Suction Feed and Cavitation after Boost Pump Failure at FL410
During an ETOPS sector at FL410, the left fuel boost pump low-pressure light illuminates. The crew leaves the pump off; engine 1 continues to run on suction feed. Fuel in the left tank is at +18 °C; total fuel is 30,000 kg, with 12,000 kg in the left tank and 18,000 kg in the right. Twenty minutes later, engine 1 fuel flow fluctuates by about ±8%, EGT rises 30 °C, N2 dips briefly, and the engine fuel low-pressure warning flickers. The engine otherwise remains stable. The nearest suitable airport is 70 minutes away; destination is 3 hours 20 minutes away. The aircraft is at FL410, and the outside air temperature is −57 °C. The QRH requires consideration of fuel temperature, suction-feed altitude limits, crossfeed, and descent. The crew must decide how to stabilize the engine and whether to continue or divert.
1. Explain why the loss of the boost pump can cause fuel-flow fluctuation and EGT rise at FL410, but often not at low altitude.
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The boost pump provides positive pressure at the inlet of the engine-driven high-pressure pump. Without it, fuel must be drawn by suction, and the pressure at the pump inlet is ambient tank pressure minus line and valve losses. At FL410 ambient pressure is only about 4.3 psi, so the available net positive suction head is small. The high-pressure pump needs a minimum NPSH to fill its pumping elements; if inlet pressure falls too low, dissolved air and light fuel fractions come out of solution and form vapor bubbles. The pump then cavitates, delivery becomes erratic, and fuel flow fluctuates.
The FADEC tries to hold the scheduled flow, so the engine may cycle slightly rich and lean; EGT rises and N2 dips as combustion efficiency varies. At low altitude ambient pressure is much higher, so the same suction-feed arrangement has adequate NPSH and the problem does not appear.
Fuel temperature is important: at +18 °C the fuel has a higher vapor pressure and releases vapor more readily than cold fuel, so the altitude at which suction feed becomes unsafe is higher. The flickering low-pressure warning is consistent with cavitation at the pump inlet rather than a complete pump failure.
2. What immediate crew actions would you take, and what is the reasoning behind each?
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First, open crossfeed and use the right boost pump to supply engine 1, restoring positive inlet pressure. This treats the cause rather than the symptom. If crossfeed is not available or not permitted in this configuration, descend to an altitude within the flight manual suction-feed limit for the current fuel temperature. Reducing thrust lowers fuel flow and pump demand, which increases the NPSH margin, but it is not a substitute for restoring boost pressure.
Monitor left and right tank quantities for imbalance and confirm that fuel is actually transferring as expected. Monitor fuel temperature; if it is rising, the suction-feed altitude limit becomes more restrictive. Do not run both engines from one tank unless the crossfeed procedure and imbalance limits allow it.
If the engine remains unstable, or if the fuel low-pressure warning becomes continuous, land at the nearest suitable airport. Continuing for 3 hours 20 minutes on a cavitating suction feed is not justified when a 70-minute diversion is available.
3. What determines the minimum altitude for safe suction feed, and how would you use the flight manual data?
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The limit is set by NPSH: the absolute pressure at the pump inlet must exceed the fuel vapor pressure plus the pump's required NPSH. Ambient pressure falls with altitude, so the margin falls. Warmer fuel has a higher vapor pressure and therefore requires a lower altitude. The flight manual presents suction-feed limits as a function of fuel temperature, sometimes with a correction for fuel flow or tank configuration.
Use the current fuel temperature, not OAT, and apply the most restrictive limit. If the aircraft is above the limit, descend, crossfeed, or land. The limit is not simply the altitude at which fuel boils; dissolved air and cavitation can occur before bulk boiling.
If the chart shows no suction-feed capability at the current temperature, the only safe options are to restore boost pressure or land.