Where does the fuel come from that powers the APU on a 737? My understanding is that it is from the left front main fuel tank, but I've also heard, hearsay, it can have it's own source. I know that the methods for getting the fuel to the APU can vary: GPU, gravity feed, main tank switch. But where does the fuel come from?
Thank you
It comes from the left side of the fuel manifold, which means it is most often fed from the #1 main tank, typically by turning on either or both boost pumps in that tank. However, it can be fed from the center tank (using the left center tank boost pump) or from the #2 main tank using a boost pump there and opening the crossfeed valve.
If no boost pumps are running, it suction (not gravity) feeds from the #1 main tank. This would be the case when starting the APU with only battery power and no external AC power. It isn't preferrd to do this for longer than necessary, though, because it puts more wear on the fuel control unit with the suction feed than when the fuel is being supplied from a boost pump.
There is no separate fuel tank dedicated to the APU. Gravity feed wouldn't work because the APU is higher than the main tanks (in the wings) and the center tank (below the cabin floor). (The exception to that would be if the aircraft is inverted, although that situation is considered undesirable for several reasons unrelated to providing fuel to the APU.)
Additionaly to what was answered by Ralph J, Boeing offers an additional DC fuel boost pump for APU, that runs whenever APU is running and pressure from the AC pumps from the left manifold is low (i.e. the pumps are off).
