737 MAX Flight Crew Operations Manual
Fuel -
System Description
12.20.2
MN-FLT-OH-201
Fuel Pumps
Each fuel tank uses two AC powered fuel pumps which are cooled and lubricated
by fuel passing through the pump. Center tank pumps produce higher pressure
than main tank pumps. This ensures that center tank fuel is used before main tank
fuel, even though all fuel pumps are operating. Individual pressure sensors
monitor the output pressure of each pump.
Each center tank pump will automatically shut off, after a short delay, when that
pump’s sensor detects low output pressure.
Note:
Fuel pump LOW PRESSURE lights may flicker when tank quantity is low
and the airplane is in a climb, descent, or on the ground with a nose-down
attitude.
Note:
Center tank fuel pump LOW PRESSURE lights may flicker when tank
quantity is low and the airplane is in cruise. One pump may indicate low
pressure sooner than the other due to aircraft attitude and/or slight variation
between pump inlet position. Low pressure indication may occur after
center tank quantity reads zero. Low pressure light flickering can continue
for as long as 5 minutes before the Fuel System Annunciator light and the
Master Caution lights are illuminated for the associated center tank pump.
Suction Feed
When main tank fuel pump pressure is low, each engine can draw fuel from its
corresponding main tank through a suction feed line that bypasses the pumps. As
the airplane climbs, dissolved air is released from the fuel in the tank due to the
decrease in air pressure. This air may collect in the suction feed line and restrict
fuel flow. At high altitude, thrust deterioration or engine flameout may occur as a
result of the fuel flow reduction.
The dissolved air in the fuel tank will eventually deplete after reaching cruise
altitude. The depletion time is dependent upon airplane altitude, fuel temperature,
and type of fuel. Once the dissolved air is depleted, the engine may be capable of
suction feed operation at cruise power.
Fuel Crossfeed
Two independent, engine fuel manifolds provide pressurized fuel to the engines.
Each engine has its own engine fuel manifold.
The engine fuel manifolds are interconnected by use of the crossfeed valve. The
valve is DC motor operated from the battery bus.
Fuel pressure can be provided from a main tank with operating fuel pumps to both
engines by opening the fuel crossfeed valve. Continued crossfeed use will result
in a progressive fuel imbalance.
March 1, 2021
