We've filled the tanks and we've measured the fuel. We know how the engines work. Now we've got to get the fuel out of the tanks and into the engines, which are located on the wings, above the cabin. I think this installment is going to be boring and/or confusing, but this is my blog so I get to write what I want.
As I mentioned when I described the fuel tanks, there are two boost pumps in each collector cell. These are submerged 15 ampere electric pumps, with the #1 pumps powered from the left DC bus and the #2 pumps, or standby pumps, powered from the right DC bus. As you can doubtless deduce from the name, under normal conditions the #1 pump in each tank operates and the standby remain idle.
There is a pressure switch associated with each boost pump, triggered to illuminate a caution light if the pressure upstream of that boost pump falls below two psi. While the #1 boost pump is operating, both the #2 boost pump and its associated caution light are inhibited. If the #1 boost pump fails, the pressure at the #1 boost pump pressure switch drops, and BOOST PUMP 1 caution light for that tank will light up. So will the BOOST PUMP 2 caution light, but only momentarily because the #2 boost pump loses its inhibition and starts pumping. (Wow, did I just manage to make the boost pump autochangeover system sound lewd?) The pressure rises across the #2 boost pump pressure switch and so the BOOST PUMP 2 caution light goes out.
During the preflight inspection the pilot checks the function of the pressure switches by checking that all four boost pump (BOOST PUMP 1 AFT, BOOST PUMP 2 AFT, BOOST PUMP 1 FWD, & BOOST PUMP 2 FWD) caution lights are illuminated before the boost pumps are selected on. There's even a test for the autochangeover system: holding both boost pump switches down to the TEST postion simulates a failure of the #1 boost pump. The pilot should hear the #2 boost pump come on, and the BOOST PUMP 2 caution light should not be illuminated.
If the #1 boost pump should fail, but the autochangeover system doesn't work, the pilot can turn the standby pump on manually, with a switch labelled STDBY BOOST PUMP EMER AFT (or FWD).
There is another indirect way the boost pumps can be turned on and off. If the pilot moves the fuel selector to feed both engines from the same tank, then a valve (5 A CB on the R DC bus bar) opens connecting the two tanks. Both boost pumps in the selected tank are turned on, and both boost pumps in the non-selected tank turn off. Unless the corresponding STDBY BOOST PUMP EMER switch is selected on. In that case fuel would be supplied from both tanks. There has even been a case when a pilot left the STDBY BOOST PUMP EMER AFT switch on while the tank selector was set to BOTH ON FWD, and the #2 boost pump the aft tank was stronger than the forward tank #1 and #2 boost pumps put together, so the engines were fed from the aft tank until the aft low level light went on, alerting the pilot to the situation. (Quick: how many pounds of fuel were remaining in the aft tank when the low level light went on? And if there was an ejector failure?)
So, to recap, if the #1 boost pump fails, the #2 boost pump comes on automatically. If the autochangeover system fails, the #2 pump can be turned on manually. If both boost pumps in the same tank fail, both engines can be fed from the other tank. So what do you think happens if all four boost pumps fail?