Why Do Some Switches Have White Circles?

In late June 2018, there was a thread on 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 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 my monthly article.

One of the major changes that took place with the F90’s introduction in 1978 was the change to an entirely new electrical system. I have no complaints with the electrical system in the previous King Air models; and the same goes for the latest 250 model of today which has a system nearly identical 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 dual generator failure, relays automatically open to remove power from the Left and Right Generator Buses. All components wired to these “Main” buses are now without power. What is the benefit of this; losing a lot of equipment in this critical time? Of course, it is to prolong battery life, since the battery is the only source of electrical power that we now have.

Before the F90-style system – often called the “Triple-Fed Bus System” or the “Five-Bus System” – it was incumbent upon the pilot to turn off the major electrical demands that he could identify and terminate to save the battery from rapid depletion. These items included the air conditioning and electric heater, the vent blower, windshield heat, propeller heat, and lip boot 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 lot more King Airs than you would think have suffered dual generator failures? Yes, it’s true … because the pilot gave the failures to himself! How? By mistakenly moving the “Ignition” and “Engine Start” switches to the top position when he/she meant to move the “Auto-Ignition” switches 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 total electrical failures in some models with the earlier-style system.

The F90’s brand-new electrical design had 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 bus ties are open and at other times means that only one is? Why does the airplane have five buses – Left Generator Bus, Right Generator Bus, Center Bus, Triple-Fed Bus and Hot Battery Bus – yet voltage can only be checked on three of them? Exactly why does the voltmeter’s “Battery” position measure the Triple-Fed Bus? How come the “Bus Sense Test” switches are labeled so peculiarly?

Six years later, in 1984, two other King Air models hit the scene – the C90A and the 300 – and both had the F90-style electrical system with almost all of the little glitches addressed and corrected. As mentioned, the 250 is the only currently-manufactured model that does not have the newer-style system, whereas the 350 and C90GTx models have the latest.

Now about those white circles …

Some clever Beech designer came up with a nifty way to indicate to the pilot which items would still be working following load shedding – paint a white circle around them. In previous King Airs, when the battery got turned on, everything in the airplane was capable of operating. It would surely drain the battery quickly to run the electric heater or air conditioner yet nothing – except commonsense – prevented that from happening. When the battery is turned on in the newer-style system, however, only the white-circled items are functional. Need to check the right pitot tube’s heat? Want to run the flaps down to wash them? If you want the battery to power these items – and many more – then you need to complete another step, in addition to turning the battery switch on: Move the Generator Ties switch momentarily up to the “Man Close” position to manually close the left and right generator bus tie relays.

I am surprised by the number of King Air pilots who are not aware of the white circle’s significance. I think training providers need to do a better job of educating their customers to the meaning of these circles since it is important to know at a glance what has not been shed when the generators are not online.

Figure 1 shows the pilot’s left subpanel on a 1988 C90A. A lot of white circles, eh? Notice the ones on the “Engine Anti-Ice” switches. Get your black Sharpie felt-tip pen out and mark over them … they should not be there! Notice that there are no circles around the “Actuators” switches. Well, if neither Standby nor Main motors are powered following load shedding, then how can Engine Anti-Ice function? It cannot, so the white circles are a design flaw / manufacturing mistake.

Now examine the same location on a newer 350 as shown in Figure 2. Notice the half-circle on the Main actuators. On the 350, the Main actuators receive power from the Triple-Fed Bus while the Standby actuators come from the Generator Buses … a better design by far.

Keep in mind that the non-circled components, the items that are shed, can be easily brought back to life by the simple action of moving the “Gen Ties” switch up to “Man Close.” (It then springs back to Norm while giving you an advisory annunciator of “Man Ties Close.”) I would strongly suggest doing this in the C90A- and F90-series if the dual generator failure happens in cruise and you need to descend through an ice-laden cloud deck for your landing. Once the ice vanes are extended, then remove the power, the extra battery drain, by moving the “Gen Ties” switch down to “Open.” I surely wish the 300 electrical designers had shared their ice vane wiring idea with the C90A group!

Figure 3 shows the pilot’s righthand subpanel on the C90A, and Figure 4 exhibits the same on a 350. Can you find the subtle differences? One nice improvement on the 350 is that the beacon will work before engine start without having to close the Generator Ties manually. Do you, like most of us, leave the beacon on when securing the airplane? That is an old trick that reminds you that you forgot to turn the battery switch off since you can see the beacon flashing as you walk away … but the trick won’t work in the C90A. Maybe you’ll need to use the Nav lights instead (and then turn them off if your next flight is in daylight).

Review the model C90A’s (Figure 5) and newer 350’s (Figure 6) co-pilot’s left-hand subpanels. Notice that in both models the pilot has manual control of Bleed Air temperature but not “Auto,” without generators. We will receive inflow for pressurization and heating only from the left side’s Flow Pack. A confusing realization in the C90A system is that both the Air Conditioning and the Electric Heater systems receive power from the Center Bus … and it is not a bus that is shed! Rest easy, however – a special circuit causes those two heavy-load items also to be unavailable.

Lastly, look at the fuel panels of the 350 (Figure 7), then the C90A (Figure 8). Oops! The memo about using the white circles must never have gotten to the silk-screening folks who did the C90A’s fuel panel! Nor did it get to the guy or gal who labels the switches on the pedestal. The later C90B and C90GT models finally did get a circle on every switch on the fuel panel … as they should have had all along.

Fred Scott, a C90A owner-pilot from Virginia, had the electroluminescent panels in his lovely King Air redone to a better-than-new condition by a shop specializing in that process. It took a while for Fred to recognize that all the circles were gone! Reading the thread on BeechTalk, then talking and emailing with me, Fred had some stick-on circles manufactured and painstakingly applied them to all the proper switches … including the fuel panel. He spent a lot of time researching the bus components and learned a lot from doing so. Figure 9 shows a photo of him positioning one of the circle stickers. The circles he added totaled up to 33. I admire your effort and results, Fred!

Here are a couple of additional switch-related comments. First, as you likely already know, the switches with the round silver base are a combination switch and circuit breaker. If excessive current flow causes too much heat, the switch will spring down to the off position. The amperage value at which it should trip is the number that you can see stamped on the end of the switch shaft.

Second, yes indeed, you can see red on some of the switches. I believe this is the color of a rubber seal that prevents dirt and grime from entering the switch. I have never observed the red seal on a CB switch, but they are quite common to find elsewhere.

I’ll close with a short quiz: Which switches on your B200 should have white circles? Depending on your point of view, the answer should be “all of them” – since everything keeps working following a dual generator failure – or “none of them,” since there is no such thing as Automatic Load Shedding and you know that it is you alone who must shed the undesired items. I am personally happy that the 200 panels are not cluttered with useless circles!

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.” 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 book, 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.ne