Page 24 - April24

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!”
22 • KING AIR MAGAZINE
APRIL 2024