Simple complexity
The Beech/Raytheon King Air series of turboprop twins has been a consistent bestseller since the fleet’s inception in 1963. The airplane has been a favorite of both pilots and passengers alike. The former like the King Air’s great handling qualities and well-thought-out panels and systems. Passengers like the comfortable cabin and feeling of security imparted by the King Air’s sturdy construction.
The King Air fuel system is fairly simple in design and operation. For the most part, it’s a set-and-forget system—in normal operations, of course. It has its own minipanel, located near your left elbow. The fuel control panel may look forbidding, what with all the dials, switches, lights, and circuit breakers, but after a little time in training you quickly understand the fuel system’s "simple complexity." Of course, as new models were introduced, changes to the fuel system were made over the years. But the basic theory behind the system essentially remains the same. What follows is a simplified description of the basics.
First, you’ve got a series of interconnected bladder tanks in the wings, all of which eventually gravity-feed to nacelle tanks near each engine. The outboard tanks feed inboard tanks, and these feed what are called center-section tanks (in the 90-series King Airs) or auxiliary tanks (in the 200- and 300-series King Airs), which, in turn, feed the nacelle tanks. From there, fuel is delivered to the engines.
Why, you may ask, do you need those Transfer Pump or Aux Transfer switches on the fuel control panel? Doesn’t the fuel make its way to the nacelles by gravity? Yes, it does, but to keep the nacelle tanks topped off to their 61-gallon (in the C90A) capacity, you need transfer pumps to keep enough fuel moving uphill from the center-section, or inboard, tanks. In 90-series King Airs, you keep the lever-lock transfer pump switches in the On or Auto position all the time; the B200s transfer fuel automatically, using self-pumping motive-flow and jet pumps. These pumps look like venturi tubes, and work rather like siphon pumps.
Here’s how the fuel transfer system works: The transfer or motive-flow pumps keep pumping to the nacelle tanks until they’re full, at which point a float switch turns the pump off. During a flight, these pumps turn on and off as fuel is burned from the nacelle tanks. In this way, the nacelle tank is always given plenty of fuel.
"Uh-oh," I hear you say, "what happens if those transfer pumps quit? Won’t the nacelle tanks run dry? And then won’t the engines flame out?" Not at all. If the pumps fail, you’ll get a No Fuel Transfer or No Transfer annunciator light when the nacelle fuel level drops to about the 22-gallon level. This signals a gravity-feed port to open, and this allows gravity flow to fill the nacelle tanks. One big caution here is that under pure gravity feed, your unusable fuel level rises to 28 gallons per side. That’s because the tanks feeding the nacelles are below the engine level, and gravity feed alone can’t get all of the fuel to the nacelle level.
In 200- and 300-series King Airs, if you get a No Transfer light and the transfer pumps are still operative (a test switch in the King Airs lets you ascertain that the pumps are working; move the Transfer Test switch to the left or right, and the transfer lights should momentarily light up to confirm this), a flow switch may have conked out. To keep the fuel moving, you move the Transfer switch from the Auto to the Override position. This puts the transfer pumps in continuous operation, and will keep the nacelle tanks topped off. Any excess fuel will be returned to the center-section tank via a vent line.
Of course, there’s another condition that will cause the No Fuel Transfer lights to come on. You guessed it: You ran the other fuel tanks empty, and now you’ve used so much fuel from the nacelle tanks that the floats turned the lights on! You’re on your last fuel, and you’d better land—quick!
To monitor fuel quantity, King Airs have large, very accurate, capacitance-type (no floats here) fuel-quantity indicators on their fuel control panels. The gauges—one for the left tanks, one for the right—read in hundreds of pounds, and a selector switch lets you read the quantity in either the main tanks or the nacelle tanks (90-series), or the main tanks and the auxiliary tanks (200- and 300-series).
As you might expect, the fuel control panel also has switches for fuel boost pumps, called boost pumps in the 90-series and standby pumps in the 200- and 300-series. These pumps help send fuel from the nacelle tanks to the engines. If a primary engine-driven fuel pump fails in a 90-series King Air (you’ll see a Fuel Press annunciation on the master caution panel), the crossfeed valve opens automatically, and the fuel pump on the other engine then supplies enough fuel pressure to take care of the problem. In the 200- and 300-series airplanes, the standby pumps must be turned on to provide fuel pressure.
The boost/standby pumps serve other functions. They’re used to minimize fuel flow problems when flying with hot fuel at altitudes above 20,000 feet, and they’re used for crossfeed operations in 90-series airplanes.
The King Air’s crossfeed switches are normally kept in the Auto or central position, depending on the model. In Auto, the failure of a 90-series King Air’s boost pump will cause the crossfeed valve to open, and allow the operating boost pump to keep the engine with the failed boost pump from flaming out.
In 200- and 300-series models with a crossfeed lever-lock switch labeled Crossfeed Flow, the pilot simply lifts up on the switch and selects the direction he or she wants the fuel to flow. This opens the crossfeed valve and lets fuel move from one side of the fuel system to the other. On the master caution panel, a green Fuel Crossfeed light comes on. Crossfeed can be used in single-engine operations when you need access to all the airplane’s available fuel. In 200- and 300-series King Airs, the standby fuel pump on the inoperative engine side must be turned off in crossfeed operations, or crossfeed fuel flow can be impaired.
There are firewall shutoff valves on the fuel control panel, too. These are activated in case of an engine fire, or as part of any other engine shutdown procedure.
Most of the rest of the King Air’s fuel system would be familiar to any general aviation pilot. There are filler caps (fill the nacelle tank first); sump drains; and recessed, heated fuel vents. The only unfamiliar items might be the fuel heaters, a fuel-filter contamination indicator (on later models), and the fuel purge system.
Fuel heaters use hot engine oil to keep fuel lines from freezing in very cold temperatures. The fuel lines are routed through what amounts to an oil cooler/radiator to keep the fuel nice and toasty. A fuel contamination indicator pops out of the fuel filters of late-model King Airs, and this should be taken as a sign that particulates or other debris are clogging up the filter; the filter can also be drained with a drain tool to check for sediment, water, and other contamination. Same thing with the sump drains. As for the fuel purge system, this is a relatively recent system, developed to comply with Environmental Protection Agency (EPA) standards. It comes into play as part of the engine shutdown procedure and works automatically.
Compressed air from the engine pressurizes a purge tank (called an EPA can by some) located in the engine nacelles. After shutdown, as engine fuel pressure subsides, a valve in the EPA can opens. This forces any residual fuel in the fuel manifold back to the combustion chamber, where it lights off with a small flourish. You hear the engine spool up slightly as the fuel is consumed, and you’ll also see a momentary surge on the gas generator (N1) indicator. The whole idea is to keep that little bit of excess fuel from leaking out of a sitting engine.
Though not as stone-simple as some other turbine-powered airplanes’ fuel systems, the King Air’s is still a model of good design. If everything’s operating normally, your only interaction with the fuel control panel will be to check the fuel quantity. If there’s a problem, all the controls and backups are right there, ready for fast action.
Links to additional information about King Airs may be found on AOPA Online ( www.aopa.org/pilot/links/links0004.shtml). E-mail the author at [email protected].
