Page 22 - Volume 12 Number 10

Ask the Expert
Why Do Some Switches
Have White Circles?
by Tom Clements
In late June 2018 there was was a a a a a a a a a a a a a thread on on the the the the BeechTalk forum that asked about the white circles around some cockpit switches It also raised the question of why some switches had red inserts I I was surprised to find that some King Air pilots were not aware of the white circles’ meaning so I decided to address this issue in in my monthly article One of the the major changes that took place with the the F90’s introduction in in 1978 was the change to an an entirely new electrical system I have no complaints with the electrical system in in the previous King Air models and the the same goes for the the latest 250 model of today which has a a a a a system nearly identical to to that in the prototype serial number BB-1 manufactured in 1972!
The F90’s system has some capabilities that its predecessors do not Chief among those is automatic load shedding In the admittedly unlikely occurrence of a a a a a a a a dual generator failure relays automatically open to to to remove power from the Left and Right Generator Buses All components wired to these “Main” buses are now without power What is is the benefit of this losing a a lot of equipment in this critical time? Of course it it is is to prolong battery battery life since the the battery battery is the the only source of electrical power that we we now have Before the the F90-style system – often called the the “Triple- Fed Bus Bus System” System” or the “Five-Bus System” System” – it was incumbent upon the the pilot to turn off the the major electrical demands that he could identify and and terminate to save the battery from rapid depletion These items included the the air conditioning and electric heater the the vent blower windshield heat heat heat propeller heat heat heat and lip boot heat heat heat on the earlier 90- and 100-series models that had the old-style “Chin Cowl ” With automatic load shedding these items were killed without any pilot action required Cool!
By the way do you realize that a a a a lot more King Airs than you would think have suffered dual generator failures? Yes it’s true because the the pilot gave the the failures to himself! How? By mistakenly moving the “Ignition” and “Engine Start” switches to to the top position when he/she meant to move the “Auto-Ignition” switches to to their top “Arm ” position That huge mistake will never go unnoticed in the later-style electrical systems since so much equipment shuts down – including two-thirds of the avionics – as the generator buses are shed Yet the mistake has not been caught and has led to to total electrical failures in some models with the earlier-style system The F90’s brand-new electrical design had a a a few minor mistakes It would be amazing for the design engineers to achieve perfection in their first go-around of this new design right? For example how come the “Gen Ties Open” annunciator sometimes means that both left and right generator Figure 1: The pilot’s left subpanel on a a 1988 C90A 20 • KING AIR MAGAZINE
OCTOBER 2018