Most C-90 pilots understand the sequence of events that transpire in the event of a boost pump failure. A red fuel pressure light illuminates and is quickly extinguished as the cross feed opens automatically allowing the functioning electric boost pump to supply fuel pressure to both high pressure engine driven pumps. This eliminates the possibility of cavitation of the high pressure pump due to suction feeding. It also prevents engine surging at high power settings and negates the need to record any time against the 10 hour overhaul requirement of the high pressure pump during suction feed operation. So far, so good. Of course, since we are talking about a King Air, there is more to the story. Did you know that boost pump failure can lead to engine failure? Here’s how:
Boost pump failure during a rapid climb-out will cause a gradual power loss on the affected engine starting at approximately 13,000 feet. This altitude varies with the fuel temperature in the tank. A higher fuel temperature will cause a gradual power loss at a lower altitude. Interestingly, a complete power loss will occur if the climb is continued under these circumstances. This power loss results from the highly aerated condition of the fuel caused by rapidly decreasing tank pressure during a rapid climb. This allows the air trapped in the fuel to expand. However, once the fuel tank pressure has stabilized and excess air has escaped from the fuel, the loss of a boost pump has less effect on the engine operation at maximum power settings. It is impossible to determine how much time is required to stabilize the fuel from the highly aerated condition because it is a function of both rate of climb and fuel temperature. Fuel stabilization should occur after a few minutes of stabilized cruising operation. If in doubt, open the crossfeed! Descents from a high altitude with the boost pump inoperative do not affect engine operation.