Page 26 - Volume 14 Number 2

there is no need to have a Philips screwdriver to open that little access door in the wing. Instead, you can do a reset via the combination yellow light/reset switch on the left-hand side of the nose wheel well. Many folks erroneously refer to this light/switch combination as a circuit breaker. It is easy to see why this is a common misconception since pushing it usually causes the system to be restored ... just like resetting a CB. But, no, unlike the CB, no excessive current caused this trip and we are not merely closing an open circuit. We are pulling a relay on a printed circuit card back to a closed state after the pressure trip had caused it to go open.
There is one reason why the latest system – the one that eliminates the need for a screwdriver – is, in my opinion, a tiny bit less desirable than the one that preceded it: There is no way to easily tell if the trip was caused by low or high pressure. Yes, you can tell, but now it involves removing the floor aisle access panel behind the main spar in the cabin to find the PCB with the red indicator lights. In most cases, this is something your A&P will be doing, not you.
Consider this scenario: You takeoff from Meacham Field (KFTW) in Ft. Worth, Texas, on a July day and fly a 2:25 flight to El Paso (KELP). The AC worked well on the ground after starting and continued doing a fine job during climb and cruise. Unfortunately, as the descent brought you down into the hot West Texas temperatures, the cabin temperature began to rise. Turning the cabin temp rheostat down, trying the Man Cool mode and holding the Man Temp switch down to DECR for 60 seconds ... nothing improved the situation so you and your passengers were sweating by the time the cabin door was opened. You have only a one-hour layover. What do you do? Here’s a time to try a reset.
It’s not as easy as many pilots think. You cannot simply find the button in the nose wheel well, push it and be done. Oh no! There’s more to it than that. Unless electric power is on and the AC system is receiving a command to operate, a reset cannot take place. Here’s a checklist for the proper steps:
1. Turn the battery switch on. For the 300-series, also select Man Close on the Gen Ties switch.
2. SelectManCoolontheCabinTempModeselector.
3. Note if the “AC N1 Low” advisory (green) annunciator is illuminated.
A. Is the light on? If so, go to Step 4.
B. If the annunciator is not on, hold the Man Temp switch down until it illuminates or until 60 seconds or more have elapsed. If you cannot make the light appear, then that is your problem. You did not have a system trip caused by under- or overpressure, but instead your AC is not receiving a command to operate. Perhaps the left bleed air bypass valve’s motor has failed causing it to never reach the “Full Cold” position or maybe the switch on this valve assembly is inoperative. Too bad; you’re
going to be hot on the return flight until you reach cold outside air temperatures aloft. Turn everything back off and go drink some ice water.
3. With the “AC N1 Low” light illuminated, leave the switches as they are and go check the nose wheel well.
4. If the reset light/button is not glowing yellow, again you are out of luck. No reset can be made. But if the light is indeed on, then push the button firmly. You should hear a “click” and the light should extinguish. Congratulations! You have done a successful reset of the pressure protection circuitry.
5. Return to the cockpit to turn everything back to its normal shutdown position.
Now the passengers have returned, you have started both engines and it’s time for some cooling! I strongly suggest you add a step or two to your normal procedure. Here’s why.
Do you know about the 10-second time delay in AC operation? As is true for any typical automobile, house or aircraft refrigerant cooling system, undesirable pressure excursions (spikes) can be caused by turning the system back on immediately after it has been shut off. It is best to wait a few moments to allow pressures in the closed- loop system to stabilize. The designers of the King Air system first used on the 200 model recognized this and envisioned a typical case in which this unwanted off-on action would occur. Consider landing on a warm day with the AC operating. In the flare, with the power levers now at Idle, the RH N1 drops below about 60% causing the AC clutch to disengage. Perhaps the pilot chooses to spool up into reverse to make that turnoff closest to the FBO. As the N1 increases above the AC clutch cut-out value and if the clutch re-engages now ... hello pressure spike and possible system damage! To handle this situation, the designers added a 10-second
The reset button in most BE-200 and all of the 300 series King Airs is located on the left-hand side of the nose wheel well.
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24 • KING AIR MAGAZINE
FEBRUARY 2020