Page 18 - Volume 15 Number 1

One can recognize that the plane was built with the new GCUs by observing whether or not the generator control switch has the third, top, “Reset” position. That reset position means GCUs are installed. With the old-style system, the generator switch was merely On-Off, two positions, up and down.
Just as a municipality’s water pressure would drop if every resident opened every spigot and tap at the same time, causing too much water demand, also no generator can maintain proper voltage when its current outflow gets too large. Although this will happen in the event of a short to ground, a more common example of this is when one generator is assisting with the start of the opposite engine. Have you ever monitored voltage while you activate a starter switch? It surely makes a momentary big drop, doesn’t it? No wonder the lights go dim for a while.
When switched on, each generator feeds directly into its own main bus, also called the generator bus. As a general rule, with minor exceptions, the main buses have the electrical components that use higher amounts of current connected to them. Landing gear motor, air conditioning motor, windshield heat, cabin electric heat (when applicable) ... are obvious candidates for being 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. These buses, for redundancy, are in turn fed not just from one side’s main bus but from both. A circuit breaker (CB) rated at 50 amps protects the wire going from each main bus into the particular 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 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 “checkvalve” for current flow, the lowly diode, is the device that allows both main buses to feed to the subpanel but do 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 made the decision of 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).
Whenever a technical writer takes a complex subject and tries to present it in an understandable manner to a non-technical reader, challenges arise. If the writer makes it too simple, often details are ignored that may be important for better understanding. 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 personally think the Beechcraft POH writers did an excellent job at this compromise, although I know others may disagree.
In the POH’s schematic, each main and each subpanel bus is rendered as if it were a single strip of metal with all associated components wired off of the bus one-by-one,side-by-side.Inthereal ›
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16 • KING AIR MAGAZINE
JANUARY 2021