The fuel system on the new C90GTx models that are being delivered in the year 2020 is nearly identical to the fuel system that first appeared on the King Air A90 when it made its appearance in 1966 … a “mere” 54 years ago. The A90’s predecessor – the Straight 90, the first King Air model of all – had a similar but different system, one that had two electric boost pumps per side instead of one. Yes, from the A90 to the C90GTx there are some significant differences – many of which will be presented in this article – but overall, the system remains the same. Whether you fly an A90, B90, C90, C90-1, C90A, C90SE, C90B, C90GT, C90GTi or C90GTx (did I forget any?), this article is for you. Other King Air models? Feel free to read these pages, but they will not be directly addressing the fuel system of other models.
I will begin by reviewing the tanks, all of which are rubberized bladders. For most of these models the POM or POH lists the nacelle tank quantity as 61 gallons and the wing tank quantity as 131 gallons. That yields 192 usable fuel gallons per side or 384 gallons total. All A90s and B90s, and C90s up until LJ-529, had individual gauges that displayed left and right nacelle quantity and left and right wing tank quantity – a total of four gauges. There were marks on the face of the gauge showing Empty and Full as well as quarter, half and three-quarter quantity; no numbers for gallons nor pounds were presented. The gauges received information from float-type sensors in the tanks. As is true for most float-style gauges, their accuracy and reliability were rather horrid. Starting with LJ-529, the gauging system was changed to one that used capacitance probes instead of floats. (Note: LJ-502, the very first C90, was the prototype test airplane for this system so it, too, has the newer style system.) The two separate gauges per side were replaced with a single gauge per side that reads in pounds of fuel. The scale went up to 1,400 pounds. Since one side’s full fuel would be 1,286 pounds – using the common conversion factor of 6.7 pounds per gallon of Jet A fuel – there should never be a situation in which 1,400 would be insufficient to display the side’s total quantity, even when the fuel is very cold and hence very dense. A two-position toggle switch lies between the two gauges. Its top position is labeled “Total” and its bottom position is labeled “Nacelle.”
I want to divert for a moment to insert a critical warning. For you pilots who perhaps fly a 200 or 350 on a regular basis and only do occasional fill-in duty in a C90, there is a nasty “gotcha” waiting for you on that fuel panel. You are used to using a switch – not a two-position toggle, but rather a two-position switch spring-loaded to the upper position – that reads the main tank quantity in its normal, “Up” position and reads aux tank quantity when held down against the spring force that holds it up. There is no “total” reading. Of course, most of the time the auxes are empty, so the main reading is the total reading.
There is a possibility that a fuel exhaustion accident that put a C90 into the Caribbean was caused by the pilot’s misunderstanding of the gauges. It would be easy to think that the 800 pounds showing with the switch in the “Up” position was main quantity and the 400 pounds showing with the switch in the “Down” position was aux. Hmmm … 800 plus 400 is 1,200. Yeah, I have close to full fuel on board, good to go. But no! The 800 pounds being read was the total on that side. Instead of 2,400 pounds the plane departed with 1,600 pounds … not quite enough to complete the long overwater leg.
Please read the labels on the selector switch carefully and remember that in the C90 system the nacelle reading is already included in the total reading.
Why do we even have a reading for nacelle quantity? So long as we know the total per side, that’s enough, right? Never let the total reach zero and we will never run out of fuel, right?
Wrong! Realize that every bit of fuel the engine ever consumes comes from a nacelle tank. If that tank reaches zero, we are either going to lose engine power or we must use crossfeed to supply the fuel from the other nacelle tank.
Since wing tank fuel is useless until it gets transferred into the nacelle tank, how is that accomplished? Two ways: (1) The electric transfer pump; (2) Gravity flow.
Since liquid “seeks its own level” the nacelle level must drop before any wing fuel flows into the nacelle. When one looks at the location of the two fuel filler caps – one about half-way out on the outboard wing and the other atop the nacelle – it seems that the nacelle level is only marginally above the top of the wing fuel. In regard to height, that is correct. But in regard to volume, it’s not even close. Realize that the nacelle tank becomes quite narrow near its bottom. Why? To allow room for the main wheel and tire to retract. Most of the nacelle tank’s volume is in the upper half. Before gravity flow moves fuel from the wings into the nacelle, the nacelle tank must decrease to about 150 pounds, or 3/8 full.
Think again of the 200- or 350-experienced pilot filling in on a C90. In the bigger models, fuel transfer from aux to main is automated, usually requiring no pilot action at all. If this pilot forgets that the C90 is different, he or she may take off without turning the transfer pump switches on. When will the mistake manifest itself? Probably not for a long, long time.
First, the no fuel transfer annunciator(s) receive no power with the transfer pump switch(es) off, so no warning light will advise of the error. Looking at the total or nacelle quantity on the gauge will show no problems for at least 30 minutes. In fact, if the pilot is not quite familiar with normal readings on the gauge, he/she could go for over two hours without noticing anything amiss. So, what would/should be noticed? This: That the nacelle tank level is not staying between 300 and 400 pounds.
Without the transfer pump keeping the nacelle relatively full, its quantity will drop to the 150-pound figure mentioned previously. A rule-of-thumb is that a PT6A-20 or -21-powered 90 will use about 600 pounds the first hour, total – 300 pounds per side (closer to 700 pounds for the -135As of the GTs). Thus, within the first hour it should be obvious that something is wrong since the nacelle will have decreased to about 150 pounds. “Gee, the nacelle is going to be empty in less than another hour if it keeps decreasing at the rate I’ve observed!” No, it won’t. Remember that the 150-pound figure represents the point at which gravity flow will begin. As the nacelle level decreases further, the wing fuel will seek its own level and start to be used. You might say that we are now drawing fuel out of a very large tank since all five bladders contain fuel at the same level. What’s wrong with this?
Actually, not much. Rarely do we land with less than 300 pounds of fuel remaining per side and usually it is quite a bit more. But here is the important fact to know: When the nacelle tank goes empty, reading zero, the total quantity should still be reading about 200 pounds. This 200 pounds – actually, 28 gallons or 188 pounds – is sitting too low in the center section wing tank to flow by gravity feed to the nacelle. The legal and comfortable fuel reserve for which you planned has been significantly compromised, hasn’t it?
What is the takeaway here? In addition to not making the mistake of forgetting to activate the transfer pumps, I am trying to emphasize the importance of regularly checking not just the total fuel quantity but the nacelle quantity also. That step should be done at least every half-hour. I tend to combine it with a deliberate check of the loadmeters and voltmeter.
These impeller-type pumps (one per side) are located at the low spot of the center section tank … the lowest spot in the wing tank complex of bladders. In normal operation the pump cycles on and off so as to keep the nacelle tank within about 10 gallons full. Since full nacelle fuel is 61 gallons or 409 pounds, when it drops to about 51 gallons or 342 pounds the pump should automatically activate to refill the tank. The pump has a 550 pounds per hour (pph) flow rate so even with the engine at a high power setting the pump will be able to supply more fuel into the nacelle tank than the engine is taking out. There are actually two high-level shutoff switches in the nacelle. If we had only one and it failed, the pump would run continuously, decreasing its expected lifetime. (There are vent lines at the top of the nacelle that allow extra fuel to return to the wing tanks so no pressurizing of the tanks nor venting overboard of fuel would occur.) One high-level switch should activate a tad below 60 gallons and the other a tad above.
Let’s be realistic. With the typically not-too-stellar accuracy of even the capacitance type fuel gauges, can you tell the difference – the difference of a couple of gallons? Of course not! But here’s what’s important: Every time you check the nacelle fuel level it should be between about 300 and 400 pounds. If ever it drops below 300, be aware that something is probably wrong … either the gauge is in error or the transfer pump is not working. (As discussed, the inoperative transfer pump may be because we forgot to turn it on!) Similarly, there is a problem if the nacelle is always reading 400 pounds … the transfer pump seems to be running continuously.
The C90A and after models use three-position transfer pump switches – override, auto and off. The previous models had only two positions – on and off. The “On” position is exactly like the “Auto” position … that the pump cycles on and off as required to keep the nacelle within about 10 gallons of full. The “Override” position keeps the pump running 100% of the time, keeping the nacelle 100% full and returning the excess back where it came from via the vent lines. (More about the usefulness of this is to come.)
If we did indeed turn the transfer pump switches to “On” or “Auto” after starting as we should, then here is how the system should operate. Unless one of the high-level switches in the nacelle is activated, then the pump will begin immediately. It does not wait for the low-level switch to be hit; it starts now unless it is broken or the nacelle is full. Once the nacelle is full – one of the two high-level switches activated – then the pump stops until triggered by the low-level switch.
In the line connecting the transfer pump to the nacelle tank is a pressure switch. The switch’s setting is at about 2.5 psi, not that it matters very much. The pump typically puts out about 25 psi of pressure. When the pump begins pumping air instead of fuel – as the wing tank complex of bladders is finally empty – the pressure drops below 2.5 psi. A signal is sent to an electrical printed circuit board (PCB) that activates a 30-second timer. Anytime within the next 30 seconds, if the pressure rises above 2.5 psi the system resets and returns to normal. This allows for the momentary lack of pressure due to sloshing fuel when the tank is approaching empty but is not yet totally there.
When the output pressure drops below 2.5 psi and stays low for the full 30 seconds, then the printed circuit board does two things. First, it turns off the pump to prevent possible damage caused by the pump spinning too fast and creating excessive heat. Second, it activates the “No Transfer” annunciator. In my opinion, this light does not warrant the warning, red status, yet in all models up to the C90B it is indeed a warning light. In the C90B and later models it was changed to a caution, yellow light. Good move, Beech!
As all of you have probably experienced near the end of a long flight – or even a short one if you have a large cabin load – it is expected and normal to have the left and right “No Transfer” annunciators illuminate when the total fuel quantity gets down to just nacelle fuel. With the main tanks empty and the low-level switch activated, the pumping of air obviously takes place. It is fun to see how much time passes between when the first side’s and second side’s light appear. How close was the consumption rate of the two engines? How even was the left and right fuel quantity when the fueler finished doing his task?
When the first “No Transfer” light appears – as was expected; no surprise – the proper action is to turn off the transfer pump switch. This does two good things: First, it assures that the pump is indeed no longer running and perhaps shortening its life. Second, it kills the annunciator. No need for that light to stay on, burning out the bulbs and making the passengers nervous. (Remember what I wrote earlier? If we never turn on the transfer pump switch the annunciator cannot operate, cannot give us a reminder of our mistake.)
Another thing to do when that first “No Transfer” light illuminates is to start a timer. If you are shut down on the ramp within no more than one hour from now, you will never run out of fuel. Yes, “never” is a powerful word and if the nacelle bladder sprang a big leak now, I reckon we could still run out of fuel. As we have presented, the nacelle should contain at least 51 gallons of fuel when the transfer pump started pumping air due to the wing tanks being empty. Even the powerful -135A engines won’t burn that much fuel in an hour, especially since some of this time will be at the lower powers associated with the descent, approach, and landing phases of the flight.
What about the times when we have more total than nacelle fuel so we are not expecting the “No Transfer” annunciator yet it illuminates? This indicates failure of that side’s transfer pump. Expect the nacelle level to drop significantly so that gravity feed begins and keep a careful eye on nacelle tank quantity. You need to be shutdown well before it reads zero, even though the total will still be reading about 200 pounds when the nacelle goes empty. (Reading the 28 gallons that won’t flow uphill.)
To desire an absolutely full fuel load is somewhat rare unless we have a max gross takeoff weight limit at or near 10,500 pounds or we have a light cabin load. But presume that we do indeed desire to top the tanks entirely on our next flight. There is a neat little trick that saves the fuelers’ time – and perhaps our nacelle’s paint – that works when we start the descent for landing if the “No Transfer” lights had not previously illuminated. If you have a C90A or after, just place the left and right transfer pump switches up to “Override.” Doing that will force the transfer pumps to run continually until we complete the shutdown checklist. By doing this, we will know that the nacelle tanks are totally full so all the lineperson must do is top off the wing fillers. There’ll be no need to drag the hose on the nacelle’s nice new paint job.
Almost the same thing can be accomplished in the earlier models that have just the “On” and “Off” transfer pump switch positions. Here’s how: Leave the switches in the ON position and tap the transfer test switch out of its spring-loaded, center position to both the left and right sides. I will talk more about this test switch in the remaining paragraphs, but for now what this action does is to start the transfer pump immediately, without waiting for the low-level switch to be activated. Once activated, the pump won’t stop until the high-level switch is hit. Since there is no way to accurately know if the nacelle tank is totally full or just relatively close to being full, tap that switch again a couple or three times during the approach and landing before you stop on the ramp. By doing so you can guarantee the nacelle tank will be darn close to full.
Now about that transfer test switch …
Somewhat like the “No Transfer” annunciator, the transfer pump switch must be “On” for this switch to have any function. What exactly does it do? Two things. First – as was taught in a previous paragraph – it starts the pump running immediately, without waiting for the low-level switch to be activated. (If you read carefully what has been previously written in this article, the same outcome could be achieved by cycling the transfer pump switch to “Off” and then back to “On.”) Second, the transfer test switch changes the operation of the “No Transfer” light. The 30-second delay and the 30-second shutting off the pump are eliminated entirely. Instead – and quite simply – the “No Transfer” light will be on if the pump’s output pressure is less than 2.5 psi and it will be off if the pressure is above that 2.5 psi switch’s setting.
Based on this insight into what the switch accomplishes, there are three different scenarios that may be encountered when the switch is tapped to the side of interest. (1) Nothing noticeable takes place; no illumination of the “No Transfer” annunciator. (2) The “No Transfer” annunciator blinks on for a moment then goes back off. (3) The “No Transfer” annunciator comes on and remains on with steady illumination.
The explanation for scenario No. 1 is that the pump was already in the process of filling the nacelle tank. The light never came on because the pressure was already above 2.5 psi. Situation No. 2 indicates that the pump was still in a “waiting” state, waiting for the low-level switch to tell it to start resupplying the nacelle. Without the pump running, there was less than 2.5 psi at the sensor switch so the annunciator came on. Immediately however, the pump began operation, output pressure rose above 2.5 psi, and the light extinguished. No. 3 indicates one of two things – try to figure out the reasons before I present them …
The first reason is that the pump is inoperative. Its motor gave up the ghost or perhaps its impeller is jammed and cannot rotate – time to have maintenance do some troubleshooting. The second reason is that the wing tanks are still empty from a previous flight or from some maintenance that involved draining the wing tanks. How did you do on this pop quiz?
Here’s another quick quiz. Why is scenario No. 2 – the blink of the annunciator when the switch is tapped – so often followed by scenario No. 1 – no light at all – when you hit the test switch more than once? Answer: The first tap of the test switch took place when the pump was waiting to operate because the low-level switch had not yet been reached. Remember that once the pump starts operating, it doesn’t stop until the high-level switch is hit. So, the pump was already running, putting out positive discharge pressure, when the subsequent taps of the switch were made.
A couple of closing thoughts:
The first-flight-of-the-day test of the transfer pumps is more satisfying and instructive by doing it in this manner: Take one hand and hold the transfer test switch to one side and use the other hand to now turn that side’s transfer pump switch to “On” or “Auto.” Unless the wing tanks are still empty, it should always result in a blink of the annunciator to verify that the pump is OK.
The second closing comment is to understand the danger inherent in the use of auto crossfeed. If you have not already done so, read “The Crossfeed Trap,” an article in the Clements’ Corner section of the King Air Academy’s website and view the King Air Academy’s YouTube video channel about that topic. I strongly encourage your standard operating practice be to leave the crossfeed switch in the bottom, closed position.