hello wk,
find here under quote of mr. boeing flight crew training manual, 3.11 fct 737ng(tm) dd. october 31, 2005.
" operation with alternate fwd cg limit for take off.
take off performance is based on the fwd cg limitations as defined in the AFM. however, t/o perfo can be improved by taking credit for an alternate (further aft) fwd cg limit if shown in the AFM. use of this data provides higher perfo-limited t/o weights than the basic AFM perfo data.
typically alternate fwd cg is used to increase perfo-limited t/o weight for field length, climb or obstacle limited departures. another potential benefit of alternate fwd cg is to allow greater thrust reduction which increases engine reliability & reduces engine maintenance costs. However, this improved perfo capability is only available if the airline has the certified data in their AFM & has approval from their regulatory agency to operate the airplane at an alternate fwd cg limit.
a more aft cg increases the lift available at a given angle of attack due to the reduction in nose up trim required from the horizontal stabilizer. this allows Vr & V2 to be reduced, which in turn reduces the field length required for t/off. reduction in field length required can also permit an increased field length limited weight. in most instances this reduction in nose up trim also results in a decrease in drag which improves the airplane's climb capability.
NOTE : the FMC calculated t/off speeds & QRH t/off speeds are not valid for operations using alternate fwd cg. t/off speeds must be calculated using alternate fwd cg perfo data normally provided by dispatch or flight operations."
hope this helps,
kind regards,
bm
