Page 22 - April24

ASK THE EXPERT
King Air Crossfeed Basics
by Tom Clements
My aim in writing this article is not to present anything new but rather to simply review some of the fuel system information that you should have already received. I am sure the fuel system was covered extensively in your initial King Air training course and, if you have been flying King Airs for a while now, I am sure it has been reviewed in some or all of your recurrent training courses.
Regardless of the particular King Air model you operate, three things must exist for fuel crossfeed to take place. Before I present those three things, let’s remember this important fact: Fuel never flows from a tank on one side to a tank on the other side ... unless we do something wrong and unusual. The term is not “CrossFLOW.” We are not flowing fuel from tank(s) on one side to tank(s) on the other side. The correct term is “CrossFEED,” since we are taking fuel from a tank on one side and feeding it to the operating ENGINE on the other side. (Sadly, I believe the switch we will be discussing, in some King Air models is, in fact, labeled “Crossflow.” That’s a demerit for the Beech switch labelers!) Additionally, never say “transfer” when you mean “Crossfeed.” In a King Air, transfer refers to moving fuel from the auxiliary tank into the main tank on the same side.
Back to the three things we need for crossfeed. They are: (1) An operating electric boost pump on the feeding side, (2) An open crossfeed line and (3) No opposing electric boost pump pressure on the receiving side.
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)
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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!
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
APRIL 2024