Page 28 - Volume 14 Number 2

time delay. No engagement of the AC compressor clutch can occur until 10 seconds after the command for AC has been received.
Try this the next time you are in your aircraft on a hot day. Because it is hot, you probably have selected High on the vent blower switch, turned the aft blower on and made sure the bleed air switches are still in the center, Envir Off position. As needed, you have also tweaked your condition levers to have enough N1 for AC operation. After you turn the mode selector to Auto, note the ITT values then tilt your head back and closely watch the loadmeters. In about 10 seconds you should observe both meters jump up about 5 or 10%. Now look at the ITTs. The right side should have increased a bit (10 to 20° is typical) since the fuel control unit has put in more fuel to keep the N1 from falling due to the drag of the Freon compressor’s load. Lastly, you possibly may hear a new whine in your headset: A little
electrical interference created by the condenser blower motor. These are the proper indications that the AC has begun working as you hoped.
Return now to our El Paso flight where we think we have done a successful system reset. Recall that one of the leading causes of an overpressure trip is an inoperative condenser blower. With both engines now started, I strongly suggest that you watch those loadmeters for the first 10 seconds after you have selected Auto or Man Cool. If you do not see the expected increase caused by the load of the condenser blower, quickly turn the mode selector back to Off. Aha! You have just probably confirmed why you had an overpressure trip – no blower.
You and your passengers will suffer on the ground due to no cooling. But relief is on the way! After takeoff with the gear up and climbing normally, return the mode
selector to Auto or Man Cool. See that RH ITT rise a tad after 10 seconds? Feel that cool air starting to flow? Ahhhhh, feels good!
Realize that the airspeed itself creates enough airflow across the condenser such that it works properly while in flight. Unless we are going very slow – on the ground or perhaps in flight doing slow flight practice with the gear and flaps extended at 80 KIAS – we never need the blower to assist the relative wind. That’s why it only operates when (1) the AC is receiving an operating command, and (2) the nose gear’s downlock switch is activated. Now you understand why the whine in the noise-canceling headset sometimes begins when the gear goes down at the outer marker, eh?
On our next leg after KELP we can enjoy the comfort of the AC system, just not on the ground. To avoid a second overpressure trip, remember to turn the mode selector off before touchdown! Time for a repair or replacement of the blower.
I hope your AC system operates flawlessly so that you will never need to use that reset button in the wheel well. But if ever you do, now I hope you can do it correctly and with confidence. Stay cool out there! KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 46 years, and is the author of “The King Air Book.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his book, contact Tom direct at twcaz@ msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@blonigen.net.
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26 • KING AIR MAGAZINE
FEBRUARY 2020