Boost pump failure and suction feed at altitude
An AS350 B2 with an Arriel 1D1 is on a scenic flight at 9,500 ft, OAT +18 °C. The fuel boost pump caution light illuminates. The pilot continues, because the engine-driven pump can suction feed. Twenty minutes later, while climbing at 85% torque, torque and TOT begin to fluctuate, then the engine flames out. The pilot enters autorotation and lands safely. On the ground the fuel tank is half full of Jet A-1 at 35 °C, the boost pump circuit breaker is not tripped, and the fuel filter bypass indicator has not popped. The manufacturer's flight manual allows suction feed only below 6,000 ft with fuel below 30 °C. The operator wants to replace the boost pump and return the aircraft to service without further checks.
1. Explain why the boost pump is fitted and how its failure can lead to a flameout at altitude, using the concepts of vapour pressure, cavitation and suction feed.
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The boost pump delivers fuel at a positive pressure to the engine-driven high-pressure pump, preventing the inlet pressure from falling too low. Jet A-1 has a low vapour pressure at normal temperatures, but when fuel is hot and ambient pressure falls with altitude, the margin between the fuel's vapour pressure and the pump inlet pressure narrows. The engine pump then has to generate suction, and any restriction in the line, a hot line run near the engine, or dissolved air coming out of solution can cause cavitation: bubbles form and collapse, fuel flow becomes erratic and the engine control sees a fluctuating supply. A boost pump keeps the inlet pressure above the vapour pressure, providing adequate net positive suction pressure. With the boost pump failed and the aircraft climbing above the permitted suction-feed altitude, and with fuel above 30 °C, the suction feed could not maintain enough pressure, so the engine flamed out. The bypass in the boost pump may allow suction feed, but only within the manufacturer's stated limits.
2. What indications would you expect and what actions should the pilot have taken when the caution light appeared?
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Expect a fuel pressure caution light (low pressure), possibly a fuel flow fluctuation, torque and TOT fluctuations, and eventually a flameout. The pilot should have treated the boost pump failure as an abnormal condition: reduce altitude, reduce fuel temperature if possible, avoid high-power climbs, and land as soon as practicable at a suitable site. Continuing the climb at 85% torque increased fuel flow and heat, worsening the suction margin, and the aircraft was already above the permitted suction-feed altitude with fuel above the permitted temperature. Once torque and TOT fluctuated, the engine was already surging or flameout was imminent; the correct action was to lower collective, maintain Nr and prepare for autorotation, not to continue the climb. The caution light was the cue to change the plan, not merely to monitor it.
3. What maintenance and airworthiness action is required before further flight?
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Replacing the boost pump alone is not enough. The operator must find out why the pump failed and why suction feed was inadequate: test the replacement pump and circuit, check the wiring and breaker, check the fuel lines for restrictions, heat shielding and correct routing, check the fuel filter and bypass indicator, verify the fuel temperature indication and the tank vent. The engine-driven pump should be bench-checked or replaced if it ran in cavitation. The fuel control and engine should be checked for damage from the lean or erratic supply and the flameout. The maintenance manual's procedures for boost pump failure and suction-feed limits must be followed, and a functional ground run and flight test done before release. If the suction-feed limit is below the planned operating altitude, the aircraft should not be released for that operation until the system is fully serviceable. Document the fault, the checks and the release.