King Air Electrical Systems Evolution

King Air Electrical Systems Evolution

This article first appeared in the January 2012 issue of this magazine. It is also a chapter in The King Air Book – Volume II. It is significant enough to merit reprinting here both to enlighten newer readers, as well as to provide a review for seasoned King Air veterans.

The original King Air, the 65-90 model that came out in 1964, was basically a Queen Air with the Lycoming engines exchanged for the first version of the Pratt & Whitney PT6A powerplant and the fuselage modified and strengthened to allow for a meager pressurization system. Two years later, the A90 model replaced the “Straight 90,” and with it came a number of very significant improvements, including: A totally redesigned cockpit layout that contained an annunciator panel; reversing propellers were offered as an option (to the best of my knowledge no A90 was manufactured without that popular option); driving the pressurization system’s air compressor (supercharger) off of the left engine’s accessory case, mechanically, instead of via a hydraulic motor driven by a hydraulic pump on that same engine; and a redesign of the electrical system into one that continues, with various modifications and improvements, as the design is still in use on the King Air 250. The aim of this article is to present an overall view of the King Air’s electrical system and to elaborate on some significant changes that occurred along the way. I will also discuss the Five-Bus system that first appeared on the F90 model in 1978 and that continues in the current C90GTx and the 350-series.

There are four sources of DC (Direct Current) electric power in a PT6-powered King Air: (1) a single battery housed in the right wing’s center section, (2) in front of the main spar, (3) two identical engine-driven generators, and (4) sometimes, an External Power Unit (EPU). The B100 model, powered by the TPE331 engine, uses two batteries, one in each wing’s center section, due to the greater starter demand of its fixed-shaft turboprops.

The first battery used was a 19-cell Nickel-Cadmium without provision for air cooling. In the mid-1970s the standard factory-installed battery became a 20-cell NiCad and the battery box including ram air cooling. This was about the same time that the battery monitoring system was included, with its battery charge annunciator that could indicate the early stage of a thermal runaway. Improved lead-acid batteries first became popular as an STC’d replacement for the more-expensive and potentially more troublesome NiCads, and in the 1990s the factory discontinued the use of NiCads and went to a Concorde VRSLAB (Valve-Regulated, Sealed, Lead-Acid Battery) replacement. The battery charge annunciator is no longer required with a lead-acid battery since that battery does not have thermal runaway potential.

The battery is typically wired directly to a bus that is always “hot” with voltage whenever the battery is installed. This bus is named the “hot battery bus” and is usually located in or very near the battery box. Some convenience items as well as some components that were considered most important by the engineers receive power from this bus. These include the door and baggage compartment lights, boost pumps and crossfeed in the LJ-series, and, often, standby fuel pumps and fuel firewall shutoff valves. The fuel-related items that receive power from this hot battery bus, for redundancy, also receive power from their own fuel panel bus after the battery and/or generator is switched on, as we will see.

The generators, which of course do double duty as the starter motors, as well, are made by Lear-Siegler and originally were rated at 200 amperes maximum continuous output. Beginning with the C90- and the 100-series, better cooling ducts were incorporated that allowed the maximum generator output to be upped to 250 amps. Instead of presenting the generator output on a gauge that was marked in amps, the decision was made to mark the gauge with decimal equivalents relating to the maximum rating. For example, with the 250 amp generators,
a gauge reading of 0.20 would indicate a current output from the generator of 50 amps (250 amps maximum
x 0.20). In more recent years these gauges – load meters – are marked in units of percent, so the 0.20 reading would now show as 20%.

It is important to realize that there really is no actual limit on maximum generator output. If a direct ground short were to occur on the generator output line, current will immediately rise to 1,000 amps or more! This excessive workload would create too much heat that could, given enough time, cause the generator to undergo an ugly and expensive death. A lot of King Air pilots and even instructors suffer under the misconception that any ground short will be handled automatically without the need for pilot action. Not true. Although the melting of a large fuse – called a current limiter – will protect the opposite generator and the battery from harm, the only thing that saves the shorted generator from its ugly demise is the pilot turning off its switch … in reaction to the load meter on that side being pegged out at more than a 100% reading.

So, when it is stated that 250 amps is the maximum continuous output, it is really saying that this is the maximum output that may be sustained, under certain conditions, while still being able to maintain the correct output voltage and without overheating taking place. The “certain conditions” referenced here has to do with how much airflow is pumped through the generator by the cooling fan built into the back of the unit, where the cooling duct connects. The two factors that most determine the airflow are N1 speed and altitude. The fan is turning at the same percentage of its maximum speed as the N1 gauge reads. At an idle speed of 60%, for example, the fan is also turning 60% of its highest speed so it cannot provide the same cooling as it would at cruise power. (Actually, that is not totally correct since the maximum Ng speed of PT6s is either 101.5% or 104%, but close enough.) Of course, since air density decreases with altitude, less airflow takes place the higher one flies, even for the same N1 speed. This explains why the POH typically shows lower sustained load meter limits on the ground with idle speeds and in flight at very high altitudes.

The previous paragraph included the phrase, “… while still being able to maintain the correct output voltage …” The target voltage is 28.25 volts with an allowable tolerance of ± 0.25 volts. Thus, any voltage between 28.0 and 28.5 is proper. No instrument can ever be totally accurate and the typical King Air voltmeter can have up to a 0.5 volt error and yet still be considered acceptable. That explains why in some POHs the statement appears that, during the electrical system After-Starting checks, the voltage must be between 27.5 and 29.0. These numbers are derived by including the possible gauge error into the correct voltage range.

Early 90- Series

Prior to LJ-678 (C90), LW-157 (E90), and B-224 (A100), the voltage output of the generators was controlled by a carbon-pile type of device that was not nearly as accurate and predictable as what we have come to expect in the modern digital age. To keep voltage within proper limits and to make the generators share their current output equally – i.e., provide good paralleling of the load meters – were difficult aims to achieve and almost impossible to maintain over any significant period of time. However, beginning with the serial numbers noted above, a solid-state Generator Control Unit (GCU) was made standard equipment and what a nice difference it made! Now voltage rarely strays out of the expected range and generator paralleling is so perfect that an imbalance between the left and right load meters becomes the exception rather than rule.

One can recognize that the plane was built with the new GCUs by observing whether or not the generator control switch has the third, top, “Reset” position. That reset position means GCUs are installed. With the old-style system, the generator switch was merely On-Off, two positions, up and down.

Just as a municipality’s water pressure would drop if every resident opened every spigot and tap at the same time, causing too much water demand, also no generator can maintain proper voltage when its current outflow gets too large. Although this will happen in the event of a short to ground, a more common example of this is when one generator is assisting with the start of the opposite engine. Have you ever monitored voltage while you activate a starter switch? It surely makes a momentary big drop, doesn’t it? No wonder the lights go dim for a while.

When switched on, each generator feeds directly into its own main bus, also called the generator bus. As a general rule, with minor exceptions, the main buses have the electrical components that use higher amounts of current connected to them. Landing gear motor, air conditioning motor, windshield heat, cabin electric heat (when applicable) … are obvious candidates for being main bus items. Not quite so obvious are avionics buses, inverters and flap motors, but they, too, almost always are fed by a main bus.

Components that use only a small amount of electric power receive that power from subpanel buses. These buses, for redundancy, are in turn fed not just from one side’s main bus but from both. A circuit breaker (CB) rated at 50 amps protects the wire going from each main bus into the particular subpanel bus. Therefore, each subpanel has two, 50 amp, feeder CBs associated with it. With nothing else, however, this would compromise the separateness of the two main buses, since now there is a bridge between left and right sides via the subpanel and its two feed wires. What’s that I see riding over the horizon to our rescue? Why, it’s Sheriff Diode!

Yes, the necessary and often-used, one-way “checkvalve” for current flow, the lowly diode, is the device that allows both main buses to feed to the subpanel but do not allow current to flow from the subpanel back into the main bus. Every subpanel feeder CB has a diode between it and the subpanel to prevent return flow.

I speculate that the designer who made the decision of what small components would be wired to which of the two subpanels had his reasons for placing things as he did, but I’ll be darned if I know what the reasons were! Only when we get to the 100-series does logic seem to dictate the selection. For the A90, B90, C90, C90-1 and E90 systems, the only way to know which items are wired to which bus is to consult the electrical system schematic in the POH or Wiring Diagram Manual (WDM).

Whenever a technical writer takes a complex subject and tries to present it in an understandable manner to a non-technical reader, challenges arise. If the writer makes it too simple, often details are ignored that may be important for better understanding. On the other hand, if he tries to describe every minute detail, the reader is quickly lost or put to sleep! Likewise, the drawing of the POH’s electrical system schematic always becomes a compromise between accuracy and understanding. I personally think the Beechcraft POH writers did an excellent job at this compromise, although I know others may disagree.

In the POH’s schematic, each main and each subpanel bus is rendered as if it were a single strip of metal with all associated components wired off of the bus one-by-one, side-by-side. In the real airplane, often that is not the case. For example, although the majority of main bus items receive power from the vicinity of the cockpit console, the inverters tap off their power from an area in the main wheel wells.

So please take what I am about to write as a compromise between accuracy and understanding, OK? It is not technically correct in all respects but it will generally be helpful. Here goes: The main buses are underneath and inside the cockpit pedestal whereas the subpanel buses are generally in the area we call the instrument panel subpanels … the hard-mounted black metal ahead of the crewmembers’ knees, where lots of switches and CBs are located. It follows that the 50-amp subpanel feeder CBs, as well as CBs for things like landing gear motor and windshield heat are located on the aft end of the pedestal, whereas most of the subpanel items involve the switches and CBs in front of our knees.

There are two more cockpit locations of electric switches and/or circuit breakers: the left and right sidepanel areas. The left sidepanel, next to the pilot’s left elbow, has traditionally been devoted to the King Air’s fuel panel. The right sidepanel is devoted mostly to CBs or fuses that protect the engine instrument circuits.

The engine instruments are definitely subpanel-connected items.The fuel panel items, however? The reasoning behind the design has probably been lost in the mists of long ago times, but the electric power received by the left and right fuel panel buses come from two of the four subpanel feeder CBs, yet it comes via a separate wire branch that has nothing to do with either of the two subpanels. The diagram, shown above, may help explain.

As the A90 evolved into the B90 and as the B90 evolved into the C90, only minor changes occurred in the electrical system. One of the more significant changes, occurring at LJ-773, LW-278, B-241, and BE-41, involved a rewiring of the external power circuit such that the battery switch must be on before the external power relay can close, allowing the external power to flow into the airplane. Beech always specified that the battery switch should be on before the external power unit was energized and kept on while the EPU was in use, but in the later serials it is a physical requirement. By having the battery on line along with the EPU we have two, not just one, power sources. An advantage of this is that a start will continue successfully even if the EPU malfunctions and cuts out before the engine had reached its self-sustaining speed. Also, the battery provides a cushioning effect, permitting the airplane to experience less of any voltage fluctuation that the EPU might provide.

100- Series

In 1969, the first long-cabin King Air was added to the model lineup: the BE-100. Although the electrical system remained nearly identical to the one in the LJ-serials (B90 model) in production at that time, the decision was made to add more logic to the location of various smaller subpanel components. Whenever there were two identical components – for example, two pitot heaters, two bleed air flow packs, two oil temperature and pressure gauges, two start switches, etc. – the left one of these would always be wired to the same subpanel bus and the right one would receive its power from the other subpanel. No longer did the electrical schematic label them “Subpanel Bus No. 1” and “Subpanel Bus No. 2.” Instead, they became the “LH Loop” and the “RH Loop,” reflecting where the identical left and right components were connected.

Two comments need to be made here. First, due to the diodes, current cannot “loop” through the subpanels from one main bus to the other side’s main bus, as we have discussed. Yet, looking merely at the path of the wires it does seem that the subpanels form loops from one side to the other, so the “Loop” terminology was used. Second, there are so very many components that are not duplicated on both left and right sides, that there is still a lot of arbitrary selection that must occur: Where do we put the single stall warning heater circuit, the single nav light circuit, the single pressure control CB? So, both the LH Loop and the RH Loop contain many items that do not fit within the left-right separation.

Further, in the 100-series, the labels above the four, 50-amp subpanel feeder CBs on the aft end of the cockpit pedestal were totally changed. In the 90-series, the four breakers have a single label that covers the entire group of four – Subpanel Feeders. When an operator wants to designate a particular one of these four, he must say, “The far left one,” or “The second one from the right,” etc.

In the 100-series, however, to keep following the Loop concept, the four breakers are now labeled, in order from left to right: LH #1, RH #1, LH #2, and RH #2. The first and third of these are the first and second feeders for the LH Loop, and the second and fourth are the first and second feeders for the RH Loop. The first feeder for both loops comes from the left main bus and the second feeder comes from the right main bus. Until this unusual CB labeling is explained and understood correctly, it can be very confusing: How come a CB labeled “RH” originates from the left main bus?! However, when it is recognized that the LH and RH labels refer to the loops being fed whereas the #1 and #2 labels refer to the source side, clarity is achieved.

Now I’ll let you in on a surprising and weird little piece of design sadism that took place. Guess which subpanel CB has the branch going to the left fuel panel? The correct answer is “RH #1.” Vice versa, the “LH #2” CB feeds the right fuel panel! Although this is not as crazy as it first seems when one associates the #1 label with left side and #2 label with right side, the designer could just as easily have used “LH #1” to feed the left fuel panel and “RH #2” to feed the right fuel panel and the labeling mismatch would have been avoided. Oh well …


In 1972 Beech started working on the first Super King Air, the wonderful model 200 that has become the best-selling of all the King Air models. In addition to the obvious improvements of more power, more fuel, a longer wingspan and more pressurization capability, the engineers were also tasked with trying to improve on all systems and to make the airplane more maintenance-friendly.

One system improvement was the elimination of the fuel panel bus weakness of the 90- and 100-series: Namely, that the items thereon were not dual fed for redundancy but received power from a single subpanel feeder CB. To correct this, Beech went from two to four subpanel buses, with the names of, not surprisingly, Dual Fed Bus #1, Dual Fed Bus #2, Dual Fed Bus #3, and Dual Fed Bus #4. To keep the left-right thing as logical as possible, all left side items are now associated with an odd-numbered bus and all right-side items with an even-numbered bus. Dual Fed Buses #1 and #2 have all of their CBs and/or CB-switches on the cockpit’s right sidewall or on the instrument subpanels; #3 and #4, however, are exclusively located on the left sidewall, where the fuel panel is located. Unlike in the past, the CBs on the left sidewall would include some items not associated with the fuel system: Flaps and ignition, for example.

Also, the 200-series moved the main buses from the cockpit pedestal to a location in the cabin aisle, just aft of the main spar. A neat panel is located there under a clear plastic overlay that is painted with labels showing exactly what’s what. As in the past, two spare 325-amp isolation limiters are installed near the main buses to be available for on-the-road replacement.

The Five Bus System

It might be said that the model 200 was a Super 100 since it had the same cabin size but offered a tremendous increase in overall performance. The Beech engineers were given the directive to create a similar change for the 90-series: To develop a Super 90 that would be head and shoulders above its C90 and E90 siblings.

Off to the drawing boards they go. Wow! Here it is 1978 and we get to modify what was first designed in 1963! What improvements we can make! Let’s redo the nose profile to eliminate the “flat face” of the King Air and go with a sleek profile like that used on the Model 60 Duke. Let’s go with a wet-wing system and eliminate the bladder tanks. Let’s go with a whole new wing but using the landing gear from the 100-series, to allow for a higher gross weight. Let’s redesign the electrical system to make it more modern, copying a lot of what Boeing did on the 737. Let’s use the T-Tail Rudder Boost system, and cockpit layout of the 200, along with its automation of the fuel transfer system. Lastly, let’s use the 200’s stronger cabin door and dual-pane cabin windows so that we can increase the pressurization differential. We’ll call this model the F90.

Not many of my readers probably remember those days of the 1970s. The Carter presidency saw nearly runaway inflation and prices were soaring dramatically. After the Beechcraft financial analysts reviewed the changes that were planned, they concluded that this proposed F90 would need to cost more than its big brother, the 200, to account for these major changes that were to be paid for in late-1970s dollars! Oops, that cannot be! So, the engineers were sent back to the drawing boards with orders to tone it down, keep the price in line with reasonable expectations.

The pressurization increase, the T-Tail, the 200-style cockpit layout, the automated fuel transfer, the landing gear from the 100-series, and the 737-like electrical system were retained. The rest was tossed. So, let’s talk about this new electrical system that first appeared on the F90 but continues in the C90A, C90B, C90GT and all its variants, as well as on the entire 300-series.

Although this article cannot go into the depth required to really “teach” this new system, I will say that one of the few similarities to the older system is the presence of left and right generator (Main) buses. Instead of subpanel buses, we now have a triple-fed bus, a center bus, and, as before, a hot battery bus … five buses in total, with 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 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 so as 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 located on the hot battery bus, the triple fed bus, and the center bus, and thereby prolongs its life a great deal. (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.)

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 is often called the “Triple-Fed Bus System,” which is OK. Some, however, leave out the “Fed” and say the “Triple Bus System.” Huh? But there are five buses! I try to be consistent and always refer to this as the Five Bus System. (The 350 kinda, sorta, has a sixth bus, the dual fed bus, but it really 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 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 is on, or an EPU is in use, or the 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 the generator buses are on line, 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 have a better understanding of how the electrical system evolved in the particular model that you are flying.

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