Page 29 - Volume 15 Number 10

what about when an actual case of excessive current flow in the wrong direction activates only one HED? The answer is that we still get the Caution, yellow GEN TIES OPEN, plural, annunciator but now it is combined with an Advisory, green annunciator that says L BUS SENS OPN or R BUS SENS OPN. I am very glad that Beech corrected these little “problems” on the F90-1 and all later five-bus systems!
Let’s return to where we were in the test procedure. We have mo- mentarily pushed the BUS SENSE switch down to TEST, released it promptly, checked for the three Cau- tion annunciators and verified that the center bus shows no voltage. (You F90s that have no display of center bus voltage: Try to inflate the deice boots. They get their power from the center bus so their being inoperative tells you the center bus died, like it should at this time.) No- where does it state this next step in Beech’s procedure, but I encourage
you to do it. Did your inverter keep working? No warning, red Inverter annunciator illuminated? Good!
You see, in the event of a dual generator failure, this newer electri- cal system will automatically shed a lot of electrical load to prolong the time your battery can supply more critical items. It does this primar- ily by “killing” both left and right generator buses. You will be left with the same items you have when you first turn the battery switch on: only items powered by the hot bat- tery bus, the triple-fed bus and the center bus. The switches with the white circles painted around them remind you what is still operative. With so much now gone, the design- ers believed (rightly so!) that los- ing the inverter at this time would not be a good idea. Consequently, each inverter has two sources of power. If and when a generator bus is powered, the inverter automati- cally gets its DC input from that side’s generator bus: No.1 Inverter
from Left Generator Bus and No. 2 Inverter from Right Generator Bus. But when the respective generator bus has no power upon it, then a relay automatically selects the in- verter’s power from the center bus.
Therefore, if your selected inverter did not fail when you tapped the BUS SENSE switch to TEST and made the center bus go dead, then you have just verified that the normal source of inverter power is available correctly. Although over the course of time as you run this test you will probably be using No.1 inverter some of the time and No. 2 inverter at other times – and thereby verify that both inverters have the normal power source from their generator buses available – why not now go ahead and move your inverter select switch to the other inverter to know that it is also wired correctly?
Another thought concerning this design of two sources of power for each inverter: Nowhere does Beech direct you to do what I am now going
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OCTOBER 2021
KING AIR MAGAZINE • 27