Ask the Expert: Pressurization Basics

I keep observing a disturbing lack of knowledge and understanding of an aircraft’s pressurization system. Let me try to set the record straight … or at least straighten it out a little bit. I will use the numbers associated with a member of the King Air B200-series. However, what I write, with minor modifications, will apply to any pressurized airplane.

Differential Pressure (∆P, “Delta P”)

Differential Pressure is simply the difference between inside and outside absolute pressures. In engineering parlance, the Greek letter Delta, ∆, is commonly used to indicate the difference between two measurements. So, expressed as a formula, ∆P = PCABIN – PAMBIENT.

If a positive amount of ∆P exists, the airplane is pressurized with more pressure inside than outside … just like a party balloon. The doors and windows are trying to be pushed open and the structures must be strong enough to withstand these forces. This is the reason why pressurized airplanes are heavier than their unpressurized predecessors.

So, like that party balloon, we push more air in than is let out and the airplane becomes pressurized, right? When doing a test in the maintenance run-up area, yes, that is correct. In a great majority of our flights, however, it doesn’t work that way. In most cases, we set the pressurization controller for a cabin altitude that is higher than the field elevation from which we departed, right? And then after takeoff the cabin is climbing to that altitude, right? Well anytime the cabin is climbing it is decreasing its pressure and the fixed-volume cabin is therefore losing air, not gaining air. The pressure inside the cabin is indeed decreasing but why we are getting pressurized is because it is not decreasing as fast as the ambient pressure outside the cabin.

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 (as well as in all of the 300-series), 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 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 seven 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 five ppm, that means that the outflow valve would be positioned by the controller to allow nine 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, however, ∆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.

Inflow

The flow packs attempt to provide constant air mass flow regardless of altitude, outside air temperature, or compressor speed (N₁ or N_g). If compressor speed is too low, however, the flow cannot keep supplying the pounds of air that it should … the air pump isn’t turning fast enough. A quick and unscientific check of your inflow and outflow is this: Can you maintain maximum ∆P with both power levers pulled back far enough to just trigger the landing gear warning horn? If the answer is no, then you can be sure that your air inflow is too low (weak or dead flow pack) or your air outflow is too high (excessive leaks) or a combination of both.

As you reduce power aggressively for a descent –  either to comply with an ATC request or to keep the speed down due to turbulence – you may observe the cabin starting to climb. In fact, I tend to watch the cabin’s vertical velocity indicator (VVI), more than torque or fuel flow, when I reduce power significantly. You may need to push the power levers back up a bit to keep supplying enough inflow to prevent the cabin from ascending. On the other hand, if you need to come down steeper, it’s time for landing gear extension and maybe, if it’s not overly turbulent, approach flaps too. Remember that the maximum allowable load factor limit is reduced when flaps are extended.

Even the relatively small portion of air that is bled from the engine’s compressor for cabin pressurization and heating in a King Air typically causes the engine to run about a 10 to 20 degrees hotter ITT than if the bleed air were shut off and allowed to remain in the engine. That explains why leaving the bleed air valve switches closed sometimes allows more takeoff power to be achieved.

Outflow

The pressurization control system is made by Honeywell Aerospace. Honeywell is the name that has survived from a long line of company acquisitions and mergers. The control system we use evolved from the very first installations used on B-29s in the latter days of World War II. The company that designed and manufactured that system was Garrett AiResearch. So even today, most of us say it is an AiResearch control system.

The system is mechanical, using springs and vacuum. Electricity plays a minor role. In the King Air, the system uses electric power primarily for Dumping: Opening a normally-closed solenoid valve that permits vacuum to suck open the Safety Valve and thereby create an opening (hole) so large that cabin pressure quickly equalizes with ambient pressure. In fact, the reason that a total loss of electric power in flight always leads to a lack of pressurization is not because the control system fails.
No, it is because the inflow of air ceases. (Electric power is needed to keep the flow packs open.)

Somewhat surprisingly, since it is rather complex, the AiResearch control system is quite reliable. The problem with an airplane that cannot maintain the cabin altitude selected is very rarely due to a bad controller. Instead, it almost always is caused by too little inflow or too much outflow or a combination.

Troubleshooting Pressurization Problems

You have discovered that your pressurization is not working as it should. For example, you cannot reach 6.4 – 6.6 psid ∆P, or you see the cabin starting to climb even though the power levers have been only slightly reduced. How can you find what’s wrong? How can you help your mechanic reduce his troubleshooting time? Here are some ideas that pilots can do in flight. Mechanics have their own and, sometimes, more accurate procedures to use.

First, you can make sure the controller is functioning properly in this manner: In level flight, set the controller’s cabin altitude for 3,000 or 4,000 feet below you. For example, fly at 10,500 feet with the cabin set for 7,000 feet. Now zoom up to 11,500 and then dive down to 9,500 without changing engine power. Does the cabin stay level as it should? Next, back in level flight, dial the cabin up to, say, 9,000 feet. Does it start climbing? Twist the rate knob to the minimum setting. Does the cabin rate of climb decrease to almost nothing? Now spin the rate knob to maximum. Does the cabin climb like a homesick angel? Next, dial the cabin down to a lower altitude and check the rate control again as it descends. In almost all cases, you will find that the controller is working perfectly. As I wrote above, it is a surprisingly robust piece of gear. By doing this test with a small difference between airplane and cabin altitude, ∆P is very low and thus the effect of excessive leaks or weak inflow will also be low.

Second, on a deadhead leg – so that passengers’ ears will not be subjected to uncomfortable pressure fluctuations – force ∆P to the maximum attainable by dialing the cabin altitude down to sea level while you are up high, typically above FL180. When the cabin stops descending, note the indicated ∆P. (Write it down or, better yet, take a picture.) You have forced ∆P to its maximum attainable value and if it is not within 0.1 psid of the ∆P gauge’s redline, then you have identified a problem.

Move the left bleed air valve switch to the center, Envir Off, position. (It doesn’t matter which side you do first, but we’ll start with the left.) Take a video of the cabin VVI while you do this or at least note and record the peak cabin climb that takes place. Maybe it hits a peak, say, of 1,600 fpm. What should next happen is that the cabin will stop its climb, go into a descent, and return to the exact altitude where it began. The King Air should be able to maintain maximum ∆P even with only one flow pack supplying air. Can yours do that? It is not at all uncommon to find the cabin will not descend back to where it started. Let’s assume that is what we see here … the cabin does not recover back to its starting altitude but keeps climbing at an ever-decreasing rate. This means either the still-operating flow pack is weak – lack of inflow – or the leaks are excessive – too much outflow – or a combination of both. When we finish this little test, we will know what the problem is.

Turn the left bleed air valve switch back on and give plenty of time for the situation to return to normal operation, with the cabin altitude and ∆P the same as they were when you began the test. An occasional flow pack is balky to reopen. Give it time. You will know it reopens when the cabin VVI shows a downward surge. ITT will also increase a little and torque will decrease a little.

Once everything is the same as it was initially, switch off the right side’s environmental bleed air and record or film those results. Let’s suppose that this time the peak cabin climb is 600 fpm and the cabin quickly reverses the climb and descends back to the original altitude. Before reading further, take a moment to think about these results and see if you can determine why there is a difference.

Tick-tock-tick-tock-tick-tock. Ok, got your answer?

The answer is that the right flow pack is much weaker than the left. We lost less air when we turned off the right pack and it, when operating alone, was not strong enough to overcome the cabin’s leaks. Yet we lost a lot of air when we terminated the left pack’s flow and it overcame the leaks just fine and was able to maintain full pressurization when operating by itself.

But even one or two strong flow packs may not be able to supply enough air to overcome massive leaks. So how do we complete this test and determine how badly your airplane leaks?

We start by turning the right pack back on and giving plenty of time for things to return to normal, at the maximum attainable ∆P. Now we turn both bleed air switches off simultaneously. (If you have three-position switches – as all of the 200- and 300-series do – make sure you go only to the center, not bottom position. You don’t want to lose the inflation pressure for the door seal.) Observe the peak on the cabin VVI.

If it is less than 2,500 fpm, then you have an airplane that meets Beech’s specifications. Congratulations! Sadly, a leak rate this low is exceedingly rare to find. You have a one-in-a-thousand, exceedingly tight airplane. More typically, you will see a leak rate of 3,500 to 5,000 fpm. Realize this, too: If the combination of weak inflow and excessive outflow prevents your airplane from attaining the proper maximum ∆P of 6.4 – 6.6 psid, then this check will not be valid since you have not attained the “push” that would exist if you could get to the proper maximum ∆P. To better explain: If you can only get 5.0 psid maximum and the peak leak rate at that ∆P is 4,000 fpm, perhaps it would be 5,500 fpm at 6.5 psid.

My personal criteria for deciding that a King Air’s pressurization system is satisfactory looks at two things: First, can either side’s flow pack alone maintain full ∆P when at cruise power? Second, can I pull both power levers back to the gear horn’s setting, with both flow packs operating, and not have the cabin start to climb? If both of these are true, then I see no reason to spend money and time on overhauling flow packs and/or finding and sealing cabin leaks.

I hope this presentation of basic rules of pressurization will help you better understand your system and troubleshoot problems when they arise.