Page 24 - Volume 13 Number 5

King Air F90 fuel panel.
On every King Air model ever built, the nacelle tank – the one behind the engine’s firewall in the nacelle area above and in front of the main wheel well – is where the fuel that is to be crossfed originates. In the 65-90, A90, B90, C90 (including all of its variants), and straight 100, the nacelle tank has its own filler cap and is labeled “Nacelle.” Duh! A gauge in the cockpit reads its quantity. In the E90, F90, A100, B100, 200 (including all of its variants) and 300 (including all of its variants) it’s not quite so simple. In these models the nacelle tank has no filler cap – with one exception that I will present in a moment – and there is no ability in the cockpit to measure its quantity. Instead, this tank is simply a part of the “main tank.” This combination of tanks includes four rubber bladder tanks and one wet-wing tank in the outboard section of the wing and one bladder tank in the nacelle, all connected so as to drain and vent together. The highest spot in this complex of tanks is at the filler cap near the wingtip and the lowest spot is at the bottom of the nacelle. By filling the cap at the tip, fuel flows downhill into all of the connected tanks, including the nacelle tank, and fills it to the brim. In the cockpit, we can read main tank quantity, but we have no exact way of knowing what is in the nacelle and what is still in the outboard wing. To us, the nacelle is merely a part of the main tank, including the main’s lowest point.
The E90 is the one exception mentioned in the previous paragraph, the one that has a nacelle filler cap even though it doesn’t need one. It was less expensive for Beech to manufacture an identical nacelle for the C90 and E90, with a filler cap, even though the newer fuel system of the E90 filled the nacelle by filling the cap at the wingtip. Never take the E90’s nacelle cap off when the Main Tank is full, unless you want to bathe your nacelle in kerosene!
22 • KING AIR MAGAZINE
Inside the nacelle tank, on its bottom, is a submerged electric boost pump that has a nominal discharge pressure of about 30 psig. This pump feeds into a pipe that exits the nacelle tank on its inboard side and immediately connects to a “T” fitting that has one pipe going forward and one pipe going aft. The forward pipe goes to the fuel firewall shut-off valve, just inches away, and the aft pipe is the start of the crossfeed line. This electric boost pump is the only pump that prevents cavitation of the engine-driven, high-pressure pump on the A90, B90 and C90. There is a secondary electric boost pump in parallel with it on the 65-90 and 100, the Straight 90 and Straight 100. The other models – E90, F90, A100, B100, 200s and 300s – have an engine-driven boost pump so the submerged, electric one in the nacelle is now called the “standby pump.”
Between the submerged pump and the exit pipe from the nacelle is a key element – a check valve. This valve permits fuel to exit the nacelle but does not allow fuel to enter the nacelle at this point.
Therefore, concerning the nacelle tank, we have reviewed (1) That its output line can send fuel in two directions: To its own engine and to the crossfeed line. (2) That fuel can come from the nacelle tank here but cannot enter into the nacelle tank due to a check valve.
Both sides, left and right, are as we have presented. This means that there is only one crossfeed line, or pipe that connects the left and right nacelle outlets to each other. This line contains the single crossfeed valve, a Normally-Closed (NC), solenoid-operated valve that uses electric power to open. In all King Air models this valve is located close to the left nacelle tank, not smack-dab half-way under the cabin aisle floorboards.
Assume that the crossfeed valve is open. That would yield a single fuel line connecting the two nacelle tanks together ... a “common fuel manifold” providing fuel to both engines. Also assume, for our discussion now that both side’s electric boost pumps/standby pumps were operating, were discharging into this common manifold. If both engines were consuming fuel at an identical rate – say, 300 pph (pounds per hour) or 45 gph (gallons per hour) – would both nacelle tanks be decreasing their fuel quantity at the same rate?
At first glance, it seems the answer should be, “Of course!” But that is not correct. Let me explain. Modern King Airs have no cockpit display of the discharge pressure from the electric boost pump. However, that Fuel Pressure gauge exists in the straight 90, A90 and B90. The green arc of normal pressures on this gauge goes from 15 to 50 psig ... quite a wide range! Since the purpose of this pressure is simply to prevent cavitation of the high pressure, engine-driven pump, any pressure in this large range does the job well. It would be highly unlikely that both left and right pumps would have identical discharge pressures. (In fact, that is likely the reason why this gauge was deleted on the C90 and later
MAY 2019