Page 25 - Volume 13 Number 5

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
MAY 2019
KING AIR MAGAZINE • 23