Yesterday I got a call from a mechanic working on a pressurization problem with a King Air C90. The pilot’s squawk was that the altitude warning light came on while at cruise; the pilot descended. The mechanic did not know if the cabin dumped all at once, but the way he told it, it didn’t sound that way. (I know this mechanic; it probably didn’t occur to him to ask the pilot.)
By the time he called me, this mechanic had already changed the pressurization controller, the safety valve and the solenoid valve. In other words, he was just throwing parts at the problem – very expensive parts, I might add. This guy had squandered $10,000 to $15,000 of the customer’s money with no fix in sight. This infuriates me and it’s not my airplane!
Confusion
I am confused as to why so many pilots and mechanics go straight to the controller, and then the outflow and safety valves when the pressurization system goes awry. Why is this?
In 45-plus years working with King Airs, I have seen maybe six pressurization controllers that needed to be changed. Most of them had suffered external damage such as a lens getting broken after something was dropped on it or coffee got spilled all over. Pressurization controllers in King Airs rarely fail. The same holds true with the outflow and safety valves. These are at the bottom of my pressurization troubleshooting list.
Flow Packs & Cabin Leaks
The vast majority of pressurization problems (95% or more) stem from one or both flow packs becoming weak and/or leaks in the cabin. Usually, it is a combination of both. You can carve that in stone.
The C90 this guy called me about is owned by an air ambulance operation and has over 9,000 hours. In my experience these medevac birds are run hard and put away wet – that’s horse talk for not properly cared for. The first place I would check is the ducting in the nose.
Crossover Duct
All King Air 90 models have a weak link: the pressurization air goes from the cabin to the nose; goes through the A/C evaporator; then goes through a crossover duct to the heater on the other side; and finally back into the cabin. After 9,000 hours these ducts get old and brittle, they split and leaks develop.
Other King Air models have a version of this crossover duct but it is plumbed differently. The 90 has the most problems with this duct, so if you’ve got a 90, put it at the top of the list when troubleshooting pressurization.
Pressurization Review
Twelve years ago, my “Maintenance Tip” for this magazine was on pressurization. It appeared in the September/October issue in 2010. Not only does it apply to the mechanic that phoned me yesterday, it also applies to a post I saw today in an online forum by a King Air owner. He was looking for recommendations on where to send his pressurization controller for repair or overhaul. My good friend and colleague, Tom Clements (who also writes for this publication), gently suggested that the controller might not be the source of his pressurization problem.
So, let’s review.
The Ins and Outs of Pressurization
Imagine this scenario: You are nearing your destination airport so you adjust the pressurization controller for the field elevation where you will be landing; you begin your descent and pull the engines back; you notice the cabin rate of climb going up instead of down. This scenario is the most common symptom of a weak pressurization system. The reduction in engine RPM on descent puts less air into the cabin via the flow packs. If you have a weak flow pack, a high cabin leak rate or both, you will experience this scenario. Further, as the cabin altitude continues to rise during descent, the cabin will dump (depressurize) as soon it converges with the aircraft altitude.
Most pilots suspect that the pressurization controller or the outflow valve went bad. But in all my years of working on King Airs I found the most common culprits to be a weak or bad flow pack, an excessive cabin leak rate or both. It is a waste of time and money to change controllers or valves without thoroughly troubleshooting the pressurization system.
Inflow and Outflow
Properly functioning pressurization achieves a balance between the inflow of air pumped into the cabin by the flow packs and the outflow of air through the outflow valve. A weak or dead flow pack can’t pump enough air into the cabin. An excessive leak rate allows too much air out at one time. The trick is figuring out if your problem is inflow (packs), or outflow (leaks) or both.
Flow Packs
Most King Airs have flow packs, except for the straight 90, the A90 and the B90 which have a supercharger for pressurization. All others, from the C90 going forward, have flow packs for pressurization. Pneumatic flow packs are found on older King Airs and electronic packs are found on newer models. Pneumatic packs have filters and orifices that become clogged, thus weakening performance. Pneumatic packs can be cleaned and recalibrated or completely overhauled to restore functionality. I have a great vendor for this. I always get excellent results with pneumatic packs after overhaul.
However, the scenario with a rising cabin altitude during descent was a fairly common complaint among King Air owners. It prompted Beech to switch to electronic flow packs which are not susceptible to clogging. Unfortunately they are a great deal more expensive to repair or replace in the event of failure.
Testing Flow Pack Efficiency
To test the efficiency of your flow packs, operate them one at a time while observing the cabin rate of climb. Start with both packs going. To test the left pack, turn the right pack off and watch the cabin climb rate; it should go up briefly while the left pack adjusts to the change in flow, then it should come back down.
A strong flow pack can pressurize the cabin by itself. If the cabin climb rate goes up to 500 feet/minute, then that left pack is getting weak. Turn the right pack back on, allow for adjustment of flow, then turn the left pack off to test the right pack. If the cabin climb rate were to go to 1,000 feet/minute or more on such a test, you may have a dead pack and/or you have an excessive cabin leak rate that is throwing off the true results of this flow pack test … more on that later.
Tip: Should you ever see a sudden split in the ITT gauges for no apparent reason, this could signal a flow pack failure. The cooler engine has the dead pack.
Identifying the Cabin Leak Rate
In troubleshooting a pressurization problem, identifying the cabin leak rate is essential. A leak rate of 2,500 feet/minute is considered ideal. The factory shoots for this on newly built aircraft. On a test flight, you want to get the cabin to maximum differential as shown on your cabin differential/cabin altitude gauge, OR you want to get the cabin differential gauge as high as it will go without triggering the cabin altitude warning. Turn off both packs at this point. Observe the cabin rate of climb – that is your cabin leak rate.
If you cannot bring your King Air to maximum differential then you already know you have a major pressurization problem, but which one? Flow pack(s)? Cabin leaks? Both?
Even if you can bring your King Air to max differential, you should still test your cabin leak rate. I find the electronic flow packs can camouflage a high leak rate. If this is the case in your King Air, and you suddenly have a flow pack failure, the remaining pack would not be able to sustain the cabin pressurization against the leaking cabin. Your cabin altitude would suddenly go up and you would have to take appropriate measures very quickly.
Finding Cabin Leaks
The only way to find leaks in the pressure vessel is with a huffer (a pressurization cart). Without one you are pouring money down the drain. You could spend a week sealing antennas and drains and barely gain a negligible improvement in the leak rate. With a huffer, the leaks are identified very quickly; then you can choose which leaks are the most cost-effective to fix.
Example: A B200 with electronic flow packs had a leak rate of 5,800 feet/minute. One pack was bad and was exchanged with an overhauled unit. Next, using the huffer, I found leaks at a number of screw holes with no screws. They were hidden beneath farings and I never would have found them without a huffer. I sealed those, plus a few more leaks that weren’t too hard to reach. Lo and behold, the leak rate came down to 3,100 feet/minute. There were a few leaks I did not fix, such as the air conditioning plenum drain, because of the major disassembly required to gain access. It would have cost a fortune for minimal gain.
Summary
If you can’t make max differential, you probably have inflow and outflow problems. I usually address the flow packs first to eliminate that variable. Then I go after cabin leaks with a huffer and I choose my battles wisely.
Caution: This is by no means a comprehensive discussion of King Air pressurization. There is much more to it. My goal here is to give you, the owner-operator, a glimpse into how your maintenance technician would approach such a problem so you can assist him/her effectively and keep your maintenance costs under control.
A Little More…
Do you have a pressurization problem? If so, chances are it’s NOT the controller. If you take away that much from this, then I’ve accomplished my mission.
If, however, you are wanting a little more information, please see my article in the December 2018 issue of this magazine, “Looking at Pressurization.” It digs a little deeper into the subject and has a few more tips. That article was inspired by a shop trying to pinpoint cabin leaks with a household vacuum cleaner. (I’m not joking.)
If you can’t find it online, please email me. I’ll be happy to send you the pdf file. Most importantly, to anyone out there with pressurization questions or any others, I’d love to hear them and see if I can help. As always, I wish you many hours of enjoyment in your King Air.