Fuel Tanks, Pumps and Crossfeed

Each wing tank feeds its own engine through a small feeder tank, and the engine itself drives the pump that normally does the work. This lesson covers the tanks and vents, the electrical and jet pumps, and the crossfeed with its memo colours.

At a glance

• Capacity per tank3 185 l (2 500 kg)
• Total capacity6 370 l (5 000 kg)
• Density for those figures0.785
• Thermal expansion allowance2 %
• Surge tank (outer wing)100 l
• Feeder tank (in each tank)200 l
• Electrical pump takes over (jet pump pressure)below 5 psi
• Electrical pump auto-off after HP fuel pressure30 s
• FEED LO PR lightbelow 4 psi
• Low level (pump auto-start)below 160 kg

Tanks and vents

The fuel system has one electrical pump and one jet pump in each of the two tanks, plus a vent system and the quantity indicating and refuel/defuel systems.

The two tanks are integral wing tanks. Maximum capacity is 3 185 l (2 500 kg) per tank, 6 370 l (5 000 kg) in total, at a density of 0.785. Extra volume allows 2 % fuel thermal expansion without spillage. Water drains at each tank's low points work up to a 3° ground slope, and the fuel temperature sensor is in the left feeder compartment.

Each tank vents through its own vent duct and a vent float valve to a 100 l surge tank in the outer wing. The surge tank is open to atmosphere through a flush NACA inlet that avoids icing obstruction. The wing center box above the fuselage stores no fuel.

Feeder tank and pumps

Inside each tank is a 200 l feeder tank, kept full against negative or lateral g, holding one electrical pump and one jet pump.

• Jet pump — driven by high-pressure fuel from the HMU, through a motive flow valve.

• Electrical pump — can supply one engine in the whole envelope. Normally used only to start the engine, then shuts down automatically; it takes over automatically if jet pump pressure drops below 5 psi.

A fire handle controls the fuel LP valve at each tank outlet. When a tank reaches low level — quantity below 160 kg, or the feeder tank not full — its electrical pump starts automatically.

PUMP pushbutton

• Pressed in — the electrical pump starts automatically on jet pump low pressure (engine not running, or a pressure drop), and stops by itself once HP fuel pressure has been available for 30 s and the 8.5 psi switch sees the jet pump working normally. RUN shows green while it runs.

• Released — pump off and motive flow valve closed, white OFF.

• FEED LO PR (amber, with caution, Master Caution and single chime) — fuel delivery pressure below 4 psi: a pump failure or fuel starvation.

Crossfeed

When the electrically actuated crossfeed valve is open, a cyan FUEL X FEED memo appears and both electrical pumps start automatically. One tank alone can be used by switching off the opposite pump.

X FEED pushbutton

• Pressed in — crossfeed valve open (green flow bar in line); both electrical pumps actuated automatically.

• Released — valve closed (bar across the line).

• Both flow bars permanently off — a valve fault.

The FUEL X FEED memo

• Cyan — normal use, crossfeed open.

• Flashing cyan — abnormal crossfeed operation: both pumps OFF, both pumps ON, or a pump OFF on the side holding more fuel.

• Amber — imbalance detected.

Check yourself

• What is the maximum capacity per tank and in total, and at what density?

• What drives the jet pump?

• When does the electrical pump take over, and when does it switch itself off?

• What two conditions start a tank's electrical pump at low level?

• What three pump configurations make the FUEL X FEED memo flash?

Study aid — always fly your aircraft's current FCOM.