View Full Version : 737 Systems Questions

14th Mar 2003, 20:21
Heya all
I have some questions regarding the 737 which I got after looking through www.b737.org.uk
Firstly, regarding the fuel system, the scavange jet pump operates when the centre pump switches are turned off for 20 minutes. What would happen if you turned the pumps off when the tank was full? Would the pump run for 20 minutes and continue to use fuel from the centre tank or does it only run when the fuel quantity is below a certain value?
Secondly on the 737NG's their is no GEN busses, the Gens power the transfers directly, so what happened to the 28v AC main and main buses? What happened to all the loadings (such as the galleys and hyd pumps) that were on these buses?
Third, when the bus transfer relay is positioned 'off' why do the transfer buses lose power? I thought that this just meant that power cannot be transfered from one transfer bus to the other should a generator fail?
Lastly, when the standby power switch is in 'auto' am I right in saying that: If power is lost from transfer bus 1 power for the standby AC bus is taken from the static inverter from the bat bus, except on the ground when the switch must be moved to 'bat' to get power.
What else is the bat position for? And one last question (I promise) is that when the stby pwr switch is moved to 'off' is the standby AC bus unpowered no matter what the situation?

14th Mar 2003, 22:24
:O I fly the thing, and NOW Im confused;) Let me look into it and I will get back to you. But a word of advise, don't get to caught up in it all, basically you turn it on and things happen, if they dont you go to the QRH, and if that does not work, well..................?

Regarding the scavange pump it only starts to operate when the number one main tank is half full and the tank one forward pump is operating. Once the scvange pump starts to run, you cant stop it for the rest of the flight:p

Cross Check
18th Mar 2003, 05:21
For –300/500’s (the Classics! :ok: )

Hej Richie

As soon as both CTR pumps are turned off the scavenge shut-off valve opens and a 20 minute timer starts. Utilising #1 FWD pump pressure the scavenge pump “sucks” CTR tank fuel (or air) into the #1 MAIN tank for the duration of the 20 minutes. The scavenge SOV regulates fuel pressure to the scavenge pump (motive flow pump), but I’ve never found out what kind of transfer flow rate it achieves … it just “sucks” :} Would the pump run for 20 minutes… Yes, that’s what it’s supposed to do. … and continue to use fuel from the centre tank… Do you mean the extraction of fuel from the CTR tank or #1 ENG … the scavenge pump operates on fuel pressure provided by #1 FWD pump and draws fuel from the CTR tank to the #1 MAIN tank. …or does it only run when the fuel quantity is below a certain value? I’ve not discovered or been made aware of any "limiting" fuel quantity governing the jet pump’s operation – the criteria is for both CTR pump to be switched to OFF (and #1 FWD pump on of course, else no motive flow := ). It is also possible to keep activating the the scavenge pump if you so desire ...

Where did you see “positioning the bus transfer relay [sic – switch] to off would cause the transfer buses to lose power”? By selecting OFF you isolate the #1 and #2 DC buses (via their respective TR’s), prevent the battery charger from switching to it’s alternate source of power (#2 MAIN bus) and prevent TRFR to GEN bus switching. The relay logic is to switch a de-powered TRFR bus (due associated GEN bus failure) to the opposite powered GEN bus – this is to protect the aircraft’s “essential” AC/DC loads. The TRFR buses cannot power each other.

STBY PWR switch in AUTO – Yes and yes; you are right as long as TR3 is powered or the BATT switch is on, and on the ground the BAT position overrides the air/ground safety logic. What else is BAT good for? Well … it’s a good way to drain the battery :uhoh: -> :ouch: … otherwise the standby power test, doing a battery start (powers the right ignitors), calling for the fuel truck on Comm#1 (instead of going for a walk) :p , refuelling on battery power, checking out how much fuel is onboard (cold aircraft) the lazy way … awww c’mon … who wants to pull out all those floatsticks :ugh: :zzz:

STBY PWR switch in OFF – the STBY AC and STBY DC buses will be de-powered

18th Mar 2003, 09:38
what ever happens it works just fine ! :cool:

19th Mar 2003, 17:10
Yes - the only 'gotcha' I have found is a full No 1 tank, fuel in the centre, put on the centre pumps, change your mind - and you can over-fill No1 tank!

The only improvement I would ask for (for Christmas) is to be able to select the destination for the scavenged fuel. It is great for balancing when No 1 is lower but when No 2 is lower it is a pain!

19th Mar 2003, 17:44
BOAC- RE the 'changing your mind' thats what I was thinking!
Thanks for your all your replies.

19th Mar 2003, 17:53
But the center pumps MUST be on if the mains are full, because of the aircrafts AOM limitation that the main tanks must be full if the center tank contains more than 453 kilos. So if you turn it off, "change your mind", you are operating against the AOM:O

19th Mar 2003, 18:46
SOPS - I do not quite undertstand your logic? I assume you realise I am referring to ground holding, otherwise the engine flow rate would avoid any over-filling of a wing tank?

Putting centre tank pumps on with lowish contents before take-off WILL give you a large fuel imbalance in the climb due to the location of the 2 centre tank pumps and their different flow rates. Therefore in my experience it is normal to leave these pumps off at certain fuel states until some point in the climb/cruise.

Cross Check
20th Mar 2003, 15:25
BOAC - haven't done it myself, but have seen it happen once ... and the heated exchange between engineer on ground and engineer in cockpit :eek:

SOPS - just a point of clarification. The three AOM limitations wrt Fuel disposition are: if more than 453kg fuel is to be loaded in the CTR Tank the Mains must be full, the lateral imbalance between #1 & #2 must be scheduled to zero and the random imbalance is not to exceed 453kg at any stage from T/O to LDG.

The normal procedure recommendation is to switch ON the CTR Pumps only if CTR Tank fuel quantity exceeds 453kg. ON is preferable anyway because you don't want extended APU ops to create an imbalance before you even get around to pushback (cf limitation #2).

I discovered pretty quickly that light-weight departures with around 10.0 FOB can get a nuisance caution on rotation because the fuel shifts back uncovering the ports in the CTR Tank - they're longitudinally displaced (the L/R CTR pumps are physically in the respective Main Tank ;) BOAC you must be on NG's) - from memory the right CTR Pump starves first, but someone please correct me..When established in a level attitude cruise, if the CTR tank contains useable fuel and the CTR tank pumps switches are off, the CTR tank pump switches should be positioned ON again. If the CTR tank contains more than 453kg, the CTR tank pump switches must be turned ON.This alone suggests that for T/O and CLM it's permissible to have the CTR pumps off - they just have to be back on by cruise. I'm sure if Boeing thought the wings were going to fold up for 500kg (x2) burnoff out of the Mains they would have been a little more strict in their wording. So you can rest in the comfort you are not defying the AOM :cool: Incidently, we turn ours off when we line up and back on once accelerating out of min manouevring to 250.

... still friends? :O

Oh yeah, BOAC - I was told the six pumps were all "identical" - same nominal flow rate, same pressure (30psi, flow rate is dependant) - the difference was all in the check valves. The CTR tank check valves are adjusted lower. The only difference comes about when you have the AUX tanks fitted in the aft hold. AUX fuel must be used before CTR fuel (can't imagine why :eek: ) and therefore has the higher pressure "mother of all pumps" to beat the rest into the left fuel manifold.

20th Mar 2003, 17:26
Cross Check - I should have been more 'precise' with my post :eek: (Trying the 'KISS' principle!) It was indeed tank 'feed' position I meant (- 1-500's).

This, from Chris Brady's site (HOPEFULLY with his permission!:eek: ) is what I was referring to

"NB On the classics, when departing with less than 1,000kg of fuel in the centre tank, an imbalance may occur during the climb. This is because the RH centre tank pump will stop feeding due to the body angle so number 2 engine fuel is drawn from main tank 2, while engine 1 is still drawing fuel from the centre tank. When this “runs dry” the scavenge pump will also transfer any remaining centre tank fuel into main tank 1, thereby exacerbating the imbalance."

'Scuse me if you were 'TIC' about the aux tank, (and it is a WHILE since I operated one) but I understand use of ctre before aux can give c of g problems (as can refuelling the aux before the ctre is full!) hence the tank output differential. I concur with the tank output v check valve analysis you post and "the location of the 2 centre tank pumps and their different flow rates" referred to the flow rates due to pitch angle.

Must try harder :(


Cross Check
20th Mar 2003, 18:29
She's right mate, no dramas. The pax are none the wiser and they'll only remember you for your silky landings anyway ;)

I forgot to agree with you on the fuel imbalance problem too - ta. All fair and vaild reasons to have the CTR's off when departing with 10.5t or less of go-juice onboard.

I've only flown a sim with AUX's and luckily it's not too hard to manage. The autopilot manages nicely to trim in the event of a "oh I forgot to turn the AUX's on" and although we didn't burn much more than half the CTR tank you could see the trim was a couple of extra units nose down. The nightmare is for the ground crews I gather - refuelling with AUX's is a right prick of a procedure :* From memory I think the CTR tank has to be full before you can pump into the AUX, but if you do something crook with the de-fuelling/crossfeed (?) valves (and have fuel in the wings) you can put the aircraft in T/O attitude at the gate :hmm: Dunno if the instructor was kidding or for real, but it did keep the session interesting.

3rd Feb 2005, 08:56
@Cross Check...but I’ve never found out what kind of transfer flow rate it achieves … it just “sucks”...A flow rate of 150kg in 20 minutes I have in mind.


Flight Detent
4th Feb 2005, 02:30
I seem to remember that there is an NG fuel feed procedure that reads something like:
"If the center tank fuel quantity is less than 5,000 KG at engine start, leave the CTR TANK fuel pumps OFF until established in cruise flight"

Something close to that.

Which avoids the problem of LOW PRESS indications at rotation and early climb!

Cheers, FD

4th Feb 2005, 16:30
Each company should have their own SOP about CTR Tank pumps for takeoff. Ours is if it has 800kg's or less, they will be off to avoid possibility of master caution on T/O, rotation. They are put on during climb or if its really low and one or both pumps won't 'take' the fuel due to body angle then the'll be up on at level off. Don't know about the 5,000kg's or less for the NG, but that's alot! At 1000kg or more, leaving centre pumps on is fine for T/O and climb to CRZ. Imbalance and therefore crossfeed after all this is just a way of life.

5th Feb 2005, 06:05
Third, when the bus transfer relay is positioned 'off' why do the transfer buses lose power? I thought that this just meant that power cannot be transfered from one transfer bus to the other should a generator fail?

This statement is most likely true only when you are supplying the aircraft with Ext or APU power (The bus ties need to be closed to get power from Ext or APU to the Xfr Buses).

Regarding the scavenge question...

The NG scavenge pumps are "hydromechanical" and are not electrically operated and do not have timers.
On the NG, there is a float valve in the Left tank which won't allow fuel to be scavenged from the centre tank unless the fuel level in the Left tank is below 4,487lbs (1990Kg).
Scavenge flow rate is between 220 to 450lbs/hr (100~200Kg/hr)
In your scenario, the scavenging won't begin until the float valve is opened. The scavenge pump should continue to operate until the left tank fwd boost pump runs dry (Obviously the scavenge rate will be insufficient to empty the Centre Tank first).

I stand to be corrected ;)