Dukes Electrical Fuel Pumps

[Robert Pastusek]

Lancair Owners,
Heads up on a very subtle service need with aircraft in service for 10 years or more...
Lancair typically provided a Dukes Corporation two-speed electrical auxiliary fuel pump with IV, ES and Legacy kits, and most builders installed a big-bore Continental engine. (Earlier Lancairs used different type pumps that do not have the failure mode below.) Over the past two months, I've talked with four owners that experienced pump failures that were not apparent but caused abnormal engine operation that was not readily associated with the aux fuel pump.
Dukes pump users flying behind big bore Continentals reported failure to maintain fuel flow during climb, erratic engine idle operation, unexplained loss of fuel, momentary loss of power for no apparent reason, and loss of power during fuel tank changes. Preliminary testing indicates failure of the seal between the motor and pump can cause any/all of these symptoms. Andy Chenous of Aero Motors LLC advises that ten years is a reasonable life for the seal between the pump and motor, and that it's somewhat independent of how much the pump is used. When this seal fails, it allows fuel to be vented overboard through a drain--specifically installed for this purpose--when the pump is operating under pressure (e.g. boost pump on). When the boost pump is off, this section of the fuel system is under a slight vacuum (fuel is being drawn by the engine-driven fuel pump from the tanks), and the leaky seal admits air into the fuel line. The seal failure is rarely sudden/abrupt--it starts leaking a small amount causing very minor engine anomalies that may go unnoticed. As the leakage rate increases, it eventually becomes hard to ignore--but still difficult to identify/isolate.
Fortunately, it's very easy to test this seal with the pump in place. With reference to the attached picture, disconnect the existing drain tube from the brass fitting at the bottom center of the pump and attach a similar tube that you can blow AND suck on. DO NOT use compressed air or a vacuum pump for this test. Either can damage a good seal. If you can pass air into this area of the pump, or draw air out, the internal seals are defective and the pump needs to be overhauled. This is by no means the only pump failure mode, but it's one that has stumped several good aircraft mechanics!
Bob Pastusek
757-286-4802

[Robert Pastusek]

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