Switches with white circle symbol

[jarek]

Electrical system in the King Air looks quite complex, as it includes powered from 5 primary buses that supply power to the different components. Some of these buses are dedicated and some, like triple-fed bus, can have multiple power sources (engine-driven generators and battery)
To help a little bit with identification what is connected to the battery, some switches received this white circle mark.

bat-on.jpg (361.64 KiB) Viewed 989 times

The first thing we want to do in cold and dark state is to apply power to the battery bus. There is a switch marked with white and red stripes on the left hand side of the left switch panel. In EMER OFF position battery is isolated. Moving it to NORM, applies power to the battery bus and some minor components are now alive (like yoke clock).
Now let's move to the switch indicated by red arrow on the picture. This is main battery switch and it is located under a flap, together with generator switches. Lets switch it to on and now two additional buses receive power - triple fed bus and center bus. Now we have power applied to all switches marked with white circle.

To check this, let's switch on some lights - right landing light, taxi and ice inspection lights. First one does not have white circle symbol. If we now look outside, only 2 lights are illuminated as expected: taxi and ice inspection.

bat-on-ext.jpg (332.42 KiB) Viewed 989 times

So how can we apply power to the right landing light that is powered from right generator bus? There is an option to do that even if engine is not running. Let's momentarily move switch described as GEN TIES to the upper position. It is spring-loaded so will return to the center position automatically. What it does - it closes circuits between Center Bus and Left/Right generator buses, so now they are powered form battery.

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Lets now look outside - and now right landing light gets illuminated. However, battery has limited capacity, so it easy to drain battery this way.

bus-ties-closed-ext.jpg (331.61 KiB) Viewed 989 times

Now last remaining mystery - there are two switches that have half circle symbol. They can be found in the engine anti-Ice section. Reason here is that main actuators are powered by the triple-fed bus while the standby actuators are powered by generator buses.

[matpietsch]

Good read, thanks a lot. I am wondering what the engineering at Beechcraft was thinking when designing this solution. I understand the circle around the beacon because you want to have it on before engine start. But why not the NAV light? Und why the Taxi light? You would think, that it would be other way around.

Could it be that they „Bombardiered“ (Dash8 and CRJ anybody) themselves here?

[KenG]

Very nice. Thank you.

Here is an electrical diagram I recreated in Powerpoint and flattened into an image for discussions related to the MV KA350i. Feel free to use it. Sorry it is not as pretty as the commercially produced stuff...

King Air 350 Electrical System

King Air 350 Electrical.png (234.85 KiB) Viewed 942 times

[KenG]

If you right-click and open the image in a new tab it is a little easier to read.

[Raptor]

Thank you Ken and Jarek! Great info!