On short final to KMHT the calm approach was interrupted by the alert tone. "Fuel" flashed up on the display. A glance at the gauges showed about 30% in each side, but the center tank had, apparently, just emptied. I switched to the overhead which, with the engines now spooling down, was lighting up with increasing problems as the generators dropped off-line. All fuel pumps were still ON, and the only anomaly I noticed was the crossfeed switch was enabled. I flipped it back and attempted a re-start - not easy with no APU running :-) I gave up the relight and barely dead-sticked it to the threshold.Two questions -- is it expected behavior for the engines to quit if the center tank empties and crossfeed is enabled? Secondly, in such a situation (losing engines on final) what, if any, is the correct procedure to follow?Thanks, Jimi

>On short final to KMHT the calm approach was interrupted by>the alert tone. "Fuel" flashed up on the display. A glance at>the gauges showed about 30% in each side, but the center tank>had, apparently, just emptied. I switched to the overhead>which, with the engines now spooling down, was lighting up>with increasing problems as the generators dropped off-line.>All fuel pumps were still ON, and the only anomaly I noticed>was the crossfeed switch was enabled. I flipped it back and>attempted a re-start - not easy with no APU running :-) I gave>up the relight and barely dead-sticked it to the threshold.>>Two questions -- is it expected behavior for the engines to>quit if the center tank empties and crossfeed is enabled?>Secondly, in such a situation (losing engines on final) what,>if any, is the correct procedure to follow?>>Thanks, JimiThis is a known problem with the routines that "fake out" the FSx fuel logic, and is supposedly fixed in SU2.When you're down to less than 1000 Lbs in the center tank, you need to shut off the fuel pumps. This is both the workaround for the bug, and the realworld procedure, as keeping some fuel in the tanks prevents any wiring from being exposed to vapors and keeps the fuel pumps cool.

Understood and noted. Another thing to keep my eye on in flight.Incidentally, thank you sir for an excellent tutorial. I feel it shortened my learning curve considerably and helped me realize what a comprehensively modeled aircraft this is.Best, Jimi

>When you're down to less than 1000 Lbs in the center tank, you>need to shut off the fuel pumps. This is both the workaround>for the bug, and the realworld procedure, as keeping some fuel>in the tanks prevents any wiring from being exposed to vapors>and keeps the fuel pumps cool.>Very interesting note. I have one question:assume we are running out of fuel from the center tank... if I don't touch anything in the fuel panel control, when the booster pumps can no longer pump fuel from the center tank, the engines should be normally fed by the other tanks and continue running normally.At that point, will the center tank be emptied "completely" by the scavenge pump ?When the center tank gauge reads "0" in the cockpit the amount of fuel in the center tank should be only enough to keep the booster pumps submerged completely ?

"when the booster pumps can no longer pump fuel from the center tank, the engines should be normally fed by the other tanks and continue running normally.At that point, will the center tank be emptied "completely" by the scavenge pump ?"Tough question, Mr/Mrs/Miss (?).... and I'm not sure I have a definitive answer for you.Note that there are two types of scavenge pump in the 737NG CWT. One is for fuel, the other is for water. The latter siphons water in small quantities (if any) from the very bottom of the tank whenever there is pressure in the engine fuel manifolds and mixes it with much larger quantities of fuel (these pump types have no way of distinguishing water from fuel, so I guess you could also say they were small fuel scavenge pumps). The proper CWT fuel scavenge pump, on the other hand, comes into operation when the Left (Main #1) Forward boost pump is running and the fuel in the Main #1 Tank is below 1990Kg (4487lbs).The pumps themselves are not located on the bottom of the tanks, but the pickup pipes which feed the pumps are usually very close to the bottom of the tank. Scavenge pumps, BTW, normally have a good filtration system to stop solid items getting sucked up.The pickups for fuel pumps can be in various locations in the tank to enable fuel to enter one or more of the pickups at most normal airplane attitudes. However, there is no way to completely empty fuel tanks using these pumps. There will be drain plugs to faciliatate this for tank maintenance. "When the center tank gauge reads "0" in the cockpit the amount of fuel in the center tank should be only enough to keep the booster pumps submerged completely ?"This question has generated endless debates on aviation forums over the years. I think you'd be hard pressed to find an "official" answer to this one. I would say that there would be a safety margin (there would always be some fuel remaing in the CWT when the gauge read zero to allow for different normal airplane attitudes, the possibility that the #1 forward main pump might fail (losing your scavenge ability), etc). Any extra fuel you can extract from the tanks via safe methods (e.g. scavenging) will probably be a bonus.Under new regulations to help prevent fuel tank explosions, aircraft are now being modified to provide visual/aural warnings of low fuel levels in tanks before you get pump low pressure indications (i.e. these warnings are generated by fuel levels rather than fuel pressure). Fuel pumps are kept cool not by submersion, but by the fuel being pumped through them, so it is necessary to ensure that the pumps are switched off before the fuel stops running through them.Hope this helps.Cheers.Ian.P.S. Edited for political correctness ;-)

For the ignorant (but interested) simmers, like me. Is it possible to explain what a scavenger pump is exactly and how these work?Thanks!JohanNL

Hi Johan,While I'm not an expert I've taken some time to get some knowledge about this. A scavenge pump is, as I understand it, a pump that will suck any residual of fluid left in a tank or sump i.e an oil scavenge pump will pump oil from the bottom of an oil-sump. The NG center tank is serviced by three pumps. Two main pumps with their inlets located left and right of the center tank. And a scavenge pump located in the left wing tankand with it's inlet in the low parts of the center tank. See the linked schematic from http://www.b737.org.uk for a brief explanation on the NG fuel system.If I'm mistaken here I'm sure one of the real experts will correct me. ;-)Hope it helps,

>Secondly, in such a situation (losing engines on final) what,>if any, is the correct procedure to follow?Well, in the event that you loose your 2 engines on final, the correct procedure essentially depends on your religion...Sorry, couldn't resist...Eric L.

I just tried a flight yesterday where I knew I would run out of fuel in the center tank.What happend was, that I lost all power on my right engine, eventhough I could see that there was still being fuel used in the right tank!! Does the left engine use fuel from both the right and left tank?

Hi,It shouldn't if you have the X-feed valve closed!TGIF,

"The NG center tank is serviced by three pumps. "This depends on your definition of pumps, Mats ;-)The NG scavenge pumps are "jet" pumps. These have no moving parts. They rely on a venturi effect: i.e. fluid/gas rushing through a large pipe which narrows at certain point speeds up the flow of the fluid, but decreases the pressure in that fluid at that point (a bit like the air going over the top of an airplane wing). If you have a smal pipe attached to the bigger pipe at this point, a suction will be created in the smaller pipe.http://members.ozemail.com.au/~b744er/737/Venturi.gifThe CWT has three of these jet pumps. Older aircraft are often fitted with an electric pump for scavenging (including older 747-400's). I believe the newer ones have all switched to jet pumps.Cheers.Ian.

Hi IanHow's things?>The NG scavenge pumps are "jet" pumps. These have no moving parts. That would be a pump! We call them venturi pumps though. I've seen these in action in heating subcentrals in southern Siberia, Irkutsk to be exact. Didn't work too good though... lolThanks for the clarification though. As always something new to learn about the systems onboard. ;-)Cheers,