A reader recently asked me some questions about the procedure for restoring the air conditioner (AC) back into working order in a B200, following a shutdown caused by too low or too high system pressure. I came to believe that this would be a worthwhile topic for discussion, so here we go.
Before I discuss the King Air model 200 and 300 systems – the only ones that have the pressure protection circuit – I want to mention the method by which other King Air AC systems may sometimes be easily fixed when they fail. The AC system on the 90- and 100-series uses an electric motor to drive the refrigerant compressor. As you know, this motor is located between the inlet and outlet louvers located on the right and left sides, respectively, of the fuselage’s nose section, forward of the avionics bay doors. The compressor motor receives its power from a circuit that includes a 150-amp current limiter or circuit breaker (CB), located in the cockpit. The CB on earlier models is visible on the floor, between the pilot’s seat and the pedestal. The limiter is under the floor and is not visible until an access panel is removed. Most of these King Air models include an AC start control printed circuit board (PCB) located near the condenser between the louvers in the nose. The purpose of the start control is to reduce the large current spike (quick increase and decrease) caused by the initial application of power to the compressor motor. It was found that the voltage drop that accompanied this current spike often caused the autopilot to make some strange and undesirable deviations. (Somewhat hard to believe but tuning an ILS frequency into Nav 1 actually locked out the AC system in some early 90-series models prior to start controls!)
Some components on the start control PCB are protected by a 1-amp CB located on the board. It can be unfortunately common for this CB to trip, which prevents the AC from starting. Many 90- and 100-series operators have learned that if their AC is inoperative, it is wise to check that CB and do a one-time reset of it before calling the maintenance troops. Often, the CB reset solves the problem and it does not appear again for a long time, if ever. Cheap fix!
Where is the CB located? High in the nose section, under the panel that allows access to the condenser. In fact, many serial numbers have a hole in that access panel, covered by a 1-inch round “bullet hole patch,” that saves a lot of time. Now the CB may be seen and reset by removing the single screw securing the patch, not by taking out the myriad screws holding the entire access panel.
Let’s now turn to the low- and high-pressure protection system on the 200- and 300-series. As you probably know, the AC compressor on these models is driven by the right engine’s accessory case, no longer by an electric motor. These King Air models received an entire redesign of their environmental system around 2007 when the “Keith” system was introduced. Although overpressure protection is retained in the Keith system, the reset procedure is automatic. There is no reset procedure to do nor button to push. Nice!
However, the thousands of 200s and 300s – including B200s and 350s – with the original environmental system are the models to which this article is directed.
In all of these airplanes, two pressure-protection switches are installed. The underpressure or low-pressure switch is installed in the line going to the AC compressor and the overpressure or high pressure switch in the line coming from the compressor. Both switches are physically located side-by-side in the leading edge of the right wing’s center section.
Low system pressure can be caused by a Freon® leak leading to there being less Freon mass in the fixed volume of all the components that make up the AC system: compressor, condenser, receiver-dryer, expansion valve, evaporator and all the lines connecting the components together. Perhaps the most common cause of a low-pressure trip is freezing in the evaporator plenum caused by a defective hot gas bypass valve. On the other hand, high system pressure is often caused by an inoperative condenser blower. There certainly are other reasons for pressure problems, such as a partial blockage in a line or component.
Allow me to run through a simple AC system review. A rule of nature is that thermal energy always flows from hotter to colder. If we drop an ice cube into a glass of water, the energy in the warmer water flows into the cooler ice cube and melts it. We obviously never see the ice cube losing thermal energy and getting cooler while the water is receiving that energy and getting warmer! So how do we take energy from our 70°F cabin air and send it out into the 110°F outside air?
We do so by utilizing the properties of a refrigerant that goes by the DuPont tradename of Freon. (R-12 and R-134A versions are the most common refrigerants in King Airs. The first is a CHC – Chlorofluorocarbon – while the second is an HFC – Hydrofluorocarbon.) Like water, this substance can be in solid, liquid or gaseous states of matter. Also, like water, the temperature at which the transition from liquid to vapor (evaporation) and from vapor to liquid (condensation) takes place changes as pressure changes. As you all know, water on the stove boils at 212°F with standard sea level pressure, but its boiling temperature is less in Denver at a high altitude under less pressure.
What the AC system does is use the Freon as a mechanism to transport thermal energy from colder to hotter – the cool cabin to the warm outside. It does this by making the Freon’s pressure very low, hence making its evaporation temperature also low. The expansion valve is the mechanism that causes this big drop in pressure and temperature. As the Freon flows through the coils of the evaporator, perhaps now with a boiling point of 30°F, the 70°F cabin air can lose energy into the Freon as it boils from liquid to gas. Next, the compressor squeezes the gas, making it hotter, say 180°F. The compressed Freon now flows through the coils of the condenser. The outside air flowing across the other side of the coils, perhaps at 110°F, receives energy from the hot Freon and blows harmlessly out the left side louvers on the nose. (Ever hold your hand near those louvers with the AC running on the ramp? Isn’t it hot air coming out?) To summarize, we make the Freon cold so that cabin air energy flows into it then we make it hot so that it may expel that energy into the outside air … the Freon is our energy transport mechanism.
There have been three versions of the underpressure/overpressure reset mechanism installed in the 200-series. The very early 200s had a fuse holder easily accessible in the right wing’s center section. Beneath the battery box, on the bottom of the wing, do you see that small hinged door with the three camlock fasteners? The fuses are in there. On these early models it contains a fuse and a spare fuse mounted beside it. If the fuse is blown (take it out and look at the element in the glass tube) merely replace it with the spare and maybe you will be lucky enough not to experience another pressure trip. If the spare fuse also blows, it’s time for maintenance help.
The second version – which began with BB-345 and could be retrofitted to earlier models – replaced the fuses with a PCB. There are two things of interest here. First, the PCB contains two small red lights. With the battery switch turned on, the illumination of one indicated that the pressure trip was caused by low pressure; the other indicated high pressure was the reason for the shutdown. The second item of interest is a reset button. Just push it once, the pressure-indication light should extinguish and we’re back in business for another try. (Make a record of what caused the trip – low or high. That information will help if further troubleshooting is required.)
The third and last of the BE-200 reset mechanisms – and the one most serial numbers now have – began with BB-729 and after, plus BB-688. (All of the 300-series also has this system.) With this version 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:
- Turn the battery switch on. For the 300-series, also select Man Close on the Gen Ties switch.
- Select Man Cool on the Cabin Temp Mode selector.
- Note if the “AC N1 Low” advisory (green) annunciator is illuminated.
- Is the light on? If so, go to Step 4.
- 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.
- With the “AC N1 Low” light illuminated, leave the switches as they are and go check the nose wheel well.
- 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.
- 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 time delay. No engagement of the AC compressor clutch can occur until 10 seconds after the command for AC has been received.
on the left-hand side of the nose wheel well.
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!