Sign in to follow this  
victorlima01

737: Should there always be fuel in center tank?

Recommended Posts

Hey guys, I just remeber reading somewhere that the 737s are supposed to always have a minimum of 500 lbs in the center fuel tank to avoid having an empty tank full of hazardous fuel vapor. I would like someone to confirm to me whether this informaiton is accurate. If it is, does that mean that I should always have 500 lbs of exta fuel in there even if I am on very short flights where only the wing tanks are enough to hold all my fuel? One thing is to use up the fuel in center tank first and close the tank valves once it reaches 500 lbs. Another thing would be to load up 500 lbs of fuel in the center tank when only wing tank fuel suffices. It doesn't make any sense to me, to carry around 500 extra lbs of something that can't even be used... Best Regards,Victor LimaSBGLhttp://www.precisionmanuals.com/images/forum/800driver.jpg

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

>Hey guys, I just remeber reading somewhere that the 737s are>supposed to always have a minimum of 500 lbs in the center>fuel tank to avoid having an empty tank full of hazardous fuel>vapor. I would like someone to confirm to me whether this>informaiton is accurate. If it is, does that mean that I>should always have 500 lbs of exta fuel in there even if I am>on very short flights where only the wing tanks are enough to>hold all my fuel? One thing is to use up the fuel in center>tank first and close the tank valves once it reaches 500 lbs.>Another thing would be to load up 500 lbs of fuel in the>center tank when only wing tank fuel suffices. It doesn't make>any sense to me, to carry around 500 extra lbs of something>that can't even be used...>> >Best Regards,>Victor Lima>>SBGLI know with Westjet this is SOPs, so yes. It is true.

Share this post


Link to post
Share on other sites

AD 2002-24-51 says:"Airplane Flight Manual (AFM) Revision: Model 737-600, -700, -700C, -800, and -900(:( For Model 737-600, -700, -700C, -800, and -900 series airplanes: Within 4 days after receipt of this AD, revise the Limitations Section of the AFM to include the following (this may be accomplished by inserting a copy of this AD into the AFM):"Certificate LimitationsThe center tank fuel pumps must be OFF for takeoff if center tank fuel is less than 5,000 pounds (2,300 kilograms) with the airplane readied for initial taxi.Both center tank fuel pump switches must be selected Off when center tank fuel quantity reaches approximately 1,000 pounds (500 kilograms) during climb and cruise or 3,000 pounds (1,400 kilograms) during descent and landing. The fuel pumps must be positioned Off at the first indication of fuel pump low pressure.The CWT fuel quantity indication system must be operative to dispatch with CWT mission fuel."Ray

Share this post


Link to post
Share on other sites

You also need around one tonne of fuel in each wing tank to use the hydrualic system too :)

Share this post


Link to post
Share on other sites

Hello fello aviators,we had the similar problem on our fleet of A320 but now the center tanks have been fixed with a modification and so there are no more restrictions on that.-Navneet(F/O A320)FS 2004 Version 9.1AMD Athlon 64 3000+ASUS 7800GTX 256MB PCIExpressMSI nVidia nForce4 Neo 3 Motherboard1GB DDR400(PC3200) Kingston RAMWestern Digital 30 GB (7200 rpm)Seagate Barracuda 120 GB (7200 rpm)Sony DVDRW - 16XHP CD12 CDRW - 12XSamsung SyncMaster 740N LCD 17'PanelCreative Sound Blaster Audigy 4Creative Inspire 5.1 5200Windows XP SP2Logitech Extreme 3D PROLogitech Cordless Optical Mousehttp://www.precisionmanuals.com/images/forum/747400.jpg-|http://www.precisionmanuals.com|-

Share this post


Link to post
Share on other sites

Curious as to what happens over multiple flights. I would guess that many flights (say Southwest for example) do not need full wing tanks for most legs. So, as Victor Lima said, do they just carry around that extra 1000# of fuel for all time? From the AD it sounds as if that fuel can not be considered reserve fuel. Way it seems once fuel is in the center tank it would always be there unless purged some how.I have heard some Boeing AC will siphon fuel from the center with the pumps off, using the fuel but avoiding issues with the pumps themselves? Interesting topic.Joe Lorenc

Share this post


Link to post
Share on other sites

Boeing wanted to keep fuel in the tanks keep the pumps lubricated to keep the pumps from overheating. When the fuel gets the low pumps sense a low pressure and shut off. This is a mod. Before the fuel pumps had to be switched to off manually. There is a mod now that has a nitrogen generator. Basically all ths does is create nitrogen. From this unit the nitrogen gets pumped into the centre tank to displace the fuel vapors to help prevent an incident such as the 747 off the coast a few years back. I am back in groundschool today so I will check to make sure but I believe we do not have to carry fuel in the centre tank anymore with these mods in place. Some companies might operate differently or might not have these mods. I know at my airline we are in the middle of having our planes installed with these nitrogen generating systems.Jack

Share this post


Link to post
Share on other sites

It's actually known as the NGS(Nitrogen Generating System). It neither creates nor produces nitrogen. What it does is extract the oxygen from the air in the centre tank while at the same time extracting the minute amount of nitrogen from that air and pumping it back into the centre tank to reduce the the oxygen level to 12% or less. It doesn't displace the vapours, it just makes them harder to ignite.It's an interesting system but doesn't completely purge the tank with inert gas. It only makes the air less oxygen rich to prevent ignition.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

Share this post


Link to post
Share on other sites

Wow, thanks for all the info. Now I've gotten myself into trouble again, having more to read at once than my limited brain can digest! I was particularly suprised by what someone else said here earlier that there should be at least 500 kg of fuel per wings for the engine driven hyd pumps to work. So to conclude this little story I should never ever land with less than 2,500 kg of fuel (500 center tank + 1000 p/wing), even in worst case scenarios? By worst case scenairo I mean diverting after having executed a missed approach and holding and using up all of your reserve+ extra fuel loaded. That is an absurd amount of extra unusable fuel to be hogging around in my opinion. I don't know why but I always thought that the minimum landing fuel for a new generation 737 was somewhere around 800-1000 kg total fuel...Best Regards,Victor LimaSBGLhttp://www.precisionmanuals.com/images/forum/800driver.jpg

Share this post


Link to post
Share on other sites

Thx John for correcting me on that one. Victor, the hydraulic pumps do NOT need fuel in the mains to work. The hydraulic system DOES use the fuel in the main tanks for cooling and lubrication of the hydraulic pumps through a heat exchanger. If you get low on fuel, obviously there will be a lack of fuel to extract heat from the hydraulic fluid therefore the hydraulic fluid may heat up. Therefore boeing wants 1675 lbs of fuel in the related main tank for when using the electric motor driven pumps. Maybe John can step in here and correct me if i am wrong on this one or even explain it better if i didnt explain it ood enough.Jack

Share this post


Link to post
Share on other sites

So to get it straight: what you really need in the center tank is 1500 lbs of fuel at all times for two reasons: for hyd fluid cooling and to avoid starting a center tank catastrophe? I really like Boeing aircraft, but it sounds like stupid designing imho. Please do enlighten me if I'm wrong, I'm no engineer or mechanic so I'm not privy to all these details. Maybe this "mandatory residual center-tank fuel" should be considered part of the aircraft's BOW...Best Regards,Victor LimaSBGLhttp://www.precisionmanuals.com/images/forum/800driver.jpg

Share this post


Link to post
Share on other sites

Ok to simplify this a bit. There are 3 tanks, 2 main and 1 centre. The hydraulic fluid has heat exchangers in the main tanks. System A hydraulic system uses the heat exchanger in the left main tank and system B hydraulic system uses the right main tank heat exhanger. The hydraulic fluid uses the fuel to dissipate the heat. Obvioulsy if there wasnt enough fuel in the tanks the hydraulic fluid could not use the fuel to dissipate the heat. The hydraulic pumps to do NEED fuel to operate. It just needs the fuel to dissipate the heat. Now I am not saying that the hydraulic fluid will ALWAYS overheat if there is no fuel, just that it MAY overheat. If the hydraulic fluid does not get cooled it MAY overheat the pumps. Now the fuel in the centre tank is a totally different topic. The fuel in the centre tank gets used first when all 6 pumps are operating simply because the pumps in the main tanks are stronger than the ones in the wing tanks. therefore the centre tank gets to empty first. Obviously there will be fumes in the main tank when it gets low. As John and I were saying, the nitrogen generators are used to displace the fumes in the centre tank and leaving the air mixture in the centre tank incombustible (well this is what it is SUPPOSED to do). The thing you were talking about NEEDING to leave fuel in the centre tank derived from the 747 off the coast of New York. If the centre tank gets to low the fuel pumps that are in the tank might now be kept cool due to a lack of fuel in the tanks. So normally when the fuel in the centre tank gets low, the pilots get a LOW PRESSURE on the centre tank pumps on the overhead panel. If the pilots ignored this the fuel pumps COULD possibly heat up. Now you have hot pumps in the fuel tank that is almost empty with a bunch of fumes. what do you think COULD potentially happen. SO two things were done about this. One being the nitrogen generation system and a mod that automatically shuts off the pumps in the centre tank when a LOW PRESSURE situation arises. That make sense now?Jack

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

>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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

>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

Share this post


Link to post
Share on other sites

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:

Share this post


Link to post
Share on other sites

>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

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this