Page 18 - Volume 14 Number 9

The current cabin temperature (2) is detected by a sensor almost always located in the cabin’s headliner. This sensor has a small fan attached to it that draws cabin air up and over the sensor to allow it to get a more accurate reading. Since the sensor is only a couple of inches away from the aluminum skin at the top of the fuselage, at cruise altitude it would tend to feel a temperature significantly colder than actual cabin temperature were it not for the fan.
Finally, the temperature of the incoming air (3) is detected by a sensor located in the duct that comes from the mixing plenum under the copilot’s rudder pedals. The mixing plenum, or chamber, is so-named since it is where new incoming bleed air mixes with recirculated cabin air. The temperature sensor is installed in the branch going from the plenum to the cabin floor outlets.
If any of these three sensors goes bad, it makes the Auto mode
operate poorly. In fact, one of the most common malfunctions is caused by failure of the fan in the ceiling. This leads to that sensor feeling too cold and thus the Auto system makes the cabin too hot as it tries to correct the too cold cabin temperature it now senses. Given enough hours flying the King Air, I can guarantee that your Auto mode will malfunction and lead you to use Man Heat or Man Cool until the problem is found and corrected. Let’s talk about those modes.
There are a few meaningful differences in the operation of the manual modes depending on the King Air model being discussed. Specifically, the model 200- and 300-series differs from the 90- and 100-series. I will begin by discussing the 90- and 100-series. From the first C90, LJ-502, to the latest C90GTx – including all E90s, all of the F90-series, and all of the 100-series – their environmental systems are virtually identical.
Three components can and do affect the cabin temperature in this group of airplanes. First, bleed air. The air that has been compressed – and thereby heated – by the engine’s compressor is known as “P3 Air” since it originates from the engine station #3, the “Combustion Chamber Inlet” which is also the compressor outlet. A relatively small portion of P3 air is tapped off of the engine and sent to the Flow Control Unit (flow pack) mounted inside the cowling on the forward side of the firewall. The flow pack often mixes some ambient air with the bleed air before sending it to a junction in the leading edge of the wing’s center section. Following one of the two paths at the junction finds the air going directly to a device called the bypass valve. Following the second of the two paths finds the air flowing across the metallic fins of an air-to- air heat exchanger. Flowing across the other side of the fins is ambient air that has entered the ductwork containing the heat exchanger from
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16 • KING AIR MAGAZINE
SEPTEMBER 2020