From the time I began to fly multiengine aircraft, I had always followed the precaution of allowing at least one minute to elapse between switching fuel tanks for each engine. I had been taught, and believed, that this procedure would reduce the chance of having both engines cut out at the same time. It always worked, until one day in 1972, when I was flying a PA-39 Twin Comanche back to Wings Field, near Philadelphia, Pennsylvania, from a visit to Lock Haven, Pennsylvania.
Not long before this flight, the aircraft had been returned to service following repair work required after a collision with a Ford Maverick automobile, which occurred as the plane was landing on the runway at Wings Field. The repaired airplane was flying perfectly and, in fact, was even better than new, since full deicing equipment had been installed during the repair work. The PA-39 was equipped with the optional 15-gallon wingtip tanks. Our normal procedure was to dispatch with full fuel in the main and tip tanks. The auxiliary tanks were filled only if we needed the plane's full eight-plus-hour endurance. This practice recognized that the fuel load in the tip tanks reduced the wing-bending moment, and therefore the weight of that fuel was an allowable addition to the maximum gross weight.
On the morning of the flight, I filled the mains and tips and did the usual, careful preflight. The uneventful trip to Loch Haven was made using fuel from the main tanks. Tip tank fuel could not be used until there was sufficient room in the mains to accommodate the return fuel flow from the injection system.
On the return flight, after reaching the 7,000-foot cruise altitude, I began the ritual of switching to the tip tanks. This entailed selecting the auxiliary tank for the left engine and operating the left tip-tank toggle switch, which was located in the floor-mounted fuel selector panel. This toggle switch actuated a solenoid valve that enabled fuel to flow from the tip tank, rather than the aux tank, and at the same time illuminated a "Tip Tank" annunciator light on the fuel selector panel. Since everything was perfectly normal at the end of the 60-second tank selection delay period, I switched the right engine to its tip tank and settled back to enjoy the rest of the flight.
About 30 seconds later, I found myself "enjoying" a flight in the quietest PA-39 in history. Both engines had quit cold. I like quiet airplanes, but this remarkable lack of noise was accompanied by an unremarkable loss of altitude that was not desirable over the Appalachian ridges. I wanted more noise.
I immediately switched both engines back to the main fuel tanks, without waiting for the normal 60-second delay period to elapse. Both Lycomings immediately began to create welcome noise. The fuel quantity indicators for both tip tanks still showed "Full." Since there was sufficient fuel in the main tanks for the trip back to Wings and everything seemed perfectly normal, I continued the trip as planned.
Once on the ground at Wings Field, we began the troubleshooting exercise. The tip tanks were indeed full of fuel. The aux tanks were bone dry. We found that when the plane had been repaired, the wires from the tip tank selector toggle switches had not been properly reconnected to the solenoid-operated fuel control valves. Only the annunciator lights had been powered. Even with tip tank fuel selected, the engines were actually drawing fuel from the auxiliary tanks. Always-perfect hindsight made it clear that the 60-plus seconds of left-engine operation and the few seconds of right-engine operation were fueled by the few ounces of fuel left in the aux tanks.
The lesson to be learned from this event is to take absolutely nothing for granted after any significant repair to an aircraft.
Charles B. Husick, AOPA 434178, is the former chariman of the General Aviation Manufacturers Association (GAMA) and former president of Narco Avionics (1971 to 1978, senior vice president of Cessna and Fairchild and Chris Craft Boats Corporation (1988 to 1989). He is a commercial pilot and flight instructor with over 6,000 hours.
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