Page 30 - Oct24

the airplane’s components distributed among them appropriately. Of special interest is the fact that three Hall effect devices (HEDs) are included. These devices— not yet available back in the ’60s—allow excessive current flow to be terminated instantaneously, based on the increase in the magnetic field around the wire, instead of waiting for the heat buildup to melt a fuse or blow a CB—a safer design.
Also, the new system includes “automatic load shedding.” Previously, in the rare situation in which both generators were lost, it was incumbent upon the pilot to rapidly turn off the unnecessary, heavy-load components to prevent the battery from being discharged too rapidly. In the new system, when the second generator fails, bus tie relays open automatically to disconnect the generator buses from the battery. Hence, the battery only supplies those items on the hot battery bus, the triple-fed bus and the center bus, prolonging its life significantly. (It is nearly impossible to explain this satisfactorily, but the heavy-load items on the center bus—electric heat and air conditioning motor—are also “shed” in this scenario.)
For some weird reason—perhaps because of the bus that is fed from the left generator, the right generator and the battery, too, with the name triple-fed bus—it seems that this new system was often called the “three bus system.” Huh? But there are five buses! I try to be consistent and always refer to this as the five-bus system, but if you read or hear the term “three bus,” it’s the same thing. (The 350 kind of has a sixth bus, the dual-fed bus, but it acts as an extension of the hot battery bus.)
As in any new design, some minor shortcomings surfaced after the system got into customers’ hands and feedback was received. The most significant 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 after models, this was corrected with a voltmeter that included the missing buses. 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 should still 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 normal whether we do or not. Realize that when the first generator is turned on, both the 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 power. For the F90, C90A and 300 models, the beacon is wired to the left generator bus. Since this bus does not get power until either the generator is on, an EPU is in use or the bus ties have been manually closed, the beacon would not be rotating prior to the first engine start. Realize that one of the Federal Aviation Administration’s (FAA) recommendations is always to 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 them 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, C90As and 300s, I encourage you to do as the POH directs and close the bus ties before the first start to make the beacon operate. (But if you don’t, no one will be harmed.) On the other hand, for the F90-1, the C90B and after series, 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 initially direct the pilot to close the ties. (Only for the model 300, the fuel quantity gauges don’t work until the generator buses are online, so it is especially important to close the ties manually to verify the amount 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 better understand how the electrical system evolved in the particular model you are flying. KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 50 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.
28 • KING AIR MAGAZINE
OCTOBER 2024