Page 22 - Volume 14 Number 2

ASK THE EXPERT
The Air Conditioner
Reset Procedure
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!
20 • KING AIR MAGAZINE
by Tom Clements
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.
›
FEBRUARY 2020