Ask the Expert: Pressurization System Abnormalities

One of our readers suggested that I discuss some of the common failures that affect the King Air’s pressurization system. I am grateful for his idea and this article will address those abnormalities.

Let me begin, however, by stating that the great majority of pressurization problems in King Airs are not the ones I will be presenting now! Lack of sufficient inflow – a weak flow pack or two, combined with excessive outflow, too many cabin leaks … these are the causes of the great majority of pressurization problems. I have said it before and I’ll say it again: I would suggest that the most common, almost universal, weakness in King Airs is that they have leak rates well in excess of what Beech specifies. And you know what? I don’t care! To find a leak rate – peak cabin climb rate when the inflow is stopped while at maximum differential pressure, ∆P – below 2,500 fpm is quite rare, although that’s the specification Beech tells us is correct. But if I see 4,000 fpm or even more, I can happily live with that if (1) I can maintain maximum ∆P on one flow pack alone at cruise power, and (2) that I can reduce power back to the gear horn warning, with both flow packs operating, without the cabin starting to climb (due, of course, to less inflow than outflow).

No, this time we are discussing problems that may exist even with a wonderfully tight pressure vessel and with strong flow packs.

First, the basics: Remember that both the outflow and the safety valves are spring-loaded to the closed position and require vacuum to suck them open. Suppose that the line going to the throat of the bleed air ejector – the vacuum source – disconnects so that we have no vacuum. What will happen?

Well, when the Bleed Air switches are turned on, air flows into an essentially closed container – the pressure vessel, cockpit and cabin. With inflow taking place and no outflow happening (except for the leaks), the cabin is gaining air mass, pressure is increasing, and altitude is decreasing. With high power on the engines – such as at takeoff – it will be normal to see the cabin’s rate-of-climb gauge hit the peg at -6,000 fpm! Whew, that gets your, and your ears’, attention!

So before the cabin dives down too far, we’ll grab the Cabin Pressure Control switch and move it forward to Dump, right? Well, sure, feel free to do that. However, in this scenario, it won’t work. Why? Because we have no vacuum, no force to suck the Safety Valve – the dump valve – open.

If we cannot control the outflow, we need to stop the inflow. The solution to this runaway cabin dive is to turn off both Bleed Air Valve switches. With the inflow now stopped, the cabin will continue to leak air out until we become unpressurized as the cabin rises to the airplane’s altitude. This could easily take in excess of 20 minutes.

If there are operators of straight 90, A90, or B90 models reading this, loss of vacuum affects your airplanes differently than what we have discussed which applies to the airplanes with flow packs. Your Flow Control Valve in the left wing center section – that regulates the flow from the supercharger by sending the proper mass flow into the cabin and dumping the rest overboard in the wing – needs vacuum to keep from dumping all of the supercharger’s air overboard. Hence, a total loss of vacuum – the hose disconnecting from the bleed air ejector’s throat – does not lead to a runaway cabin dive, but instead leads to a cabin climb. This is due to the fact that the Flow Control Valve stops sending any air into the pressure vessel, combined with the unavoidable pressure vessel leaks that exist.

The next malfunction I will discuss is failure of the Preset Solenoid Valve. This is the valve (not installed on B90 and earlier models) that goes from its Normally-Open (N.O.) state to a closed condition when we are on the ground and/or when we move the Pressurization Control switch to Dump. The purpose of this valve – as its name indicates – is to allow the pilot to preset the pressurization controller for the desired cabin altitude prior to takeoff. Most of us do this during our Before Takeoff procedure. Yet, with the Preset Solenoid preventing suction from getting to the Controller until after we lift off, it cannot actually start sending the cabin to the desired altitude until we leave the ground.

For example, let’s say we are departing from a sea level airport and have the desired cabin altitude for cruise set for 7,000 feet. After a couple of initial surges, we should observe the cabin climbing obediently toward 7,000 when we check the gauges after takeoff. If the rate of climb is too fast or too slow, we adjust the Controller’s Rate knob appropriately.

If the Preset Solenoid had a loose connection and did not energize closed on the ground as it should, the result after takeoff would be the cabin climbing right along with the airplane: Same climb rate; zero ∆P. Oops. After consulting the checklist and verifying that the Bleed Air Valve switches are on and that the Pressurization Control switch is not in the Dump position, we have just about decided that we’ll need to return to the departure airport to have our mechanic find the problem when – Voila! – the cabin stops climbing, ∆P starts increasing, and all is well once again.

What happened here is that the Controller began trying to climb the cabin to the selected altitude as soon as we set it during our Before Takeoff procedure. Since the actual cabin altitude may never be above the airplane’s altitude – that would represent a negative ∆P value, which is impossible to have – what I call a “Phantom Cabin” exists somewhere above us, that the Controller is trying to reach. Only when we climb above this Phantom Cabin, do things start working 100% normally. Unless you took off quite quickly following your setting of the Controller, in most cases you will fly all the way up to the selected cabin altitude – 7,000 feet, in our previous example – before the cabin stops its unpressurized climb and levels off.

Until the loose connection or bad solenoid valve gets repaired, this problem is relatively easy to abide: Simply leave the Controller where it had been for your landing at this airport and don’t dial in the new cabin altitude for cruise until after departure. This is exactly what the 90, A90, and B90 pilots must do routinely, since those models have no Preset Solenoid.

If the Preset Solenoid valve can fail open as we have just discussed – the more likely scenario since that is its de-energized state – then it can also fail the other way, stuck in the closed position, failing to open after liftoff. Now what?

The closed valve prevents suction from reaching the Controller so it, in turn, cannot regulate that suction and modulate the Outflow Valve with it. Hence, the Outflow Valve remains in its normally-closed position and we again have the runaway cabin dive after liftoff … just as we did with no vacuum at all. The only difference between these two situations is that now the Dump switch will work. So we have two methods for depressurization: Dump – allowing vacuum to suck the Safety Valve wide open – or turn off the Bleed Air Valve switches and allow the cabin leak rate to gradually depressurize the airplane.

The next malfunction I will discuss is the Dump Solenoid valve itself. Like the Preset Solenoid valve, this receives power when the airplane is on the ground but is unpowered after liftoff. However, it is the opposite type of valve as compared to its Preset cousin: Normally-Closed (N.C.), and using electric power to go open.

If a wire to this valve comes loose in flight, we will observe nothing amiss. It remains in its de-energized, closed state. Only if we chose to move the Control switch to Dump – perhaps responding to heavy smoke in the cabin – would we find anything unusual when no dumping took place.

Ah, but after we land…now there is an interesting and potentially dangerous situation!

On the ground, both the Preset and the Dump solenoid valves should be energized, each going to their proper positions: The Preset Solenoid going closed, preventing any vacuum from reaching the Controller, and the Dump Solenoid going open, allowing vacuum to reach the Safety Valve and suck it wide open.

If the Dump Solenoid has a loose wire and does not energize like it should, then neither the Outflow nor the Safety Valve can receive vacuum and they both go to their spring-loaded, closed position. If the Bleed Air Valve switches are on, air is now flowing into an essentially closed box and we will begin gaining air mass in the pressure vessel, meaning that cabin pressure is going up and cabin altitude is, consequently, going down. Although this is an unregulated cabin dive, it is not as severe nor as noticeable as the runaway ∆P after takeoff, since the engines are at Idle and not much air is being supplied by the Flow Packs. More than one crew has taxied in after landing with the cabin slowly re-pressurizing yet did not notice it due to the rather gradual rate of cabin descent.

The shutdown is completed, someone steps back to open the cabin door, wonders why the release button is a little more stiff than normal, pushes it harder, rotates the handle, and finds himself sailing through the air onto the hard tarmac, having been unceremoniously ejected from the pressurized cabin. Realize that even 0.5 psid will cause the door to experience about 700 pounds of opening force!

Although preventing the cabin from re-pressurizing as we taxi in is not the primary reason for turning off the Bleed Air switches prior to shutdown, it is one more advantage of doing so. Some pilots make a habit of always opening their cockpit vent window prior to heading back to open the cabin door, to know for sure that no ∆P remains. That may be a little excessive, but it surely isn’t a bad idea! Another method is simply to check the ∆P gauge and make sure it’s showing 0 psid before anyone operates the door.

The next pressurization abnormality I will mention is one that I’ll wager a lot of you have experienced. It’s not dangerous, just irritating, and somewhat uncomfortable. This phenomenon is almost exclusively experienced in the E90- and F90-series, later members of the C90-series, as well as in the 100-series.

There you are, enjoying a satisfyingly high ground speed as you descend near the barber pole redline airspeed on this smooth flying day. Then, boom! What was that?! Why is the cabin showing a dive of 2,000 fpm or more! Even after things stabilize, the cabin is still descending rapidly and the rate knob on the Controller has no effect. Dang!

You just joined the “I Blew Open the Ram Air Door” club. The ram air door that allows outside air to get into the pressure vessel – via the air conditioner’s evaporator plenum beneath the avionics bay on the left side of the nose wheel well – is normally prevented from opening by three things. First, there is a spring. Second, there is an electro-magnet helping keep it closed. Third, in most cases, there is enough ∆P to also force it closed. Yet when the airspeed is near redline – and the F90, with its higher VMO is the most notorious offender here – while ∆P is close to zero, the ram air force overcomes the spring and the magnet. That sudden inrush of outside air surely does dive the cabin!

Although you could avoid this by flying slower, who wants to do that on these rare smooth-air days?! Instead, you must make sure that ∆P doesn’t get too low – like below 1 psid – while zooming along near redline. How do you do this? By keeping an eye on the pressurization indicators during your descent and, specifically, making sure you are using sufficient cabin rate of descent so that the airplane’s altitude is not catching up to the cabin’s altitude. It’s having the airplane “catch the cabin” that is causing the problem.

It is tempting to use too low of a cabin rate-of-descent in an attempt to treat the passengers’ ears as gently as possible. That’s a great goal, but we must not overdo it. Unless you use a 400 to 500 fpm rate of cabin descent, there is an excellent chance that you may experience the irritating situation that we are discussing. There are exceptions to every rule – i.e., if you are landing at Aspen or Lake Tahoe, the cabin has so little altitude to lose that a low cabin rate-of-descent will probably work out fine – but sticking with the 400 to 500 fpm rate is almost always safe and not problematic for most passengers’ ears.

I am sure you have noticed also that to achieve a 500 fpm cabin climb and a 500 fpm cabin descent, it is the rare Controller in which the rate knob can remain in the same position! Invariably, a higher setting – maybe 1:00 o’clock – will be needed to get the climb but a lower setting – maybe 11:00 o’clock – will be needed for the descent. Sorry to increase your workload, but it is a fact of King Air life.

Have you noticed the little access panel in the upholstery on the right side of the baggage compartment down just above the floor? Know what’s inside? Well, looky there, it’s a petcock drain of some sort, just like the static line drains behind the right side upholstery in the cockpit.

But this is not a static line drain. Instead, it is a drain at the low spot of the line between the Controller in the cockpit and the Outflow Valve on the back wall of the baggage compartment. If it is inadvertently left open, then all of the carefully-regulated vacuum that the Controller is using to modulate the Outflow Valve gets overwhelmed by cabin pressure leaking into the line. The controller loses its ability to work the Outflow valve correctly. Although the most typical result is runaway ∆P to maximum – turn off the Flow Packs when you are ready to depressurize for landing – we have also heard of cases in which cabin altitude merely “stuck” at one value and could not be raised or lowered.

I will close with this reminder: Both the Outflow and the Safety valves have self-contained Maximum Differential Pressure and Negative Differential Pressure relief functions. It is almost impossible for these functions to fail so long as the valve is installed in the aft pressure bulkhead correctly. That is the reason this discussion of abnormalities has not mentioned what to do if you exceed maximum ∆P…it just won’t happen.

To repeat what I wrote at the start: The malfunctions I have reviewed here, although possible, are quite rare. But being unable to reach maximum ∆P because of lack of sufficient inflow and too much outflow – leaks – that’s where the bulk of pressurization problems lie.