Reducing thrust in cruise for overspeed

[LNIDA]

I fly the NG on some of the longest sectors operated by any Boeing NG operator in the world DBX/OSL & TOS/LPA/TFS we routinely leave the ramp at 79.2T (MTOM 78999) high altitude trips down to the Canary isle will often cross multiple significant changes in OAT/wind direction and we need to get high to get the endurance. My routine is bank angle selector 10/15 and MAX CON on limit page.

I'm not in the least concerned about filing a report for an exceeding a limit, but I'm very nervous about getting slow the FMC generated fast/slow limits are all based the aircraft mass being the 'actual' aircraft mass we often use a mix of charter weights/actual baggage weights/standard baggage weights, we sell an allowance of 20kg bag weight yet use an average of 13kg so a 100 bags could put your actual 700kg above your paper weight, for this reason i will not go above the FMC generated OPT flight level unless I'm very sure the wind and temp are stable and i always aim to be at least 1000' below the FMC generated MAX flight level.

I have only once seen the aircraft come back to the top of the slow box following a temp change and even with 100% we could not get away from slow box and had to descend.

The auto throttle logic on the NG is very slow to reduce thrust, much quicker to add thrust and with rapid speed increase due to wind/temp will likely result in an incursion into the barbers pole zone, there is ,if i can find it a Boeing bulletin on this subject with a warning that thrust recovery can be very slow when slowed to F/I at high altitude. It will be interesting to see how the B738MAX fares in this regard given my companies intent to use them on 8 hour + sectors across the ITCZ !!!

Small level changes will not normally result in flight idle thrust, in any event the aircraft pitch will pitch up as it starts to acquire the new altitude and increase thrust so by the time its back in level flight thrust will be at required

NSB 38 covers this subject, in short if thrust is reduced to F/I for less than 60 sec then the EEC software reduces the acceleration rate of heat soaked engines to improve stall margins, guard the thrust levers from reducing below 60% below this figure spool up time will be much longer with a possible speed loss above 20-30 knots, this only applies above FL300, if the thrust levers have been at F/I for longer than 60 sec then acceleration of the engines will be normal