Maintenance Tip: Beware of Black Death

During the hot summer months, nearly everyone is running their air conditioner, and it would be an awful thing for it to stop working. Black Death could be the culprit and it isn’t what you want to happen to your King Air. Black Death is a sludgy disgusting goo that gums up the air conditioning compressor, condenser, evaporator and everything else in the system. Essentially, it destroys all the components. Everything must be replaced and the lines flushed clean – this is a miserable and expensive process.

On King Air 200s and 300s, the air conditioner (AC) compressor is on the R/H engine and the plumbing goes through the wing root, so the R/H leading edge must come off. It’s a nightmare.

The crux of the problem is moisture in the system. AC systems are sealed to keep moisture out. When it is opened to replace a switch or a component, a vacuum pump is used to remove all the air, and the moisture in it, before recharging the system with Freon. Some think Black Death only affects R134a systems, but it can bring down an R12 system too.

Freon and Moisture

Freon becomes acidic when mixed with moisture, and the acid corrodes the aluminum in the condenser, the evaporator and certain parts of the compressor. Also, in aircraft AC systems, the lines (tubing) are aluminum. Think of the black residue on your rag after polishing the aluminum wheels on your car or the aluminum spinners on your King Air. That’s the “black” in Black Death. The AC system is being eaten away from the inside out. Corroded aluminum mixes with the Freon and oil in the system and this mixture is subjected to extremes of temperature. The result is a gooey black sludge that eventually chokes the system to death, quite literally.

R12 and R134a

Dichlorodiflouromethane (R12) boils at -21.6º F; Tetrafluoroethane (R134a) boils at the warmer temperature of -15.34º F. This is why R134a doesn’t cool as well as R12. Another difference is that R134a is slightly acidic to begin with, and adding moisture to it increases its acidity. This could be why Black Death rears its ugly head more frequently in R134a systems. That, and the fact that there are fewer R12 systems that exist these days.

I was never a fan of converting R12 systems to R134a because R12 cools so much better. The R134a systems in cars and airplanes today, however, are greatly improved over the anemic systems of 20 years ago. As long as I could procure R12 – good R12 – I kept servicing those systems. R12, although pricey, continues to be available today, but you must search for a reputable supplier.

Freon – the Good, the Bad and the Ugly

Ten years ago, I had a customer who owned a couple of King Airs with R12 systems. He chafed at the cost of R12, so he brought me his own cylinder. I knew that he flew frequently to Mexico and was worried his R12 was purchased there, so I was careful not to mix his Freon with my own. In refrigerant circles, Freon from Mexico and other countries has a reputation for impurities that muck up AC systems. I heard that Mexican R12 has butane added as a filler. Pure R12 is non-flammable.

There is nothing wrong with recycled Freon, whether it’s R12 or 134a, as long as it is good quality (clean and pure). The air conditioning machines used by knowledgeable AC technicians pull Freon out of an AC system and store it. The Freon is filtered during this process so it’s safe to be reused. These machines have built-in sensors to indicate when the filters need to be changed.

Anyone buying Freon must (a) be licensed to do so, and (b) be very discerning. My supplier weighs each incoming cylinder of R12 before accepting it to ensure it’s not tainted. Freon is heavier than air; if any air is cut into the Freon to dilute it, the cylinder won’t weigh enough. Watch out for Freon substitutes, they can become highly corrosive. They often contain methyl chloride which turns more acidic with less moisture than R134a or R12.

FR12, also known as FrigC, sometimes creates confusion. FR12 is R134a with butane added; it’s used in older cars. It comes in a white cylinder, which is the odd part. Refrigerant bottles are color coded and white is reserved for R12, sky blue is for R134a. Once I was buying R12, but got a cylinder of FR12 by mistake. When I looked closely I saw “Tetrafluoroethane” on the cylinder which threw me for a loop and wondering what R134a was doing in a white cylinder. Well, it’s a funky cylinder, constructed upside down compared to R12 and R134a. These days R12 is becoming harder to come by. Those white cylinders are a sight for sore eyes, provided it’s pure R12 and not FR12.

Mercury and Microns

A vacuum pump is used to evacuate the Freon from a system, then a stronger vacuum is applied and left on the system for a period of time. Moisture boils more readily under a vacuum. This is how moisture is purged from AC systems – the moisture is boiled away and the dry air is pulled out by vacuum.

The King Air manual calls for a vacuum pump that pulls at least 29 Hg (inches of mercury) and 125 microns. A proper AC machine in good condition meets this criteria. But on occasion, I have used a vacuum pump pulling 250 microns when I suspected a lot of moisture in a system after repairing a large leak. The manual also states the system must be left under vacuum for up to four hours. I tend to leave a King Air system under vacuum for much longer.

An Open System Invites Trouble

I’m very particular about not leaving the system “open” while waiting for replacement parts to arrive. After troubleshooting and identifying the problem (let’s say it’s a bad expansion valve), I do not remove the bad valve. I wait until I have the new valve in my hot little hand before I remove the old one. The system is only open during the time it takes to remove and replace the expansion valve. This keeps the amount of air (moisture) entering the system to a minimum.

If an AC system was left open for a period of time, for whatever reason, I would change the receiver-drier. It contains a desiccant. Receiver-driers come with plugs on both ends. I never remove those plugs until just before I install it; otherwise, the desiccant starts pulling moisture from the ambient air, reducing the effectivity of the new receiver-drier.

Confession

I went to air conditioning school when I worked for an automotive shop during high school. In the many years since, I’ve worked on a wide variety of auto and aircraft AC systems but, lucky for me, I’ve never had to deal with Black Death. Yes, I have been working for many years in a desert climate which is less conducive to Black Death than high humidity, but I don’t know if that tells the whole story.

My “partner in crime” for AC troubleshooting and repair hasn’t dealt with Black Death in his long career either. Together we’ve troubleshot and repaired King Air AC systems that others gave up for dead, but none with Black Death. Some of those King Airs lived
in the desert, but others lived in coastal environments.

Here’s what I do know: He and I are on the same page regarding pulling vacuum – the more time, the better. When it comes to purging moisture from AC systems we routinely exceed the maintenance manual requirements for time under vacuum. And we’re vigilant on keeping systems closed until the replacement part is in hand, ready for install.

What Can You Do?

If your King Air is in the shop with an AC squawk, don’t be in a hurry. Consider this typical scenario: Your AC is inop and you have a trip coming up. You get your King Air to the shop at the last minute for a quick fix. They troubleshoot it and find the compressor is bad. They have a new one coming, but they know you’re in a hurry so they take the old one off in preparation for replacement. The system sits open for one to two days. The shop receives and installs the new compressor as soon as it arrives; then they suck the system down with vacuum for an hour or so. They check for leaks and don’t find any, so they top off the Freon and send you on your way. Most likely your AC will blow ice cubes, but the door was opened for moisture to get into the system and this could hamper AC performance down the road.

If you find your AC blows cold, then warms up, then gets cold again – get your aircraft to a shop. In my experience, this intermittent cooling indicates moisture in the system. AC techs can diagnose this with their gauges. If you’re really pressed for time or far from home base, you can have the AC serviced with Freon as a stop-gap measure. But as soon as you can schedule it, put the aircraft down for a thorough check and fix of the AC. Allow the time.

The Good Guys

Besides patience, you need a shop with a good AC guy. Anyone can throw Freon in a system, but proper servicing and effective troubleshooting requires specialized equipment, knowledge and experience. A good AC tech will have all three, plus he or she will know where to source quality Freon.

I’m in a pickle here, because there are good guys out there that know what they’re doing and they go by the book. Maybe they’ve faced an AC system with Black Death and maybe not. But, I don’t want to give the impression that I’ve prevented Black Death because I went beyond the book requirements. That could be a factor but I don’t know it for a fact. Extended vacuum time has worked for me and I don’t see a downside to it.

The bottom line is that an AC system contaminated with moisture won’t function properly and Black Death could eventually be a result. A humid climate, bad Freon, a leaky system, a system left open to the air – all of these factors could contribute to the rise of Black Death in a King Air AC system. The goal? Get and keep the moisture out and keep your cool!