From Markoz in reply to wiltzei's question about the fuel system....

The problem with turning the Fuel Boost Pumps for an empty tank seem to prevent/override the fuel flow into the engine from the crossflow feeds of the other tanks. I always leave the Boost Pumps from empty tanks OFF and so the engines remain running. The side effect of that, is that when I'm doing long haul flights, engines will shut down IF I don't turn those Boost Pumps OFF immediately when the tank empties. Is it a bug? I don't know. It doesn't seem like one to me.

Hummm,

The engine driven fuel pump - the one mounted on the engine - is a two stage pump.

If both stages were normal the engine would actually suction feed fuel out of its own tank without boost pump pressure. If one of the stages was inop, the engine might quit. The fuel boost pumps for the tank are there to insure positive pressure to the engine at high power settings and to crossfeed fuel from other tanks to other engines. In older Boeing jets there is no fuel transfer, just x-feed.

Turning off the boost pump switch after the low pressure light is on is really just matching the condition of the pump since the pump is already off. That by itself should not make the engine quit. During fuel x-feed in the 707 you would need to turn off the boost pumps in the tank of the engine you were x-feeding to so fuel would move from the supply tank to the engine.

FYI - In the 757 & 767, the center fuel boost pumps are override pumps and would push the fuel from tank 2 to the other tanks even if the other tank boost pumps were left on.

Example: Tank 1 has 10,000 pounds and tank 4 has 9,000 pounds. One way to balance this situation would be ....
1.  Turn on engine ignition.
2.  Turn on both fuel boost pumps in tank 1
3.  Tank 1 x-feed open
4.  Tank 4 x-feed open
5.  Turn off fuel boost pump in tank 4 one at a time while observing engine
6.  Monitor fuel balance
7.  When fuel in tank 1 equals tank 4.... Turn on both fuel boost pumps in tank 4 while observing engine
8.  Close x-feed in tank 4
9.  Engine ignition off

The flight engineer would calculate the x-feed time for the balance procedure and advise the captain before and after this operation.

If the x-feeds were closed and the respective engine tank goes dry, the engine would quit since there is no path to supply fuel to that engine. However, if the x-feed valve for the engine is open and a x-feed valve to a tank with fuel is also open and boost pump pressure from the tank with fuel is available, the engine would keep running.

In playing with the CS-707, the engines will not keep running without boost pump pressure even though there is fuel in the respective tank. I am OK with that since it makes you have to keep an eye on the fuel panel and not run a tank dry, but engines with good engine driven fuel pumps would suction feed if fuel was in its respective tank. If the tank runs dry and you're not on x-feed, it get's quiet.

FWIW - The boost pump in the wing that pushes the fuel to the engine is about the size of a can of Fosters beer.  :o

Lou