Originally Posted by
herbert23
not for fuel saving but for increasing flex temp to reduce the possibility of an engine failure. also reducing maintenance costs. packs on apu or off increases flex temp around 3 degrees.
Never did I say that FLEX reduces fuel consumption. In fact I am led to believe that opposite is true, however is that irrelevant to our topic.
I do agree that more flex is better, or better phrased any flex is good flex. In your assumtion you speculated that the departure could have been
with apu on to enable higher flex. In my opinion, this simply makes no sense (such as that APU fuel burn would offset any maintenance savings you speak so fondly about, ...). manufacturer retains this option (packs on APU) for extremely hot conditions (not the case), or a leftover for those not retrained thoroughly from 737 habits.
Interestingly enough, I attempted to check some data and our calculations also do not match.
I loaded 155 pax and 3 tons for bags / cargo and get ZFW of 60,3 not 58.
with 8 t fuel just to remain comparable, TOW is 68,2 i.e. still below max structural by almost 7 t below max. As I do not have KLGA in my database I had used LGKR / Corfu, one of my network most obstacle ridden departures and shortened the runway to 2100 m. 6 knot tailwind, 40 deg C, Q990hPa. Still: Conf 3, 45 flex (27,5 k engines) V2 132.
For ISA conditions and your weight the calculations is still flap 3 V2 132, but flex 54. Naturally all calculated with packs off.
Packs off is a good idea also for me. Packs on APU make no sense.