King Air fuel system: The inside story

Moving fuel in the world’s most popular turboprop twin

October 1, 2010

The fuel system used in the C90-series King Airs is designed with simplicity in mind, but the systems’ workings can sound complicated. There are differences among the various C90 models, but their fuel systems share basic similarities—and the same can be said for the fuel systems in the 200- and 300-series King Airs as well. Let’s take a simplified tour of the fuel system in late-1980s/early 1990s C90A and C90B models for an introduction to the basics.

Turbine Edition Table of Contents

First, a look at some fuel tank anatomy. There are wing tanks (as many as four per side) as well as nacelle tanks. The nacelle tanks—which each hold 61 gallons—are located right behind the King Air’s Pratt & Whitney engines, so it’s only logical that these tanks directly feed the engines. A low-pressure boost pump within each nacelle tank sends fuel to the Pratts, which in turn use high-pressure pumps to send it to their combustion chambers.

Some 192 gallons of fuel are stored in the wing tanks, so the system’s big job is to move all that fuel “uphill” to those high-riding nacelle tanks. This is where the fuel system’s transfer pumps—located in the airplane’s wing center-section tanks—come into play. These transfer fuel automatically to the nacelle tanks when the transfer pump switches on the fuel control panel are in the Auto position. The transfer pumps top off the nacelle tanks, then shut off. They come back on again when the nacelle fuel quantity drops by about 10 gallons.

Those transfer pumps are absolutely vital to access all the ship’s fuel, so there are backups in the rare event they fail. You’ll know something’s wrong if the NO FUEL XFR lights illuminate on the fuel control panel. You can also tell if fuel isn’t transferring to the nacelle tanks by holding the spring-loaded fuel quantity switch to the Nacelle position. Now you’ll see the gauges showing the nacelles’ fuel quantities. If the transfer pumps have truly conked out, then nacelle fuel quantity would be dropping through about 340 pounds.

A Transfer Test switch lets you verify that fuel is indeed transferring; if the NO FUEL XFR annunciator doesn’t flash, then fuel is flowing, but your flow switch is bad. To remedy this, move the transfer pump switch to the Override position. Now the pumps will run continuously, and the nacelle tanks will top off. Any excess fuel sent to the nacelle tanks will be returned to the wing tanks via a vent line.

In case of a nacelle boost pump failure—another potentially dangerous condition—the system automatically switches to crossfeed operation. Now the opposite nacelle boost pump is providing the pressure to move fuel. Another way of dealing with a boost pump failure is to move the crossfeed valve switch on the affected side from Auto to the Closed position. In this method, suction created by the engine-driven fuel pump draws fuel from the nacelle tank. This suction-feed operation is limited to 10 hours total time between fuel pump overhaul periods.

King Air fuel systems also come with the firewall shutoff valves, vents, drains, and circuit breakers you’d expect to see in any other turbine twin. Some models have standby pumps and other added features, but the basic idea is the same: Gravity feeds fuel through interconnected wing tanks to a low point, transfer pumps send it to the nacelle tanks, then boost pumps move it to the engines.

In all, the King Air fuel system is pretty much set-and-forget. Unless there’s a pump or transfer problem (in which case you’d see those warning lights) you keep on trucking, occasionally checking the total and nacelle fuel quantities, and making sure that your nacelle fuel doesn’t drop below safe levels. In low-fuel situations where fuel in the wing tanks has been exhausted—in which case those NO FUEL XFR lights will come on—your only remaining fuel is in the nacelles. With fuel flows in the 250 to 300 pph (per side) range, that 61 gallons probably won’t last an hour—assuming full tanks. That’s when nacelle fuel becomes “bingo” fuel.

E-mail the author at tom.horne@aopa.org.