PMC Off Performance
Hi LEM
thats a nice concern u got but if u look harder into yur QRH the answer is right there. i dont have -400 handy but i can give u an example of -800W. on the NGs there is some thing called as EEC (Electronic Eng Control) works similar to the PMC with more added features.
now as u rightly said that there MAYBE an increase in N1 as much as 7%...the question is 'do u really want it ???' when u know there could be an overboost !!! so if u do a full thrust TO u could end up overboosting your engine. maybe causing exceedances.
what the QRH does is that it gives u a reduced N1 in the first place, taking in consideratoin there could be an overboost. but since we are never sure that u would necessarily get a extra bit of thrust, u have to take the WT for that perticular reduced N1
Example -800W (26k)
EEC OFF @ 30deg Sea Lvl is 98.1
where as EEC ON @ 30deg Sea Lvl is 100.3
Hope this will ans yur Question
GLA