Page 23 - Volume 15 Number 1

As in any new design, some minor shortcomings surfaced after the system got into customers’ hands and feedback was received. The most significant of these was that the voltmeter did not allow inspection of all five buses, only three (triple fed and left and right generator buses).
Beginning with the F90-1 in 1982, all the 300-series and the C90A and models after, this was corrected with a voltmeter that included the missing buses. Actually, all voltmeters on five bus airplanes also include an extra position that allows EPU voltage to be measured and assessed at the plug, before the pilot turns on a new- to-this-design external power switch to allow the external power relay to close and introduce the EPU power into the airplane. With this switch, although the battery still should be on while using external power, the EPU connection will work with or without the battery switch on, just like in the early King Airs.
A controversial question that arises concerning the five-bus system is, “Do we, or do we not, manually close the generator bus ties prior to start?” In all of these systems, the start will be absolutely normal whether we do or do not. Realize that when the first generator is turned on, both left and right generator bus ties close automatically so from that point on there is zero difference. Yet, some POHs direct us to close the ties prior to start while others don’t. Why the difference?
The answer to this “discrepancy” has to do with where the rotating beacon(s) receive their power. For the F90, C90A-series and 300 models, the beacon is wired to the left generator bus. Since this bus does not get power until either generator ison,oranEPUisinuse,orthe bus ties have been manually closed, it means the beacon would not be rotating prior to the first engine start. Realize that one of the FAA’s recommendations is to always have the beacon on before a propeller rotates ... as a safety measure to let people know that they should “step away from the airplane!”
Now, just between you and me, if an innocent bystander were standing within the arc of a PT6’s propeller when someone hit the start switch, I think that the initially- very-slow-to-turn, free-turbine engine’s propeller would probably bump into the person with a gentle nudge, just enough to encourage him or her to step away. No big deal, unlike the catastrophe that would have happened with a piston engine or a fixed-shaft turboprop!
So, yes, for you operators of F90s,
C90A-series and 300s, I encourage
you to do as the POH directs and to
close the bus ties prior to the first
start, for the purpose of making the
beacon operate. (But if you don’t,
no one is going to be harmed.) On
the other hand, for the F90-1 and
the 350-series, Beechcraft relocated
the beacon to the triple-fed bus, so
it works regardless of whether the
generator bus ties are closed or not.
Hence, those POH procedures do
not direct the pilot to close the ties
initially. (For the model 300 only, the
fuel quantity gauges don’t work until 21 the generator buses are on line, so it
11 ties manually to verify the amount 25
is especially important to close the of fuel onboard before starting.)
For nighttime starts, for all five- bus models, it is best to always close the generator ties prior to starting so that all external and internal lighting is available, as desired.
I hope this review of the history of the King Air’s electrical design has been of interest, allowing you to have a better understanding of how the electrical system evolved in the particular model that you are flying. KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 46 years and is the author of “The King Air Book” and “The King Air Book II.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his books, contact Tom direct at twcaz@msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@blonigen.net.
JANUARY 2021
KING AIR MAGAZINE • 21