Coming to terms with tip tanks, transfer pumps, and imbalances
Many pilots seem to look upon Mitsubishi MU–2s with a mixture of awe and fear. Awe at their performance (up to a 320-knot cruise in the later Solitaire models). And fear of their reputation as ultra-quirky machines with dubious safety records. But as with any truly complex, powerful airplane, the key to safely flying an MU–2 is training, diligent practice, and a high level of respect for any gotchas that may lurk within its systems or procedures.
The MU–2’s fuel system is an example of one of general aviation’s more interesting sets of plumbing and hardware. In concept this system is simple, and there are some ingenious touches. But try to explain it and you soon realize that there’s a lot going on in an MU fuel system.
There are five fuel tanks in an MU–2. Those huge tip tanks carry 93 gallons (90 usable) each. A pair of outboard wing tanks—called outer tanks—can hold either 15 gallons (older MU–2N and -P models) or 35 gallons/34.5 usable (newer models) each. Finally, there’s a main tank, which holds a maximum of 159 gallons (154 usable). The main tank is divided into center and outboard sections, so you can think of the main tank as a three-in-one setup. Check valves make sure that fuel can only flow from the outboard tanks to the center tank. This all adds up to a lot of fuel capacity: 364 gallons/about 2,475 lbs. in the -N and -P versions, or 403 gallons/2,740 lbs. in the newer Solitaires (the ones with the short fuselages) and Marquises (the long-bodies).
To simplify, here’s how the MU’s fuel system works. (Check the airplane’s pilot’s operating handbook or approved airplane flight manual for the complete lowdown on a specific airplane’s workings. This article is no substitute for the POH or AFM; the advice herein is general in nature and mustn’t be used for operational purposes.)
First, engine bleed air pressurizes the tip tanks, and tip-tank fuel is fed from the tips to the main tanks. When the tip tanks empty, and after a two-minute timer counts down, a transfer pump in the outboard tanks begins pumping fuel into the mains. When the outer tanks empty, the fuel in the mains begins to be consumed.
That’s with the fuel control toggle switches set to the Auto position on the fuel control panel. Set the switches to the Tip Manual position, and the fuel burn-off sequence bypasses the outer tanks. Instead, tip fuel is consumed first, then fuel in the main tanks. In some models, you can select the outer tanks.
Why do this? If you have a short trip, the linemen need only fill four tanks (there are two filler ports for the main tanks) to top off the airplane. Equally important is that you must burn fuel out of the tips first. With all that gas way out there on a huge moment-arm on the wing tips, you can twist the wings to deformation and overstress the tip tank attach points if you land with a load of fuel in the tips—especially if you pull a lot of reverse thrust. For this reason, there’s a limitation on landing with fuel in the tips: There shouldn’t be more than 400 pounds in each tank, and the maximum fuel imbalance is 250 pounds of fuel.
In addition to the Auto and Tip Manual transfer switches, there are two fuel boost pumps in the main tanks. These send fuel to the engines. There are main fuel valve shutoff switches that stop the flow of fuel to the engines and test switches that light up to prove that the transfer pumps in the outer tanks are functioning.
Newer MU–2s have fuel gauges that show the fuel levels in the tips, outers, and main tanks. With most older models, there’s no gauge for the outer tanks. The only way of really knowing what’s in them kicks in when the Outer Fuel Empty annunciators come on.
What are the quirks? One is the sniffle valve on the tip tanks. Because the tip tanks are pressurized, you need a way to depressurize them before you refuel. Press on the sniffle valve, recessed in the side of each tip tank, and you release the pressure.
One trap can spring during a careless refueling. Fill all the tanks on one side first, and the MU’s fuel load can cause the airplane to tip radically to one side. A memorable slide in a FlightSafety International MU–2 class presentation shows the roof of an owner’s Mercedes crushed beneath an MU’s tip tank. He had parked under the wing, and linemen had filled that side of the airplane first. Another slide showed a ladder firmly wedged under an MU–2’s wing. Same deal. So the proper refueling sequence has you filling through the main tank’s left fuel filler port first, followed by the main tank’s right filler port, then filling the outer tanks, and finally filling the tips gradually, by filling one tip, then the other, in increments.
Another problem can come about should a fuel load be contaminated. Both engines draw fuel from the main tanks (there is no crossfeed), so both engines can come to grief if you take on bad fuel.
Remember that you should keep the fuel load balanced between wings—to within 22 gallons or about 250 pounds’ worth of fuel—at all times. Outer and tip tanks can transfer fuel at slightly different rates, so one wing can become lighter than the other. This causes an autopilot to deploy the light wing’s spoileron in an effort to keep the airplane level, and this causes a lot of drag. If you don’t turn off the light wing’s transfer pump, you could find yourself looking at a cocked control yoke and an out-of-center rudder ball in level flight, and face a lot of retrimming while you wait for the heavy side to burn off fuel.
Another scenario has to be kept in mind—especially on longer trips. Let’s say that one of your transfer pumps fails. You can forget about using all that fuel in the tip and outer fuel tanks. Now you’re flying an airplane with a 154-gallon fuel capacity (the amount of fuel in a full main tank) and will have to alter your flight plan accordingly. Remember that you’re limited to a 22-gallon imbalance, so you can’t use all the fuel you want to out of any single tip or outer tank. Finally, how do you know the transfer pumps have failed? On older models, the "outer fuel empty" annunciator lights are the only way—and it can light up for four different reasons: as part of the preflight cockpit checks; when a tank really is empty; momentarily, when a transfer pump first starts up; or if a transfer pump fails, causing low fuel pressure. Newer models with gauges for all tanks give you a much better, graphical picture of the fuel burn situation.
Confusing? Not if you pay attention in class, and not if you stay current in the airplane. "Those who really know the MU–2 just love it," says Reece Howell, of Howell Enterprises. "Those who don’t are scared of it. When you think about it, the MU–2 fuel system is really very simple," he says. Howell’s Smyrna, Tennessee, firm (telephone 615/459-6001; e-mail [email protected]) specializes in MU–2 training.
Links to additional information on fuel systems can be found on AOPA Online ( www.aopa.org/pilot/links/links0006.shtml). E-mail the author at [email protected].
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