King Air Crossfeed Basics

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) 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 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 models: It is bothersome to have all the other engine gauges in close agreement and yet the fuel pressure gauges reading very different values!)

If that crossfeed line – the common fuel manifold that is feeding both engines – were fed on the left end by 20 psi and on the right end by 40 psi, what would happen? No, the answer is not that the right would supply twice as much fuel as the left. The correct answer is that the right would supply all of the fuel that both engines are consuming! Think of a tug-of-war game but this time imagine pushing instead of pulling. The stronger side always wins. The manifold, pressurized to 40 psi from the right pump, would cause the left check valve to close and thereby prevent any of the 20-psi fuel being sent by the left pump from entering the manifold. The end result is that the left boost pump’s impeller would merely be spinning in its own fuel “wake” with no discharge passing the closed check valve while the right boost pump would keep filling the crossfeed line to replenish what both engines were consuming from it. Using our numbers above, the right nacelle quantity would be decreasing at the rate of 90 gph while the left nacelle quantity would be constant, not decreasing at all.

I hope this now makes it obvious why only one electric boost pump can be operating during crossfeed operation. You, the pilot, must control which pump is the stronger and which is the weaker by having one running and the other not running.

“Uh, wait a minute, Tom. You are discussing a situation in which both engines are consuming fuel from one nacelle tank. But that is a violation of a POH limitation. We can only crossfeed when an engine is shutdown in flight!”

You are exactly right: Most POHs do indeed contain this Fuel System Limitation. From an engineering design standpoint, however, I am also right. Namely, the size of the crossfeed line and the supply capability of the boost pump allows for both engines to be fed from one side’s nacelle tank. Please realize, as has been stated by me in previous articles that the POH’s crossfeed limitation comes from legal, not engineering, concerns. Let’s look at this in a bit more detail.

Suppose that one day the FBO’s Jet-A truck breaks down after it had topped the filler caps on the left side but had not yet finished filling the right side. We are left with, say, a 500-pound imbalance. The 300-series has a 300-pound imbalance limitation and the C90B and C90GT-series have a 200-pound limit. (Both of which, in my opinion, are unnecessary.) For the other models, we are still good to go with the 500-pound imbalance. So off we go – with a little aileron trim cranked in – and when safely in cruise we decide to balance the fuel. We do this by crossfeeding, sending fuel from the side with more fuel to both engines until the greater fuel quantity equalizes with the lesser quantity. Step 1: Make sure the electric pump on the feeding side is on. Step 2: Open the crossfeed valve. Step 3: Make sure the electric pump on the other side is off. Step 4: Monitor the fuel quantity gauges over a period of at least 15 to 30 minutes to verify that indeed the side with more fuel is going down and the side with less fuel is not changing. Step 5: Keep monitoring fuel quantity and stop crossfeeding when balance is achieved!

OK, I admit it. You caught me again, caught me in not following the manufacturer’s checklist. Realize that this article is meant to cover every King Air model. The individual differences are many. For example, in the F90-, 200-, and 300-series, moving the crossfeed switch laterally toward the side to which you want to feed the fuel not only should send power to the NC crossfeed valve and cause it to open but also should turn on the feeding side’s standby pump. It does not, however, turn off the receiving side’s standby pump if it happened to be already on. Another example of differences: In the C90 and earlier style systems the pilot will definitely have to turn off the receiving side’s boost pump since it is normally running at all times. The E90, A100, and B100 probably follow the procedure written above most accurately.

But realize this, readers: There is never a Step 4 or Step 5 in the POH and they are both critically important! There is absolutely no way to truly know that fuel is going from the high side to both engines until the decrease in the higher fuel level is confirmed! Does the POH address this? No! Have you been taught this? Maybe.

As for Step 5, this is where most of the legal team’s liability worries arise. Suppose that an asleep-at-the-wheel pilot gets distracted and fails to stop crossfeeding when balance is achieved. In fact, he forgets the fuel panel totally and runs the feeding side’s nacelle tank dry. Since this tank is feeding both engines, they both quit nearly simultaneously! Darn! I hate it when that happens!

“But wait!” says our hapless pilot. “I still have fuel on the other side. I’ll use that to get the engines running again!”

Quiz time: Is it easier for the engine-driven pump to suck vapor (air) or liquid (fuel)? I think we can all agree that the engine-driven pumps will draw air before fuel. Only if we turn on the electric boost pump/standby pump on the side with fuel remaining – so that our common fuel manifold, our crossfeed line, is full of pressurized liquid and no air – do we have a chance for an airstart to be successful. Hard to do? No! A critical step that is easy to overlook? Yes!

Now let’s review the “proper” use of crossfeed. Suppose we are returning from Europe and on our leg from Reykjavik to Goose Bay we lose oil pressure and shut down the right engine. The airports in Greenland are below minimums and we have enough fuel to continue to Goose Bay. (By the way, in most cases our range just went up, not down!) As we continue with only the left engine running, the left fuel is decreasing while the right fuel is remaining at the level it had when the engine was secured.

When the left side gets down to, say, 500 pounds, but with the right side still showing 800, we decide to send the fuel from the right side to the left engine. Easy. Right boost pump on, crossfeed open, left boost pump off. Now the right fuel quantity starts decreasing and the left fuel quantity does not change … just as it should.

A reminder: For you fortunate pilots flying a member of the F90-, 200- or 300-series, the “right boost pump on, crossfeed open” steps mentioned above are both done by merely moving the crossfeed switch left toward the engine we wish to feed.

“But wait, something’s wrong! The right fuel pressure warning annunciator is still illuminated!” Relax. That’s normal. When you conduct your first-flight-of-the-day fuel panel checks it is correct for both left and right fuel pressure annunciators to extinguish. But with the right engine actually shutdown and all of the proper checklist “cleanup” steps completed, the right Fuel Firewall Shut-off Valve has been closed. Thus, the pressure from the operating pump cannot be felt at the pressure switch since it is mounted on the fuel filter downstream of the shut-off valve. So how do we know the right pump is really pumping? For all of the models with engine-driven boost pumps, we don’t know … until enough time has elapsed to confirm that the proper side’s fuel level is decreasing. (For the C90-style fuel system – the system without engine-driven boost pumps – we know the right pump is operating since the left fuel pressure annunciator remained extinguished after we turned off the left boost pump.)

We fly for another couple of hours and now the gauges read 500 pounds left and 200 pounds right. We stop crossfeeding and return to feeding the left engine from its own nacelle tank. Ah, there’s Goose Bay! We make an uneventful single-engine landing and now face the hardest task … getting to the ramp on one engine!

One last point to mention: Do not worry about fuel being lost even if we failed to close the Fuel Firewall Shut-off Valve on the engine we secured. The condition lever being in cutoff will prevent any fuel from reaching the dead engine’s combustion chamber and then draining overboard.

I hope this review has been
enlightening. Questions? Please write and ask them; I will be happy to respond.