How Fuel Boost Pumps Are Operated: A Practical Guide for Pilots and Aircraft Owners
The bottom line: Fuel boost pumps are electrically driven pumps that provide pressurized fuel to the engine during critical phases like starting, low-pressure conditions, crossfeed operations, and as a backup to engine-driven pumps. Their operation is typically automatic, but pilots must understand the manual controls, normal versus abnormal indications, and the consequences of improper use to ensure safe flight.
What Fuel Boost Pumps Do
Fuel boost pumps serve several essential functions in an aircraft's fuel system. They are primarily used for engine starting, because engine-driven fuel pumps are only marginally capable of providing suction at low speeds. During start, the boost pump ensures fuel reaches the engine before the engine-driven pump can take over. They also act as a backup when the primary ejector pump output is insufficient, and they provide fuel pressure for crossfeed operations when transferring fuel between tanks.
On many aircraft, the boost pump is also critical for transfer operations, moving fuel from outboard bays or auxiliary tanks into the main feed chamber. Without a functioning boost pump, the engine may not receive adequate fuel pressure during low-fuel situations or negative-G maneuvers.
How Boost Pumps Are Controlled
Automatic Operation
Most modern aircraft use a three-position switch for boost pump control: OFF, NORM, and ON. In the NORM position, the pump operates automatically under specific conditions:
- During engine start, when the start button is pressed
- When fuel feed line pressure drops below a set threshold, typically 5 PSI
- When crossfeed is selected
- When the fuel feed chamber volume falls below full
On some aircraft like the T-45A, the boost pumps automatically turn on when the GTS start button is pressed and remain on for 30 seconds after GTS shutdown, unless the generator comes online. A time delay relay keeps the pumps operating for 30 seconds after loss of signals or generator failure.
Manual Override
In the ON position, the pump runs continuously regardless of throttle position or system pressure. This is used when the pilot suspects a primary pump failure or when low fuel conditions exist. For example, on the Citation Encore, the boost pump switches must be positioned ON when the low fuel lights illuminate or at 180 pounds or less indicated fuel.
The OFF position disables automatic activation. On some aircraft, when the throttle is in cut-off, the boost pump is disabled in both OFF and NORM positions to prevent pump activation by low-pressure sensing during shutdown.
Boost Pump Indications
Annunciator Lights
When a boost pump receives power, an annunciator light typically illuminates. On the Citation Bravo, the FUEL BOOST L/R annunciator lights up when the respective pump is powered. This is normal during engine start or crossfeed but requires attention if it illuminates unexpectedly in flight.
Low Fuel Pressure Warnings
If the LO FUEL PRESS annunciator illuminates, it indicates fuel pressure is low in the left or right system. The pressure switch should have automatically activated the boost pump. If the light comes on, the pilot should turn the affected boost pump switch ON, then back to NORM to reset the system. If the pump remains on, a low-pressure condition still exists, and turning the boost pump switch OFF could result in an engine flameout.
Common Operational Scenarios
Engine Start
During engine start, the electric boost pump is brought online when the start button is pressed. This provides immediate fuel pressure to the engine until the engine-driven pump can generate sufficient suction. On some aircraft, the boost pump runs for 30 seconds after start initiation.
Crossfeed Operations
When transferring fuel between tanks, the boost pump on the supply side is activated. The crossfeed valve opens, and fuel is drawn from the supply tank through the boost pump and delivered to the opposite engine. On the Citation Bravo, the crossfeed process involves selecting the crossfeed switch to L or R TANK and operating the appropriate boost pump until the LO FUEL PRESS annunciator illuminates.
Generator Failure
After a generator failure, the boost pumps are designed to run for 30 seconds. This provides a critical window for the pilot to diagnose the issue and take corrective action. On airstarts with a failed generator or generator offline, the boost pumps also run for 30 seconds after GTS shutdown.
Fuel Transfer
In aircraft with multiple tanks, the boost pump on the selected supply side is activated to transfer fuel to the opposite engine feed bay. The transferred fuel exits through the inactive boost pump in the receiving bay. This is typically pilot-controlled through a selector knob.
Abnormal Procedures and Warnings
When the Boost Pump Switch Is Off
If the FUEL BOOST switch is in OFF, the pump will not operate automatically during low-pressure situations. This can lead to a fuel starvation condition if the primary pump fails. The switch should normally be in NORM during a normal flight profile.
When the Low Fuel Light Illuminates
When the low fuel lights illuminate, the boost pump switches must be positioned ON to ensure uninterrupted fuel flow. This is critical because the electric boost pump provides the necessary pressure when the fuel level is too low for the primary ejector pump to function.
The 30-Second Rule
Turning either cockpit ENGINE switch to OFF secures the number 2 boost pump, causing the FUEL PRESS caution light to illuminate 30 seconds later. Pilots should not rush the shutdown checklist by turning off the ENGINE switch while taxiing to chocks, as this can cause an unnecessary caution light.
Maintenance Considerations
Running Dry
Leaving a fuel pump switched on with low fuel quantity is an explosion risk since the pump is likely to overheat. If a pump is left running dry for extended periods, typically 10 minutes, it will have insufficient fuel for priming and will be rendered inoperative when the tank is subsequently refueled.
Testing
Operational tests of the fuel booster pump should never be performed dry. A minimum fuel quantity, typically 22 to 24 US gallons, must be in the fuel cell. The pump should operate audibly when switched on, and the low fuel pressure indication should clear.
Types of Boost Pumps
Electric Boost Pumps
These are the most common type, driven by an AC motor with power supplied from the essential bus. On some aircraft, the boost pump is powered by a 28 VDC essential bus to a dedicated inverter. On larger aircraft, 115 VAC three-phase motors are used.
DC Start Pumps
Some aircraft, like the MD-80, have a DC-powered start pump installed in the right main tank to provide fuel pressure for starting the APU or an engine if AC power is not available.
Two-Stage Pumps
On some systems, the boost pump selector switch has a 'low' and 'high' setting. At low setting, the motor runs at a low speed through a resistor. When the throttle is advanced, the resistor is bypassed, and full power is applied to the motor.
Why Understanding Boost Pump Operation Matters
Improper operation of fuel boost pumps has been a contributing factor in accidents. The NTSB has investigated accidents where fuel system limitations, including boost pump-related issues, played a role. Understanding when and how to use the boost pump, what the annunciator lights mean, and how to respond to abnormal indications can prevent fuel starvation and engine failure.
Recommended Fuel Pump: KEMSO Brand
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