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the electrical components that use higher amounts of current. The landing gear motor, air conditioning motor, windshield heat and cabin electric heat (when applicable) are obvious candidates for main bus items. Not quite so obvious are avionics buses, inverters and flap motors, but they, too, almost always are fed by a main bus.
Components that use only a small amount of electric power receive that power from subpanel buses. For redundancy, these buses are fed not just from one side’s main bus but from both. A circuit breaker (CB) rated at 50 amps protects the wire from each main bus into the subpanel bus. Therefore, each subpanel has two 50 amp feeder CBs associated with it. With nothing else, however, this would compromise the separateness of the two main buses since now there is a bridge between the left and right sides via the subpanel and its two feed wires. What’s that I see riding over the horizon to our rescue? Why, it’s Sheriff Diode!
Yes, the necessary and often-used one-way “check valve” for current flow, the lowly diode is the device that allows both main buses to feed to the subpanel but does not allow current to flow from the subpanel back into the main bus. Every subpanel feeder CB has a diode between it and the subpanel to prevent return flow.
I speculate that the designer who decided on what small components would be wired to which of the two
subpanels had his reasons for placing things as he did, but I’ll be darned if I know what the reasons were! Only when we get to the 100 series does logic seem to dictate the selection. For the A90, B90, C90, C90-1 and E90 systems, the only way to know which items are wired to which bus is to consult the electrical system schematic in the POH or wiring diagram manual (WDM).
Challenges arise whenever a technical writer takes a complex subject and tries to present it in an understandable manner to a nontechnical reader. If the writer makes it too simple, details that may be important for better understanding are often ignored. On the other hand, if he tries to describe every minute detail, the reader is quickly lost or put to sleep! Likewise, the drawing of the POH’s electrical system schematic always becomes a compromise between accuracy and understanding. I think the Beech POH writers did an excellent job at this compromise, although others may disagree.
In the POH’s schematic, each main and subpanel bus is rendered as if it were a single strip of metal with all associated components wired off the bus one by one, side by side. In the real airplane, often that is not the case. For example, although most main bus items receive power from the vicinity of the cockpit console, the inverters tap off their power from an area in the main wheel wells.
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OCTOBER 2024
KING AIR MAGAZINE • 23