The expanded, first-flight-of-the-day checklist procedures in the Pilot’s Operating Handbook (POH) include a somewhat lengthy and hard-to-understand series of steps for confirming that the electrical system is fit for duty with all its safety devices operating properly. My guess is that this procedure is executed quite rarely and, when it is done, the pilot is going through the steps without truly understanding what is being accomplished. The aim of this month’s article is to provide some insight and clarity into what is being tested in this series of steps.
The five-bus system is the name I use to refer to the newer King Air electrical system that first appeared in 1978 on the F90 model. Since that time it has become the system on the C90A and after series, as well as the 300-series – both the “straight” 300 and the B300, which is known as the 350 (and now, 360). The latest members of the 200-series, including the 260, retain the electrical system of their predecessors. Hall-effect devices, automatic load-shedding and bus tie relays are unknown on BE-200s, as well as on all the 100-series and the earlier members of the 90-series. Skip this article if your King Air does not contain a “triple-fed bus.” (Many folks refer to this newer system as the triple-fed bus system. In fact, the 350 and 360 even have a sixth bus, but overall the system functions exactly like it’s five-bus brothers.)
The full test procedure spelled out in the POH takes place after both engines are started and both generators are operating. Of course, we should never turn on a generator while using an External Power Unit (EPU) so if we started with such a unit, it needs to be turned off and disconnected before proceeding. In fact, since observing the battery charging or not charging is a part of the tests to come, I recommend that we not use an EPU for this start. Make it a normal battery start of the first engine and a generator-assisted start of the second engine. That leads to the battery charging at a rather high level.
During the test procedure we want the inverter to be on if we have one. (Both inverters in 350s that have two.) The latest models have gone entirely DC (Direct Current) with no need for an inverter providing AC (Alternating Current). However, we do not want the avionics on yet. Not to say that they will be damaged during the tests to come, but since there will be momentary bus power interruptions it is preferable to not have the avionics subjected to any power disturbances.
Also, do not activate the environmental system yet. Leave the Cabin Temp Mode selector in the OFF position. Otherwise, we may cycle the air conditioning compressor off and on during the process.
The procedure begins by moving the GEN TIES switch – located on the pilot’s left subpanel – down to the OPEN, bottom position. This switch has three positions: OPEN at the bottom, NORMal in the center, and MANual CLOSE at the top. This is a lever-lock style of switch that must be pulled out before it will move. However, it springs from the top back to the center when momentarily selected to MAN CLOSE. In the case of the OPEN position, it must be pulled both to go there and to get out of that position.
When the switch is in OPEN, both the left and right generator bus tie relays should open. Both left and right Gen Tie Open Caution (yellow) annunciators should illuminate, and we check that they have appeared. I like to teach that we have now created two separate, single-engine airplanes. Yes, it’s corny, but it helps get across the idea that the left and right sides of the electrical system are no longer connected. They aren’t communicating with each other. Also, since the battery connects to the center bus – a bus that can no longer receive power from either generator due to the ties being open – it should no longer be charging. If your particular serial number King Air has a dedicated battery ammeter in the overhead panel then merely verify that the charging current is now zero.
For earlier models that do not have a battery ammeter, the way we must verify that the battery is indeed no longer charging is to note the loadmeter readings before we move the Gen Ties switch to OPEN and then note the reading afterward. There should be a major drop in generator load, generator output. Afterall, except for some lights perhaps, we haven’t activated any aircraft system, so the battery’s charging is the only significant electrical draw that we have.
One time in a zillion we may find that the battery is still charging. Not good! This is an important squawk for maintenance to repair. Either one of the bus tie relays is still closed – which is highly unlikely if the annunciator appeared – or else a diode is allowing current to flow in the direction it shouldn’t.
To truly verify that our two single-engine airplanes are no longer connected, we turn on one side’s windshield heat switch and verify that the loadmeter increases only on that side. Next, turn off that side’s windshield heat switch and turn on the other side to see the other loadmeter increase. Keep in mind that if the windshield’s temperature sensor is already feeling more than 100°F the heat will not activate. Wait for a cooler day!
Next, to verify that each of our single-engine airplanes has a functioning voltage regulator, we check the reading on the Left Gen and Right Gen positions of our six-position voltmeter in the overhead panel. Both should show correct, 28.25 ± 0.25 volts. (As if we can read it that accurately!) Voltage regulation is one of the many functions of the left and right Generator Control Unit (GCU). We are making sure that function works correctly even when the GCUs are not trying to parallel their generator’s output loads.
The next step is to move the GEN TIES switch from OPEN back to NORM. Both Bus Tie Open annunciators should disappear and the loadmeters should nicely parallel. Momentarily turn on either side’s windshield heat and verify that both loadmeters show a slight increase. Good! Our two single-engine airplanes have become one twin-engine again.
Now we reach for the switch just to the left of the GEN TIES switch, the one labeled BUS SENSE. It is spring-loaded to the center, normal position. Down is labeled TEST and up is labeled RESET. Tap the switch down to TEST.
What is being accomplished here is making the Hall-effect devices (HEDs) experience a magnetic field similar in strength to the field they would feel when 275 amperes or more passes through them. These HEDs – we have three of them – are the protective devices that will open an associated relay to stop the flow of excessive electrical current in the “wrong” direction. Unlike a fuse, current limiter or circuit breaker, these protective devices do not react to heat buildup. As you have experienced driving your old car under a high-power set of wires with the AM radio playing, electric current through a wire creates a magnetic field around the wire. The stronger the current the stronger the field. Mr. Hall, of Johns Hopkins University, was the fellow who first quantified this relationship. Not until the 20th century were devices invented that could measure current flow extremely accurately by measuring the magnetic field strength. When the price of these devices made them within reach of aircraft designers – Voila! – HEDs appeared on some airplanes’ electrical systems, including King Airs. They provide an increased level of safety since they react so fast, before the temperature even begins to rise to the level needed to melt a fuse or to pop a circuit breaker.
In the previous paragraph I used the term “wrong” while referring to the direction of current flow. HEDs only respond to excessive current in the undesired, wrong direction. To explain further: Except for the power needed by the starter to turn the engine, it is undesirable for the battery to discharge quickly at a high rate of current flow. On the other hand, after the battery has been partially depleted during the start, it is desirable to have it receive current at a high rate for recharging itself. Similarly, since the charging current comes from the generators, it is normal to have a very high current flowing from the generators into the battery after starting. To get from the source – the generators – to the draw – the battery – current must flow from each generator through its respective generator bus to the center bus to which the battery connects. So at times it is normal and expected to have high current flow from a generator bus into the center bus.
But having high current flow from the center bus into a generator bus is rare and almost always would be due to a problem: That missing wrench finally bridged the metal of that bus to the airframe … a direct ground short. To conclude, all three HEDs – the two between the respective generator buses and the center bus and the one between the battery and the center bus – only react when more than 275 amps of current flows in the wrong or bad direction.
The time that it is normal to experience very high current flow out of the battery is, of course, during engine starting. All three HEDs are “desensitized” – they take a coffee break and won’t work – whenever either starter switch is on.
Back to the test procedure: Since the HEDs react instantaneously, the BUS SENSE switch should only be momentarily depressed – I used the word “tap” above – and then released. The HED can be damaged if it senses the strong magnetic field too long.
After we “tap” the switch, all three HEDs should cause their respective relays to open, and we should now see three yellow annunciators: the same two we saw before – Left and Right GEN TIE OPEN – and also now BAT TIE OPEN. With these three relays open, the center bus should be totally isolated from the three sources of power, two generators and the battery. To verify that it is indeed “gone,” we again direct our attention to the voltmeter and confirm that there is zero voltage on the center bus. If voltage is still showing, then one of our annunciators is lying. A more likely malfunction is that one of the three annunciators did not appear when we used the BUS SENSE switch’s TEST position: Probably a bad HED.
A note to F90 operators: In the electrical system installed on earlier F90s – the very first attempt Beech made to use this newer system – some problems exist. I won’t call them “mistakes,” but the temptation is strong to do so! The two test switches are not labeled the same as I have described although they operate identically. The voltmeter is missing two positions, including the center bus. The annunciators are really weird! Instead of the simplicity of separate Left and Right GEN TIE OPEN lights, you have a single GEN TIES OPEN light. Notice that its plural, not singular. When both ties are open – such as before start and after we activate the BUS SENSE OPEN switch – the annunciator is correct in being plural. But 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 momentarily pushed the BUS SENSE switch down to TEST, released it promptly, checked for the three Caution 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.) Nowhere 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 electrical system will automatically shed a lot of electrical load to prolong the time your battery can supply more critical items. It does this primarily 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 battery 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 designers believed (rightly so!) that losing 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 automatically 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 inverter’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 to suggest. But it is super easy and it verifies that the other source of inverter power, the alternate source, is working properly. Remember, if the inverter keeps working after the center bus goes dead, we have learned that the normal inverter power source is OK. To verify that the alternate source is also OK, simply turn on one inverter with only the battery switch on with no bus ties closed. Does the inverter light extinguish, and the inverter works fine? Great! Now check the other inverter and, I hope, find that it also can operate. Twice in my King Air instructing travels I found F90s in which the inverter would not operate without the generator buses being powered. They had lost their alternate inverter power source. I ask you, maybe a couple of times a year, to verify that both of your inverters operate after you have turned the battery switch on but nothing else. Do it occasionally as part of your cockpit checks in the hangar.
To complete the Beech After Start electrical checks procedure, all that remains is to momentarily tap the BUS SENSE switch up to RESET to allow all three bus tie relays to return to their closed status. Ah, now at last it’s time for avionics and air conditioning! The electrical checks are finished.
I won’t ask for a show of hands to indicate how many of you run this check on your day’s first flight. There wouldn’t be many raised hands, would there? Guess what? My hand wouldn’t be up either.
My educated opinion concerning this check as well as run-up items like autofeather, rudder boost and overspeed governor checks is that even if they have been tested 20 times in the last hour, it is impossible to know if they will work the next time. Before going in for a Phase check, especially coming out of a Phase check or other invasive maintenance procedure, take the time and run ALL of the checks completely, slowly and with understanding of what is right and wrong. Normal operation, however? I am not an advocate of the time, fuel and noise it takes to do all checks routinely. On a deadhead leg once a month or so? Now you’re making sense!