For some reason the Engine Bleed switches of the B737 should be in the "ON" position before engine start. I see no specific reason for this. I know that the valves are only open if there is engine bleed pressure, and anyway the valves are closing when the starter switch is in "GROUND", going to the selected position after starter cut-out. But what is the reason why we may not select the bleed to "OFF" before engine start?
