Page 15 - August 2015 Volume 9, Number 8

do next, so they began replacing components. Eight thousand dollars later (yes, $8,000!), the shop told the owner the next step would be to gut the interior and replace each and every line in the system; also to replace the evaporators and the condenser in order to find and fix the problem. Their estimate for this project was another $15,000. The owner refused this offer.
He brought the aircraft to me and we found the receiver dryer totally plugged with flush. When they flushed the system, they had flushed the whole system, including everything downstream of the receiver dryer! After drowning the system in flush, they failed to get it all out. It took three more receiver dryers to capture all that residual flush that was mucking up the system. Finally, once the system was completely purged of flush, we were able to properly service it with the correct type and amount of Freon. It’s been blowing icicles ever since.
What’s Driving It?
It’s one thing to understand the principles of air conditioning, but that’s just the beginning. King Air A/C systems are a bit convoluted. I hate to say it, but the A/C in a King Air is not its strongest point.
If you have a model 90 or 100, your A/C is driven by an electric motor in the nose. If you have a 200, 300 or 350 model, your A/C is driven by the right-hand engine. Three years ago, I wrote an article for this magazine on troubleshooting air conditioning problems; it was focused strictly on the engine-driven systems in the
Engine-Driven
With the advent of the 200 series King Air, and the 300s and 350s that followed, Beech desperately wanted to shed the generator loads so they ran the A/C system off the right-hand engine. While on the ground and running, the condenser blower draws 40-50 amps, but as soon as you break ground, the condenser blower shuts off. The moment you are airborne, the scoop on the right side of the nose puts air across the condenser and that air cools the Freon.
Once in the air, the draw on the generators from A/C-related components is negligible. You would think that condenser blowers would last much longer in the engine-driven systems because they only run 20-30 minutes on the ground and never in the air; whereas the motor-driven systems are going on the ground and in the air (at least until your OAT goes below 50o F). In my experience, the condenser blowers in a 200 wear out as fast as they do in a 90. Go figure.
Automotive Ancestry
The A/C in the early model 200s had a six-cylinder compressor. It was an A6 compressor like those found in a Cadillac Coupe de Ville or a Chevy Monte Carlo. They worked pretty well considering they were cooling
200s/300s/350s. If you missed it, look for the May/June issue of 2012.
Electric Motor-Driven
Air conditioning in the King Air model 90s and 100s works pretty well as far as cooling the cabin is concerned. Plus, while you are pre-flighting the aircraft, you can plug in external power and run the A/C on the ground to pre-cool the cabin before the passengers arrive. This is positively blissful in triple digit temperatures. Another perk? It’s easy to troubleshoot the system – just plug in the GPU.
The problem is the huge load this electric motor draws off the generators. Today I was performing a routine ground run on a C90. The generators were showing 15% until I turned on the A/C; then they showed 40% per side – quite a load. Beech was concerned that in instances of losing an engine, a massive load would be born by one generator, leaving little room for everything else needing juice in a clutch situation.
Another concern was that the motor-driven A/C worked hard all the time. That motor is either on or it’s off. When it runs, it is always at the same speed, providing optimum output continuously, so these motors can wear out.
Early 90 and 100 models had a two-cylinder compressor. This was later changed to a five-cylinder compressor which used the same amount of juice off the generators, but it made colder air and it made it faster. No complaints there.
A closer look at the hole in the compressor case shown at left.
AUGUST 2015
KING AIR MAGAZINE • 13