Just a quick question about the center fuel tanks. I've watched on many real 737 videos that the center tank will have fuel in it while the centre pump switch is off. Are you not supposed to have your center tanks empty or are these just special circumstances? I often load my NGX with the 1/3 setting which leaves center tanks empty and I can't imagine PMDG would do that if it is not realistic. Anybody know what the "science" is behind having fuel in the tanks, but no pump switch on?

I think that has to do with reducing the chance of the center tank exploding. The tank in TWA800 (or one of the TWA8XXs) didn't have any fuel in it and the pumps were on. The friction from the pumps against the metal fuel tank ignited fuel vapors and blew the plane apart.

The centre tank on the 737 is the one which is drained first when all the fuel pumps are on, because it pumps at a higher pressure than the wing tanks, so if all pumps are on, the centre tank will be drained first.

 

As Kenny noted, unless the aircraft has a nitrogen inerting system in the centre tank to prevent the build up of explosive fuel vapours in an empty fuel tank (this is an expensive option on the 737, and not many of them have it, although it will apparently be standard on the 787), then it is prudent to leave some fuel in there, since more fuel means less room for an explosive vapour to form.

 

However, some of what you have seen on cockpit videos might be force of habit from crews used to operating older 737 models, since up until the mid 1980s, the centre fuel pump did not have an automatic cut out, which meant that if you left it running until the tank emptied, the pump itself would fail to function, and would have to be manually primed with fuel by maintenance crews in order for it to start working again, assuming it had not also been damaged by pumping when empty. So quite a few crews tended to be wary about leaving it on, especially if they'd ever been on the receiving end of a stern talking to for having made it necessary to manually prime the fuel pump on the centre tank, which was not a five minute job.

 

Al

Thanks for the quick reply gents. I just did'nt know if it had anything to do with center of gravity or handling of the aircraft to keep some weight in there. Thanks again.

There are two versions of the centre pumps on our fleet.

 

The early aircraft have the old ones. If theres 2300kg or more in there for takeoff, they would be ON. Otherwise switched on at 10,000ft. When the fuel quantity decreases to 950kg, open the crossfeed and close the L CTR fuel pump and on first indication of a low pressure light close the R pump and close the crossfeed.

 

On the new ones, no specific rules for quantity before takeoff but 500kg is suggested as we could get a FUEL master caution during climb which is distracting in a very busy phase of flight.

 

Henry Lidster