Ctr fuel pump restriction on GND, 737NG
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I would think the answer should be found in the passage about the center tank pumps:
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.
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.
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.
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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.
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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.
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.
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.
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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.
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And then the restrictions on T/O and descent wouldn't be as much as 2300 kgs and 1400 kgs respectively I'd guess..
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?
Maybe Boeing chose 453 because it's above the limit where they run dry and a nice round number in lbs...
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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
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
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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
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
Last edited by rudderrudderrat; 20th Feb 2012 at 20:50.
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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.
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 with2) At takeoff with less than 2300 kg.
3) In cruise when the low pressure light illuminates
4) In descend with less than 1400 kg.
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).4) In descend they auto shut off if they run dry momentarily
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.
hmmmm, I think I'm with cosmo on this.
And this.....but my money is on 423kg 
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...
Maybe Boeing chose 453 because it's above the limit where they run dry and a nice round number in lbs...
