B737NG engine start

[zth]

That's excellent, thanks guys: the last two replies summed it all up extremely well. Much appreciated.
Cheers

[xetroV]

Originally Posted by waffles

Now to consider the case when the wing tanks are full but the centre tank contains >453 kgs but <2300 kgs.
The fuel pumps/pressure 'sensors' are located towards the front of the centre tank. When there is less than 2300 kgs in the centre tank, at take-off and during the initial stages of the climb when there is a high body-angle, the pumps/sensors may become exposed.
This situation would cause a 'low pressure' caution on the flight deck. If this were to occur on take-off/early climb, the distraction may cause problems.
The reason for the large proportion of SOPs to ask for Centre pumps off with less than 2300 kgs in the centre tank is to avoid this undesired distraction - to the best of my knowledge, there is no other reason.

AFAIK, the 2300 kg figure only came into force when Boeing changed their SOPs to prevent unmodified fuel pumps from running dry (TWA 800). Before that, the procedure was to switch off center tank pumps during take-offs with less than 453 kg (1000 lbs) in the center tank, indeed to avoid unnecessary distraction during take-off and early climb. The 3/4/500 series still use the 453 kg figure, not 2300 kg; I guess due to differences in plumbing and/or pump design.