Page 26 - Volume 13 Number 12

Figure 2: The E90 Fuel Schematic from the POM. The Main tank includes the nacelle tank and all five tanks in the outboard wing. The Aux tank is the single inboard tank.
top – before the Aux tank overflows and starts venting onto the ramp or hangar floor. On the other hand, if the Aux was already filled when this leakage began ... expect the angry call from the FBO about the mess your airplane is causing!
There is one other path by which fuel can migrate from Main to Aux: Via the jet pump, through a defective check valve in the line going from the Aux tank to the jet pump. This situation is worse than the leaky valve(s) at the top of the nacelle because more fuel can migrate than the Aux tank can hold! Both the jet pump and the check valve are quite easily accessible in the main wheel well. (The previous sentence is not true for the 200­ and 300­series since their jet pumps are located
over­pressurized condition and plane you find that one side – say, inside of their Aux tanks.) I am
stopping the venting.
However, the very best idea is to avoid pressurizing the Main tank in the first place! Simply don’t transfer until there is some space available in the Main tank. Leave the left and right Aux Transfer switches off until the Main quantity is 1,100 pounds or less. (Full is about 1,300 pounds per side.)
You are correct that the factory procedure is to fill the Aux tanks last and to use them first. This is to avoid excessive wing bending stress under the worst possible combination of wing loading and fuel/payload distribution. Friends, if you ever experience spine­crushing positive G­forces while at the same time with every cabin seat full, lots of heavy baggage, full Aux tanks and almost empty Mains, I’d fret a bit. But waiting to transfer until the Mains are down about 15%? I suggest losing no sleep over that one!
the left – is showing 1,300 pounds in its Main and 0 pounds in its Aux ... just as you anticipated. However, the right side is showing 1,100 in the Main and 200 in the Aux. How did that happen?!
There is a vent line connecting the top of the nacelle tank to the center­ section (the Aux) tank (Figure 2 shows the line). This line has three parallel paths that connect it to the nacelle: (1) A float­operated valve that should open only when the nacelle’s fuel level starts dropping; (2) A vacuum relief valve that will be sucked open to allow air to enter the tank in the event that the float­ operated valve sticks in the closed position; and (3) A pressure relief valve to relieve the pressure buildup caused by thermal expansion or by fuel transfer. If any of these three connections is defective such that it allows fuel to leak past it at all times, then fuel in the nacelle portion of the Main tank will find its way downhill into the Aux. It’s time for maintenance to remove the access cover on top of the nacelle and check the three valves underneath.
happy to report that the problem of fuel migration from Main into Aux is not a common occurrence.
In conclusion, the addition of the engine­driven boost pump, the simplified and more reliable fuel transfer system, the extra fuel capacity ... the E90­style fuel system, in my mind, is a real winner that is highly improved over its C90­ style predecessor. Its only downside is the tendency to over­pressurize the main tanks and vent a little fuel overboard when one begins Aux fuel transfer while the Mains are already full. Avoid that and you, too, will find this system nearly perfect! KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 46 years, and is the author of “The King Air Book.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his book, contact Tom direct at twcaz@msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
Another problem that sometimes
crops up with the E90­style fuel
system is the migration of fuel from
the Main tank into the Aux tank.
Suppose that you had the line person
“fill the tips” but ignore the inboards
... a very common fuel order when
your payload does not allow for full
fuel. The next morning when you
show up and start checking the the level of the nacelle portion’s blonigen.net.
It is common that the fuel migration will cease – because the level in the Main complex of interconnected tanks has reached
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@
24 • KING AIR MAGAZINE
DECEMBER 2019