet while the airplane climbs to 25,000 feet. PCABIN goes from 14.7 psia (SL) to 10.1 psia (10,000 feet) but PAMBIENT goes from 14.7 psia (SL) to 5.5 psia (25,000 feet). So ∆P went from 0 psid (14.7 – 14.7) to 4.6 psid (10.1 – 5.5).
In a King Air B200 the maximum certified ∆P is 6.5 psid (pounds per square inch differential). As in everything that is mechanical in nature, there must be some tolerance and the allowable tolerance in maximum ∆P is plus or minus 0.1 psid. In other words, when running on the maximum ∆P relief, any ∆P between 6.4 and 6.6 means that your King Air is doing what it was designed to do. Of course, Beechcraft marketeers, seeing that the maximum is 6.6, were quick to put that figure in the sales brochures.
The Pressurization Controller
The purpose of the pressurization controller is merely to be a governor of cabin altitude. Within its capabilities it will make the cabin climb or descend to a newly selected cabin altitude value at the rate the rate knob is set for and then keep the cabin at that altitude the best it can. Just like a propeller governor cannot always maintain the selected RPM – for example, propeller speed decreases on landing as the governor causes the blades to flatten as far as they can go – likewise the pressurization controller cannot always maintain the selected cabin altitude. Two things will prevent this: First, the cabin can never be higher than the airplane. That would cause a negative differential pressure – ∆P would be a negative number since PCABIN is less than PAMBIENT – and negative ∆P is prevented by dedicated relief valve portions contained identically within both the outflow and safety valves. Second, the cabin cannot maintain the selected altitude if doing so would cause maximum attainable ∆P to be exceeded. That “maximum attainable ∆P” is often not the maximum certified ∆P, as I will explain.
To maintain the cabin at any selected altitude, all that must occur is for total air mass inflow to equal total air mass outflow. In the B200, as in most all pressurized airplanes, the incoming flow is regulated to be as constant as possible, and all control of cabin altitude and rates of climb and descent are accomplished by varying the outflow through the outflow valve. Of course, what exits through the outflow valve is not the total outflow ... we must consider the contributions of all the little and big leaks. Here’s where the conceptualization gets tricky. How much mass flow exits through the leaks depends upon ∆P. If there is a low ∆P, then the “push” that causes air to flow through the leak hole is small and hence the flow is small. But when ∆P is large, then the mass flow across the leak is also large, even though the leak size has not changed.
Let me apply some numbers to an example. Suppose that both the left and right inflow systems – the Bleed
Air Flow Control Packages or Flow Packs – were pumping in 7 pounds per minute (ppm) of air, for a total of 14 ppm. To keep the cabin from climbing or descending, a total outflow of 14 ppm must be taking place. If, at 6.5 psid, the leaks accounted for a total of 5 ppm, that means that the Outflow valve would be positioned by the controller to allow 9 ppm to escape (14 ppm in, 5 + 9 ppm out) ... we’re in balance and the cabin is holding its altitude, maintaining a constant cabin pressure.
Now let’s make the leaks add up to 20 ppm at 6.5 psid. (Don’t ask me how we got to 6.5, because we won’t be staying there, as you’ll see.) Since now, even with the outflow valve totally closed, there is more air exiting (20) than entering (14) a net loss of cabin air is taking place and the cabin must be losing air molecules, losing pressure and hence climbing. As the cabin climbs while the airplane flies level, ∆P is decreasing and hence the mass flow through the leaks is also decreasing. As the cabin goes up and ∆P goes down, eventually a perfect balance will be reached, wherein the leaks total 14 ppm, equal to the inflow. At that point, the cabin stops climbing. But now you see two common but incorrect indications: First, the cabin is higher than the altitude you’ve dialed into the controller, and second, your maximum attainable ∆P is well below the correct 6.5 psid value.
6 26 18
MARCH 2024
KING AIR MAGAZINE • 19