Page 24 - Volume 13 Number 12

is higher than the wing’s filler cap. So why do we even have a filler cap on the wing? Why not just fill the highest spot atop the nacelle?
Here’s the answer: All fuel the engine consumes comes from the nacelle tank only. All other fuel is useless until it gets transferred into the nacelle, so once it gets there the designers don’t want it to be able to easily escape. Thus, a flapper­type of check valve permits fuel to flow to the nacelle but prevents it from easily flowing from the nacelle back into the wing. It would take hours and hours and hours to patiently fill the wing tanks from the nacelle cap.
Liquid doesn’t want to flow uphill, right? Hence there must be a system designed to push or pull this fuel uphill to make it all usable to the engine by getting it into the nacelle tank. Hence, the bladder in the wing center section is the “Aux” tank, it has its own filler on top, and the fuel it contains is useless until it gets transferred into the nacelle portion of the Main complex of tanks. For the E90, the Main holds 196 gallons per side of usable fuel and the Aux holds 41 gallons ... a total for both sides is 474 usable gallons, 90 more than the C90’s 384.
In the C90, transferring fuel from the wing tanks to the nacelle tank can be done by two methods. First, gravity­flow works well but the last 28 gallons (about 200 pounds) cannot be transferred by gravity flow ... it would need to flow uphill. Second, a submerged electric pump – located in the wing’s center section tank, the lowest spot of the wing tanks – cycles on and off as required
10 gallons of full. So long as the transfer pump is working properly – as it usually is – then no gravity­ flow is necessary and none takes place. Although the electric transfer pumps in the C90 have proven to be quite reliable, when and if they fail then the airplane loses 28 gallons of usable fuel on that side.
The transfer system was simplified and made more reliable on the A100 and similar, later designs. A jet transfer pump is now used. The line taking fuel from the boost pump toward the engine­driven, high­ pressure fuel pump has a tap­off that sends some of the boost pump’s discharge to the jet pump. What a simple device! It is merely a venturi with no moving parts. Bernoulli’s principal comes into action causing the fuel pressure to be reduced as the fuel’s speed accelerates due to its need to squeeze through the venturi’s throat. This fuel from the boost pump is what causes or motivates the venturi to create suction that can pull the fuel from the aux tank. That explains why the fuel from the boost pump that flows to the jet pump is called “motive flow” since the venturi is not motivated to create suction until flow passes through it. A normally­closed (N.C.) solenoid valve – the Motive Flow Valve – is the only moving part of this transfer system. Whenever fuel is available downstream of the boost pump, the simple action of energizing the Motive Flow Valve to its open position – by moving the Aux Transfer switch from Off to On – begins the fuel transfer action. Simple. Reliable. Nearly fool proof.
In my opinion, there is an even bigger improvement in this newer fuel system than the simplicity and reliability of the transfer system ... it is the fact that an engine­driven mechanical boost pump was added to both left and right engines. The drive pad on the aft accessory case of the PT6 to which the boost pump may be installed has always existed. Yet, strangely, not until the A100 – and then the E90 and others – was it ever utilized! What a waste! (Are you reading this, you great King Air modifiers? I still think there would
When the word “Main” is applied
to the E90­style fuel tank system, it
includes all fuel in the wing tanks
outboard of the nacelle as well as
the nacelle tank itself. Why is the
bladder tank in the wing’s center
section – between the nacelle and
the fuselage – not included? Because
it includes a portion that is too low
to gravity­flow into the nacelle. to keep the nacelle within about
22 • KING AIR MAGAZINE
DECEMBER 2019