Monday, June 20, 2005

Fuel Pumps

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?


Anoynmous said...

(The aft low fuel light comes on at 110 pounds in the collector cell, 440 pounds total if the ejector has failed.)

All four boost pumps failing would seem to be so rare as to be not worth worrying about, except that they're all electric and would all quit simultaneously if DC power went away. I am guessing there is another non-electric way to get the fuel to the engines, and I'm not counting flying inverted to let gravity do the job.

Aviatrix said...

Hey, you've been paying attention.

The left and right DC bus bars are powered by separate generators, so all DC power going away would be two failures.

And this is an airplane we're talking about. Even the backup systems have backups.

Lord Hutton said...

Isn't there a sepcial tube that the pilot can suck at to create a siphon?

Aviatrix said...

Clever guess, Andy, but no.

Anoynmous said...

It occurs to me that these things are named boost pumps, and in flight they are probably just supplementing a mechanical fuel pump driven from the turbine. So if all of them fail, you would still have fuel flow as long as the engine is running (and the engine will usually run as long as it has fuel).

I'm also remembering hearing my father reading off the checklist in his Bonanza, where the boost pump switch was turned on only during engine start, takeoff, and landing. It's off for cruise, so obviously there is a "regular" fuel pump to handle normal requirements.