If you are not currently flying a member of the 200-series family, up to serial number BB-1459, then this article is not for you. You won’t hurt my feelings if you immediately jump to the next article in this magazine. But since this quite important topic applies to well over 1,000 airplanes, I think it merits my addressing the topic in this article.
Many of you may recall the horrific crash of a King Air 200 that happened January 27, 2001. The airplane was departing Denver’s Jefferson County airport (KBJC, now known as Rocky Mountain Metro) at night on an IFR flight to Stillwater, Oklahoma, (KSWO) with eight seats filled with players, coaches and broadcasters of the Oklahoma State University (OSU) Cowboys basketball team. Two pilots were in front but the right seater was quite new to the 200 and was primarily going along to gain experience. The left seater was an experienced pilot who had flown this exact airplane often. The airplane crashed at high speed into a field about 40 miles from the airport.
The NTSB report states that the probable cause was “the pilot’s spatial disorientation resulting from his failure to maintain positive manual control of the airplane with the available flight instrumentation. Contributing to the cause of the accident was the loss of AC electrical power during instrument meteorological conditions.”
We will never know the exact set of conditions that brought this BE-200 to its doom, yet I have an idea of the circumstances involved. Based on what you are about to read here, I hope that I can make a convincing argument for you to select the No. 2 inverter for your flights in darkness.
Before continuing, let me be clear that what I am about to describe is very unlikely – a possibility that would be rare to experience even once in a lifetime of flying. Yet the Beech engineers recognized that this possibility does indeed exist and based on that recognition safety devices – in this case, circuit breakers (CBs) – were included in the design. Specifically, we are talking about a “ground fault” on one of the dual fed buses.
A quick review of the 200’s electrical system: Two generators and a single battery are the three normal sources of DC (Direct Current) power. The generators feed directly into their respective left and right generator buses or main buses. These buses include most of the electrical components that consume larger amounts of power, such as the landing gear motor, windshield heat, avionics and inverters. (Although modern avionics and inverters are not nearly as electrically “thirsty” as their older predecessors.) The smaller electrical users are distributed among four “dual fed” buses, so named since they are fed from both the left and the right generator buses. Dual fed buses No.1 and No. 2 are mostly located on the right sidewall of the cockpit whereas Dual fed buses No. 3 and No. 4 are on the left sidewall, near the fuel panel. The two feeder wires to each bus has a 50-amp circuit breaker designed to trip, stop current flow and hence protect the wiring from getting so hot as to cause a fire in the event of an electrical short on the bus itself. (That missing screwdriver finally touched the bus bar to the airframe!)
Diodes – one-way devices for electrical current flow – permit both generator buses to feed the subpanel buses through the 50-amp CBs but prevent one generator bus from feeding back into the other generator bus … which would compromise redundancy and safety.
I have found some errors in the subpanel wiring throughout my many years of providing King Air instruction. It has been quite common to discover that one of the dual fed buses is receiving power from only one of its feeds. Once, I even found an airplane in which the No. 1 and No. 2 dual fed buses had been bridged together! All four feeder CBs were applying power to both buses.
Although the wiring should be checked during some of the maintenance checks, it is such an easy thing to do that I encourage you pilots to conduct the checks yourself, occasionally. Here’s how: Find the two 50-amp CBs for dual fed bus No. 1 on the right sidewall. Turn on the airplane’s battery, then pull one of the two CBs … let’s say the right one. Nothing should happen. This verifies that the other feeder CB – the left one that isn’t pulled yet – is indeed providing power to the bus. Now also pull the left feeder CB. By doing this, the bus should “die.” You may notice the left oil temperature and pressure gauge drop to zero … but why did the avionics just come on?! Because the avionics master CB gets power from the No. 1 fuel fed bus – the one that just died – and the avionics power relays are the normally closed type, not the more common normally open type. (I am sure you have been taught that pulling the avionics master CB is equivalent to turning ON the avionics master switch. Right? If the switch ever shorts out internally, causing the avionics to die, this is the “workaround” fix.)
By noting that the No. 1 dual fed bus did not fail when we pulled the first (right) feeder CB, we know that the left CB is able to power the bus. But for all we know the right CB was dead at the start. To verify that is not the case – that the bus is truly dual fed – reset the right CB first and make sure the bus again receives power. See how easy it is to check that the dual fed bus indeed has its two sources? So now continue doing this for the remaining No. 2 dual fed bus on the right sidewall, as well as dual fed fuses No. 3 and No. 4 on the left sidewall. The fuel gauges are the easiest way to tell if the No. 3 or No. 4 dual fed buses is or is not receiving power.
(There is a training video about these CBs on the
King Air Academy’s website at:
https://youtu.be/Iw8wvxxHIe4)
Back to the OSU 200 crash. Here is pure speculation on my part about what might have precipitated the tragedy. Although the No. 1 inverter receives its power from the left main bus, the inverter control switch’s No. 1 “up” position gets powered by the No. 1 dual fed bus. A loss of power to that switch will result in a dead inverter.
Here’s a possible scenario: As the airplane’s autopilot is just leveling off at FL230, perhaps something happens in the cabin that causes the pilot to turn in his seat to address the cabin situation. The right-seater, being new to the benefits of crew coordination, does not realize how important it is for him to keep “minding the store” and he, too, turns his attention aft. Now the unthinkable happens: The No. 1 dual fed bus dies. The No. 1 inverter loses power, causing the autopilot to trip offline. The accident investigation showed that the altitude encoder stopped sending altitude information to ATC moments before the spiral dive began. The encoder is dependent on AC power, lending credence to the inverter failure theory.
Finally, the pilot returns his view to the cockpit … perhaps wondering why the air noise has increased and/or why he’s feeling some G-forces. What the …?! The only lighting he sees are the engine instruments and the avionics panel! Both his side’s flight instruments are dark as well as the copilot’s side! Quick! Let’s turn the overhead flood light and instrument indirect light rheostats clockwise to the full bright position. But they’re dead also! By the time a flashlight is located, the spiral drive has found the Colorado ground.
Yes, friends, it is difficult to understand the designers’ thinking of why so much cockpit lighting would receive power from the same source! Thanks to BeechTalk contributor Matt Cox I think this wiring design was changed starting with BB-1459, a 1993 model. That has not been verified.
So here’s your homework assignment: Go to your 200/B200 in a dark hangar and turn on the battery and all of your cockpit lighting rheostats … all the ones in the row beside the master lights rocker switch, as well as the two in the top row. Select No. 1 inverter and make sure it begins working. Now pull both of the 50-amp CBs for the No. 1 dual fed bus. I predict you will find yourself in the precarious situation I described above … inverter failure and almost total panel lights failure. I hope I’m wrong! I hope many/most of you will find that enough lighting is still available to hand-fly the airplane successfully until you can select Inverter No. 2, allow the gyros enough time to re-erect, and then restore the autopilot. For those serial numbers past BB-1458 I am almost certain you’ll be OK.
For the others, who find what I have described is indeed factual, then I have a strong request of you: Use the No. 2 inverter for night flights!
By doing so, a failure of the No. 1 dual fed bus will kill your imp-ortant lights, yes, but it won’t fail the inverter and autopilot. Good ol’ “George” can keep handling the flying while you locate the flashlight and get it aimed as you like. And please make sure the batteries in the flashlight are in good shape, yes?
In closing, it surely seems to me that Textron or Raisbeck or Blackhawk or ???? should offer a Service Bulletin or a kit to relocate at least the indirect and flood lights to dual fed bus No. 2 on the 200s and B200s that exhibit this potentially dangerous behavior.