Ctr fuel pump restriction on GND, 737NG

[KristianNorway]

Hey guys

How come you may empty the center tank fuel while in the air but not on the ground?

On the ground you have a restriction of 453 kgs (1000 Lbs) to turn the pumps on before startup. Then you'd have to turn them off again before T/O, but that I understand.

Any good answers?

[theflyingbus]

nuisance/distracting master caution on take off?

[BOAC]

Is this a new limitation? I was not aware of it in 2008. I'm sure you can run them for defuelling or fuel transfer. Otherwise as the bus says. They would almost certainly illuminate on take-off at that state.

[punk3029]

Yes, new since last update

But since 2008 there have been a number of special procedures with the 737 fuel systmem We operated them under a AMOC for a long time.

[ImbracableCrunk]

How come you may empty the center tank fuel while in the air but not on the ground?

How about because if the plane is in the air, there's probably (hopefully) someone in the cockpit? That may not be the case on the ground, even though it's required.

[Breakthesilence]

It is a cooling matter.

[twochai]

It is a cooling matter

Ever since one or two center fuel tank explosions on the ground, the most recent being a Thai Airways B734 at Don Muang in 2001, running the center fuel tank pump with low fuel level in the tank has been a concern.

Here's the link: Thai 737-400 Fuel Tank Explosion - 3 Mar 2001 (http://www.b737.org.uk/thai737news.htm)

[Denti]

Since then a few things have changed. Center tank fuel pumps will switch themselves off if output pressure is low (after a short delay) and airplanes have an NGS fitted.

I would think it is a cooling matter indeed, the explosion risk of dry running pumps has been fixed.

[KristianNorway]

Do any of you guys have any document references?

[BOAC]

I'm pretty sure the 'cooling thing' is a red herring since I understand the NG wing pumps are in the centre tank too, so that would make committing any form of aviation in an NG with an empty Ctre tank somewhat difficult:)

I would have expected EITHER the AFM un-modded pump restrictions (2300kg) OR NO restriction for modded pumps. I remain convinced that (if it is not an editing error) it is to prevent 'nuisance' alerts.

[EW73]

Yep...BOAC, you're right, all the main and centre fuel boost pumps are physically located in the centre tank space. But that has no effect on running the main tank pumps because they will be cooled by the fuel they are drawing from the main tanks 1 and 2.
Unlike the centre tank pumps, which would not be cooled at all when the centre tank runs dry.
Cheers..EW73

[BOAC]

Thanks for that, EW, but I would have thought with 'modded pumps' the problem of 'cooling' would not arise since they shut off or would be shut-off by crew, or have I misunderstood the mod?

My understanding of the AD was

''Center Tank Fuel Pumps
Intentional dry running of a center tank fuel pump (low pressure light illuminated) is prohibited.''

and that the 'no ground running <453kg' was only listed as an alternative means of compliance so the primary compliance should be ok?

[Schiller]

In the 732 the wing pumps are in dry bays in the bottom of the wing, thus allowing for their removal without defuelling the wing tanks. The centre tank pumps are in the leading edge of the wing, for the same reason. Has this changed in the NG?

The min fuel requirement for running the centre tank pumps is, I suspect, to avoid the possibility of the tank being run dry and the pump overheating - the suspected cause of the explosions in the centre tank on the ground. In the air, one is encouraged to turn the pumps OFF as soon as the low pressure light illuminates.

[BOAC]

Schiller - see #10, #12 and #13

[Denti]

Running dry can't happen anymore as the pump switch themselves off. Apart from that there is no combustible gas mixture inside the centre tank since the nitrogen generation system fills the center tank with mostly nitrogen, not completely, but oxygen is well below below the required level. Apart from that, yes, center tank pumps have to be switched off manually when the low pressure light (and master caution) comes, not only in the air but on the ground as well of course.

Unmodded airplanes have somewhat different limitations, they should have a placard with those in the field of view of the operating crew inside the flightdeck.

Kinda interesting, the 453kgs limitation is only found in the the cockpit preparation part of our manual, it is not listed in the limitations part.

[Schiller]

If the position of the pumps in the NG is different from the 732, why the change? Does anyone know? It seems odd to move them to a position which makes maintainance more difficult.

[Green Guard]

How come you may empty the center tank fuel while in the air but not on the ground?

Into what or where would you PUMP the center tank fuel while on ground ?

running the center fuel tank pump with low fuel level in the tank has been a concern.


If it is a concern then it must be a concern for both situations, in the air AND on ground.

[captjns]

Into what or where would you PUMP the center tank fuel while on ground ?

Into the wings , possibly for maintanance.

[BOAC]

Into what or where would you PUMP the center tank fuel while on ground ? - or the engines?

[Breakthesilence]

I guess the difference in "ground" and "air" limitation exists because in the air fuel is cooler than on the ground, so running the CTR pumps with less than 453 Kgs in flight is not as dangerous as running them on the ground.

Kinda interesting, the 453kgs limitation is only found in the the cockpit preparation part of our manual, it is not listed in the limitations part.

The "Limitations" section of the FCOM 1 doesn't contain limitations that are found in the other sections of the FCOM (such as Limitations contained in the Normal procedures, Systems sections and so on).

[cosmo kramer]

I would think the answer should be found in the passage about the center tank pumps:
Note: Fuel pump LOW PRESSURE lights may flicker when tank quantity is low and the airplane is in a climb, descent, or on the ground with a nose-down attitude.

Some time ago when picking up an aircraft from maintenance it was still in the hangar and the engineers were finishing up when we got there. Apparently they had at some point during the maintenance emptied the fuel load of the wings into the center tank, and were now transferring it back using the center pumps. They were unable to pump the last barely 500 kg from the center tank back to the wings (the pumps where not taking any fuel and had low pressure lights illuminated).

Hence, my personal theory is that the pumps probably run dry with less than 453 kg on ground in certain situations when the aircraft is parked on a downslope or maybe even just when empty and the gear struts extended. Probably for that reason Boeing decided to mandate that they at all times must be kept off if below this value.

With any automatic system of any kind, it's normally good practice to set the relevant manual switch to the agreeing position. Automatic systems can fail and the auto shut off feature of the fuel pump should be regarded as an extra protection, in case someone negligently forgets to turn the pumps off. It is therefore also still required that the flight deck is manned as long as the center tank fuel pumps are operating.

P.s.
And not all NG's have the nitrogen generation system, probably far from since it wasn't introduced until 2006.

[HAWK21M]

You run the CTR pumps till one LOW light illuminates.But post mod Auto switchoff to be Incororated.

[BOAC]

cosmo - I see your logic, but the same 'flickering' could apply in descent or with flap out at <500kg, could it not, hence the 'pre-mod' restriction - which is the only one I have worked with. Of course, in 'Normal' ops, the last 500 kg will be scavenged as the mains drop.

[KristianNorway]

And then the restrictions on T/O and descent wouldn't be as much as 2300 kgs and 1400 kgs respectively I'd guess..

One technician told me rather hastily that if one emptied the CTR tanks on ground one needed several tonnes of fuel to be refilled in the CTR tanks again to get them running afterwards. Why this isn't the case in air I have no idea about.

That's why I'm asking. I was told it may have been listed in the previous FCOM, prior to Boeing taking away a lot of information, but that was before I started flying the 737.

[framer]

Hence, my personal theory is that the pumps probably run dry with less than 453 kg on ground in certain situations when the aircraft is parked on a downslope or maybe even just when empty and the gear struts extended.
I can see what you're saying there cosmo but then wouldn't it be a massive fluke that they just happen to run dry at 453 kg which is pretty much 1000lbs? Why wasn't it 353kg or 553kg? Is it a fluke that 453kg is also the firgure boeing used for max imbalance ?To me, It feels like a handy round number the've used rather than when pumps actually run dry.

[Denti]

Seems you fly unmodded NGs from the restrictions you cite there Kristian. There are a few changes since then for those flying unmodded planes. Anyway, we fly a mix of both, but SOPs are already for modded ones, unmodded ones have a flightdeck placard with the restrictions.

[cosmo kramer]

And then the restrictions on T/O and descent wouldn't be as much as 2300 kgs and 1400 kgs respectively I'd guess..
Why not? You have a significantly higher and lower pitch respectively than on the ground.

I can see what you're saying there cosmo but then wouldn't it be a massive fluke that they just happen to run dry at 453 kg which is pretty much 1000lbs? Why wasn't it 353kg or 553kg?
Who is to say that it's not 447 or 436?
Maybe Boeing chose 453 because it's above the limit where they run dry and a nice round number in lbs...:)

[KristianNorway]

We fly both types, both modded and non-modded. The restriction regarding T/O and descent is of course applicable to the unmodded, you're right. But that doesn't clarify the question. Both types have the restriction of 453 kgs on ground.
If the flickering light On T/O is the problem, why not set the pumps on before taxi like in the situation with the unmodded pumps (and fuel less than 2300 kgs), and turn them off before T/O?
And if it's the pumps going dry, why is that all of a sudden not a problem in the air? There we fly them until the LOW PRESS flickers and then turn them off.
CTR tank fuel isn't all that much cooler either from what I know.
Asked a technician today. Got lots of theories but no really good explanations there either :)

[rudderrudderrat]

Hi KristianNorway,

It's a problem with the heating of the centre tank fuel by the air conditioning packs when on the ground.

"The criticism came as the board ruled that fuel vapors in the center wing tank of TWA Flight 800 had been heated to explosive levels because the air conditioning units on the 747 had run for more than two hours while the 747 was parked on the ground."

"The air conditioning units are located under the center fuel tank on Boeing-made planes, and the heat given off during ground operations can be get into the tank, raising the temperature of fuel vapors."

FAA to act on fuel tank hazard on Boeing 737s (http://www.angelfire.com/wi/wired/hotdayinbkk.html)

[cosmo kramer]

And if it's the pumps going dry, why is that all of a sudden not a problem in the air? There we fly them until the LOW PRESS flickers and then turn them off.
You are not going to let them run dry, since you switch them off in all situations were there is a risk that they will do so:

1) On ground with less than 453 kg
2) At takeoff with less than 2300 kg.
3) In cruise when the low pressure light illuminates
4) In descend with less than 1400 kg.


With the modded pump replace 2 and 4 with
2) At takeoff they auto shut off if they run dry momentarily
4) In descend they auto shut off if they run dry momentarily

Additionally, with the modded pumps you have the protection as well if you forget in 1) and 3).

The problem in climb and descend with the unmodded pump is the master caution logic. If only one low pressure light illuminates you don't get a master caution light. Hence, in climb and descend you are not getting any warning if only one pump is running dry (only on recall). Due to the pitch attitude, one pump may run dry for a prolonged period, with no master caution because the other pump may still be covered.

With the modded pump you rely on the auto shut off feature to shut the one pump off that may be running dry.

For the 3) cruise, the pitch attitude is such that the pumps will be able to reach all the fuel and they run dry almost simultaneously. Normally one low pressure light flickers for a short while (a minute tops) followed by the other along with a master caution. Boeing must have found it acceptable that one pump is running dry in this situation because it will be for a very short period of time.

Disclaimer
The above comprises my theory on the matter and should not be taken as a fact.

[framer]

hmmmm, I think I'm with cosmo on this.

Who is to say that it's not 447 or 436?
Maybe Boeing chose 453 because it's above the limit where they run dry and a nice round number in lbs...http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

And this.....but my money is on 423kg ;)