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Wednesday, May 16, 2007

Mentor - Complex fuel systems in light twins



All planes have fuel drains. Some have more. The Cessna 320 I was flying had ten.

The Cessna 320 I flew for a student, who happened to be both my student pilot and a practicing psychologist, was an amazingly wonderful plane. Twin turbocharged Lycoming 6 cylinder engines. Able to do 295 mph true airspeed. An absolute joy to fly and getting used to the speed after slower planes was always a bit of an eye-opener. The plane was co-owned, actually, by a psychologist who liked to go fast and a Tuna boat captain who thought high speed was 10 knots. It made for some interesting conversations on trips!

The most interesting aspect was without a doubt the fuel system. Each wing had a wingtip tank able to hold 50 gallons, and an inboard reserve tank of about 15 gallons, as I recall. The inboards had a unique characteristic; they could pump fuel out twice as fast as the engine could burn it. That meant you had to manage the fuel in a certain way.

On take off, both engines would burn from the mains. the rate of consumption on the fuel gauges was about what you would expect from those fuel injected turbocharged engines, but it was twice what you would expect when you switched over to the auxiliary tanks! To keep from porting fuel overboard, you would need to burn down your mains at least 10 gallons a side, then switch to auxiliaries and run them nearly dry. While you were doing this, your auxiliary fuel gauges would be dropping as though you had a hole in the bottom of each tank, and at the same time the main fuel gauges would be showing the tanks filling up!

For such an interesting fuel system, you would not be disappointed when you went underneath to work the system drains. A drain on each main. One on each auxillary. One on each engine. That's 6 so far. One on each cross-feed bringing the total to 8 and two in the system feeding fuel to the heater in the nose, for a total of 10 fuel drains to check before each flight!

All in all, flying that plane was worth it. It hummed and sang and flew straight and true. There were a few other idiosyncrasies, but I'll save those for another day.


onward & upward! ~ rfb