Fuel pumps.

The fuel system is designed to provide the engines with continuous flow of fuel under all conditions and attitudes encountered during normal flight operations. The system is designed to permit fueling from a single point. There are four tanks in DC9-41; LEFT and RIGHT MAIN tanks, CENTER tank and AUXILLIARY tank (placed forward of the center tank). There is no auxiliary tank in DC9-21. Engine fuel feed is provided by electrically driven fuel boost pumps. Normally, two parallel-mounted boost pumps in each main tank independently supply the left and right engines respectively. The fuel carried in the center tank is fed directly and simultaneously to both engines by two series-mounted boost pumps. With the FUEL CROSSFEED LEVER (on the forward pedestal) in ON position it is possible to route fuel from either tank to both engines and the APU. AUX TANK TRANS Auxiliary tank transfer switch. - When held in TEST position the auxiliary tank transfer pump will start and the auxiliary tank transfer pump low light (on the annunciator) will illuminate momentarily until fuel pressure has built up. - In AUTO position the transfer pump is armed and will automatically start fuel transfer into the center tank when the fuel level in center tank decreases to approx. 900kg. Left and right main tank BOOST PUMP SWITCHES. Each main tank contains two fuel boost pumps designated as AFT and FWD. When a switch is placed in ON position, the corresponding boost pump is activated. The boost pumps operates in parallel. Center tank BOOST PUMP SWITCHES. The two center tank boost pumps operate in series. When placed in ON the center tank fuel will be used before the fuel in the main tanks due to the higher pump pressure.