737: Should there always be fuel in center tank?

[victorlima01 2]

Certainly does Jack. Thank you very much. However my quesiton still stands. Take a 737 that hasn't been retrofitted with the nitrogen pumping system. Will the operators always carry around that extra useless fuel to avoid a potential center tank explosion? It sounds like they should and they probably do. My second question is whether this residual fuel in the center tank is also necessary for other aircraft, such as the 777 and 767. Thanks for all the help shown so far! Best Regards,Victor LimaSBGLhttp://www.precisionmanuals.com/images/forum/800driver.jpg

[Jack C 8]

>Interesting thread - thanks, everyone!>>According to the flight manual, the wing tank fuel requirement>for the operation of the electric hydraulic pumps is as>follows:>>"Minimum fuel for stationary ground operations of electric>hydraulic pumps on ground is 1,676 lbs. in each wing tank.">>Since the requirement applies only to ground ops(why, I don't>know?), I assume the fuel becomes useable once in the air?>>I might add a couple of additional questions:>>1. Are the electric hydraulic pumps dedicated to pumping fuel>only or do they serve other functions?>>2. How are the engine driven hydraulic pumps cooled?>>Thanks,>Frank>>Hi Frank. OK i think you got things a bit mixed up there. Lets start with the 1675 lbs in the main tanks. I am going to take a stab at this one here but since i am the type of guy that has to know WHY things happen on aircraft i will definately find that answer out for you. BUT for now i will stab at it. You obviously seen a 737 sitting on the ground right. the have a slant forward kind of look to them right? i mean the nose sits low and the tail sits high. I am guessing with this attitude on the ground, the fuel sits at the front of the wing tanks. therefore if there was only 800 lbs of fuel in the wing tanks the fuel might not be sufficient enough to be used in the heat exchanger. therefore you need at least 1675 lbs for the heat exchanger to be engulfed in fuel. Again that is my stab at it but i am going into the hangar today and will find a definate answer. Now you ask "Are the electric hydraulic pumps dedicated to pumping fuel only or do they serve other functions?". I think you are gettign confused here. The hydraulic pumps DO NOT pump any fuel. Think of it like a radiator in your car. The coolant needs to be cooled right? So it passes through the radiator where the airflow over the radiator extracts heat from the coolant and the coolant gets sent back to the engine core to cool the block. Well the hydraulic fluid needs to be cooled so it goes through a radiator that is placed in the fuel tank and the fuel absorbs the heat from the hydraulic fluid. Since the fuel is fairly cool and a liquid is a better conductor of heat than air the fuel is a good source to extract the heat from the hydraulic fluid. As for the engine driven hydraulic pumps? I am guessing they are cooled the same but i do not think they generate as much heat as the electric driven ones. Again i will find out for sure.Jack

[zubart 5]

Most 737NG's can burn all the fuel out of the center tank now. It's been this way for a couple of years now.One of the Procedure's to burn all fuel from center tank in some airplanes:-When Center Fuel Tank Quantity reaches 2000 lbs -Crossfeed Valve - Open-One Center Fuel tank Fuel pump switch - Off-When Center tank fuel quantity reads 0-Other center fuel tank switch - Off-Crossfeed Valve - CloseGood luck,Floyd

[Guest 102301636]

This is certainly a very interesting topic and there is plenty to learn from all the postings. i'm surprised all the same by those who think it's crazy to carry around so much fuel which technically isn't usable.A good analogy would be if you owned a car and had a tank of gas that was full, it doesn't mean you use the tank til it's dry you fill up beforehand, especially when it's a multi-million dollar piece of equipment you're talking about and not a car :)The same goes for aircraft except obviously the amount of fuel in question is larger

[Jack C 8]

Just to clear that up. It IS usable. you can suck the centre tank dry.

[CF104 0]

>1. Are the electric hydraulic pumps dedicated to pumping fuel>only or do they serve other functions?The electric 'A' and 'B' system hydrualic pumps only pump hydrualic fluid and have no connection with the fuel system.>>2. How are the engine driven hydraulic pumps cooled?>You have to remember that it's only the fluid that's getting cooled and not the pumps. The pumps themselves are cooled by the hydraulic fluid they pump. The heat exchangers are specific to each hydraulic system and not only the electric pumps. 'A' system is cooled by #1 main fuel tank and 'B' is cooled by #2 main fuel tank. The heat exchangers are located on the return line back to the reserviors.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpg

[Guest frankathl]

Jack C and John - Geez, I'm embarassed; that was such an elementary mistake!!! :-( I guess my brain froze a bit after being away from FS for a time and I got confused? Anyway, "Yes, of course!", and thanks a bunch for putting me right and your other explanations.Jack C, your idea that the amount of fuel required to cool the hydraulic fluid passing through the heat exchangers is related to the attitude of the airplane makes sense to me, and is I think backed up by references to the relevant Airworthiness Directives in the flight manual:

[CF104 0]

>Jack C and John - Geez, I'm embarassed; that was such an>elementary mistake!!! :-( I guess my brain froze a bit after>being away from FS for a time and I got confused? Anyway,>"Yes, of course!", and thanks a bunch for putting me right and>your other explanations.No problem Frank. We all have those moments.>John, thanks for clarifications on how the pumps and the>hydraulic fluid itself is cooled. I promise not the forget the>purpose of the hydraulic pumps in the future!;-)>>A related, slightly off topic, question(just a clarification,>really)?. Since there is no AUTO setting for the electric>hydraulic pumps, I assume they are always operating when the>hydraulic pumps switches are ON? That is, they are not demand>pumps, like in the B767, for example? Presuming the answer is>Yes(always operating), is it because the engine driven pumps>are insufficient on their own to meet system requirements, or>is it to provide backup in case the engine driven pumps should>fail?There are several differences between the B767 and the B73NG. On the NG, the electric pumps are on and providing pressure with the switch ON. The engine driven pumps supply about 4 times the fluid volume that the electric pumps produce so they are the main producers of hydraulic power. The reasoning behind having engine and electric pumps is for redundency due to certification and ETOPS requirements.On the B767, the Primary engine pumps(L&R) and electric centre pumps© are always on in normal operation. They're just like the B73NG but the Demand electric and air pumps are there to supplement the Primary L,R&C pumps in times of high volume usage if they can't keep up with the demand. Therefore they are known as Demand pumps.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpg

[Guest frankathl]

Thanks once again, John!Frank