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be a market for an STC that would allow an engine­ driven boost pump to be installed on A90s, B90s and the C90­series.) Unlike the earlier fuel system, no longer is it SOP (Standard Operating Practice) to operate with the electric boost pump on at all times for the purpose of cavitation avoidance. Instead, the electric pump becomes a Standby Boost Pump, rarely needed at all. How nice!
When I spoke briefly to Mr. Randall on the telephone, he confirmed what I had suspected was his complaint about his E90’s fuel system: Sometimes it vented copious amounts of fuel onto the ramp out of the heated fuel vent under the wing outboard of the nacelle. This only happened following short flight with all tanks topped before departure.
I am certain that many of you have read previous articles I have written about this problem, especially as it applies to the F90­, 200­, and 300­series, the ones in which fuel transfer is automated. For these models, I suggest that SOP be to operate with the left and right Aux Transfer circuit breakers pulled, only pushing them in if the aux tanks contain fuel and only when the mains tanks are at least 200 pounds less than full. For the E90, A100, and B100 the solution to the venting problem is very similar: Simply do not turn on the Aux Transfer switches, left and right, until the main tanks are down a bit ... like at Top of Climb, after reaching cruise speed.
You see, the transfer system sends fuel into the Main tank faster than the engine is burning fuel from the Main tank. The net increase in Main tank fuel quantity causes a pressure buildup in the Main tank. In theory, the pressure should be prevented from reaching an excessive level by sending some of the fuel through a pressure relief valve and into the vent line from the Main tank back into the Aux tank. Almost always, however, some of that fuel, instead of returning to the Aux, vents overboard. How much? Not much. No one knows with certainty but my educated guess is that less than a gallon or so per side would be vented during the time it takes for the Auxes to empty. Unless someone were flying in close formation with you, the minor amount of fuel streaming out of the heated vents will likely go unnoticed. But in the rather rare case in which you land before the Aux tanks are empty? Now that dripping fuel will be very obvious, perhaps causing the hazmat folks to pay you a nasty visit.
A common response is to open the filler cap at the wing tip to relieve the tank’s pressure. Yes, that is effective ... but comes with the nasty downside of gushing fuel out of the filler onto the wing, onto yourself or the lineperson, and onto the ramp! A better way? Leave the transfer switches off while running the engine for 5 to 10 minutes. That will take fuel from the main tank without it being replenished, thus eliminating the
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DECEMBER 2019 KING AIR MAGAZINE • 23