Page 24 - Volume 15 Number 10

ASK THE EXPERT
After Start Electrical Checks for the Five-Bus System
by Tom Clements
The expanded, first-flight- of-the-day checklist pro- cedures in the Pilot’s Op- erating Handbook (POH) include a somewhat lengthy and hard-to-understand series of steps for confirming that the electrical system is fit for duty with all its safety devices operating properly. My guess is that this procedure is executed quite rarely and, when it is done, the pilot is going through the steps without truly understand- ing what is being accomplished. The aim of this month’s article is to provide some insight and clarity into what is being tested in this series of steps.
The five-bus system is the name I use to refer to the newer King Air electrical system that first appeared in 1978 on the F90 model. Since that time it has become the system on the C90A and after series, as well as the 300-series – both the “straight” 300 and the B300, which is known as the 350 (and now, 360). The latest members of the 200-series, including the 260, retain the electrical system
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The pilot's left subpanel where the after start electrical check procedure begins.
of their predecessors. Hall-effect devices, automatic load-shedding and bus tie relays are unknown on BE-200s, as well as on all the 100-series and the earlier members of the 90-series. Skip this article if your King Air does not contain a “triple-fed bus.” (Many folks refer to this newer system as the triple-fed bus system. In fact, the 350 and 360 even have a sixth bus, but overall
the system functions exactly like it’s five-bus brothers.)
The full test procedure spelled out in the POH takes place after both engines are started and both generators are operating. Of course, we should never turn on a generator while using an External Power Unit (EPU) so if we started with such a unit, it needs to be turned off and disconnected before proceeding.
OCTOBER 2021