Troubleshooting Tips for King Air AC

Troubleshooting Tips  for King Air AC

Troubleshooting Tips for King Air AC

How’s the air conditioning (AC) in your King Air right now? This time of year, my phone is burning up with air conditioning problems due to everything from low Freon to the dreaded Black Death. If your AC is acting up a little or not working at all, there are things an owner/operator can check before calling the shop. The more info you can give your mechanic, the more efficiently they can fix the problem.

Please don’t mistake my intentions here; this is not a replacement for your POH, nor is it official training of any kind. I’m just offering some troubleshooting tips for the more typical problems found in King Air air conditioning systems.

Low on Freon

The most common problem with weak AC is low Freon. You need a shop with the proper equipment to measure how much is in your system and whether it needs more Freon added or taken away. Over-servicing Freon is as bad as not having enough. Make sure you know whether your system takes R12 or R134a. Many King Airs built with an R12 system have been converted to R134a and there should be a corresponding STC in the POH. The Maintenance Manual or the STC will specify the exact amount of Freon your system takes. A pound over or under will greatly affect performance.

If the system is empty or very low, your shop needs a sniffer to find where the Freon is leaking. Such leaks need to be fixed. If the system is down just a little, it’s probably the normal seepage inherent in most AC systems – rarely are they completely airtight.

The balance of this article applies to the 200/300/350 series King Airs. The AC system in these models has more components and therefore more places where the system can break down. My apologies to the
King Air 90/100 drivers, I’ll catch you on the next one.

Belt and Quill Shaft

Troubleshooting is like peeling an onion layer by layer. The first layer is the belt and quill shaft – an easy check. Open the righthand engine aft inboard cowl. To do this you may need a screwdriver – model 200s have two camlocks on the bottom just forward of the front latch, these camlocks could require a common or Phillips screwdriver; 300s have three screws on the cowl door; 350s have latches only.

The compressor is right there so look for the belt. If the belt is shredded or missing, that’s the first thing to fix. The belt might be the only issue, or it could mean the compressor is frozen. If the belt is intact, try to move it back and forth. It should only move about one-fourth inch and the clanking noise you hear is normal. If the belt moves freely without clanking, the quill shaft has sheared and must be replaced. If the belt and quill shaft are good, then you must cut into the next layer of that onion.

N1 Low Light

In the cockpit, turn the environment switch to “Auto” (you can do this without the engines running) and look for the green “Air Cond N1 Low” light on the annunciator panel. If you don’t see it, turn the environment switch to “Manual Cool” and look again for the N1 Low light. If you still don’t observe it, toggle the Increase/Decrease switch to the “Decrease” position for at least 90 seconds and look again for that N1 Low light. If you finally get the light at this stage it means you may have a temp sensor problem – the system thinks it is already cold enough, but the sweat dripping from your brow indicates otherwise. If you get the N1 Low light right off the bat in “Auto” or in “Manual Cool,” it means your AC problems are elsewhere and you must delve deeper still.

The N1 Low Light on the annunciator panel is helpful when troubleshooting air conditioning issues on your King Air.

Reset Switch – Nose Gear Wheel Well

Go to the AC reset switch in the nose wheel well on the left-hand side. A yellow light in the switch indicates the AC has tripped, so if you see the light you should press the switch to reset the AC system; the light should go out. If there is no light, press the reset switch anyway. Many times, the switch has tripped, but the bulb is burned out so you have no yellow light.

After pressing the reset switch, you need to get back in the cockpit, turn everything off and fire up the right-hand engine, bringing it up to approximately 64% N1. Once your load meter indication goes below 40%, turn the AC to “Auto” and keep your eye on that load meter. Within 7-15 seconds you should see a spike in the load meter telling you that the condenser blower just turned on. If you do not see a spike, turn the AC to “Manual Cool” and look again for a load meter spike after 7-15 seconds. If the AC will not turn on in “Auto” or “Manual,” then most likely one of the pressure switches is bad or there is no power going to the system’s printed circuit board (PCB).

If you get the load meter spike in “Auto” or “Manual,” run the engine for 10-12 minutes then shut it down; get out of the cockpit and check the reset switch in the nose wheel well again. If that yellow light is on again you now know your system is tripping on the low-pressure side of the system. You are likely low on Freon. If you never saw a load meter spike then the condenser blower never came on; most likely the blower is bad causing the system to trip on the high-pressure side.

No Yellow Light

The AC reset switch is handy for troubleshooting when it is working, but the yellow lamp inside the switch was not designed to survive in an environment like the nose wheel well, so they tend to burn out easily. As mentioned, if there is no yellow light, press the reset button anyway to see if the AC will come back on. If it does, then that yellow light needs to be replaced. If the AC goes offline after a few minutes, then you have other problems to be addressed that are difficult to troubleshoot in the field.

Reset Switch – Righthand Wing Center Section

In older King Airs (think straight 200s) the AC reset switch is in the righthand center section of the wing inside a six-inch by six-inch panel with three camlock screws. Remove the panel and look in and toward the outboard side where you will see a printed circuit board with two red lights; one is labeled “Low” and the other “High.” If either red light is on, make a note which one it is so you can tell your shop and they will know where to start. You will also see a little red push-button switch which will turn either red light off.

Sometimes, pushing the air conditioning reset switch in the nose wheel well is all that is needed to get the air conditioning running again.

I miss these reset switches in the wing. It is a simple, straightforward system that saves a lot of time and money searching for the AC system problem. I’m consulting on a 200 right now with this configuration. The AC was inoperative for a of couple of years – there were several problems at play – finally the mess is being unraveled.

50 Degrees Fahrenheit Threshold

Don’t forget that the AC will not turn on if the outside air temp is below 50 degrees Fahrenheit. If you depart a hot climate with everyone in the aircraft melting from the heat, then you land in a cooler climate where a front just passed through and it’s 48 degrees Fahrenheit outside, you’ll have no luck with troubleshooting your AC problem. The OAT needs to be above 50 degrees Fahrenheit.


Let’s review: Try the AC in “Auto” first; if it doesn’t work, go to “Manual” and toggle the Increase/Decrease switch. If the AC will work in “Manual” then the system itself is fine and the only thing that needs attention is Auto Mode.

If your belt and quill shaft are intact but the AC isn’t working, you may have issues with a pressure switch or the power supply to the PCB.

If your reset switch is working and tells you the system is tripping on the high-pressure side, the shop knows to look at the condenser blower. If the reset switch indicates it is tripping on the low-pressure side, the shop knows to check the Freon level.

If you cannot get an “N1 Low” light without toggling the Increase/Decrease switch, you most likely have a temp sensor issue.

Remember my goal here is to help you jump-start the repair on your air conditioning system. Your technician needs a pilot’s perspective on the problem. The more information you offer about what your system is doing or not doing, the more quickly your shop can zero in on the crux of the matter, saving both time and money.

Stay healthy, fly safely and keep cool.

Note: This article was adapted from one in the April 2012 issue. New King Air owners have requested I revisit some of my earlier topics.

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