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.
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.
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.
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.
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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.
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.
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
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?
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.
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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.
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.
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.
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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).
