When deciding whether or not to us flex on top of derate should depend not only on runway conditions, temperature, winds etc. but how the PIC feels. You must also take into consideration of the age of the aircraft too. Believe it or not on older jets, the BOW of the aircraft may heavier than indicated than what is contained in your manuals. The older type of insulation used alone absorbs condensation between checks. While hard to believe, your jet does gain weight between heavy checks. The BOW of a narrow body aircraft can increase by as much as 1.5 to 2.0 tons between heavy checks. As all of you know for the matter of expedience, airlines use average weights (summer/winter/vacation) for passengers and their bags. Take a look at some of your passengers and their bags traveling with you on your aircraft. If you have time, take a walk outside while the bags are being loaded into the cargo pits of your aircraft. A number of airlines use the bag count method to determine baggage weights. Therefore, the larger the aircraft, the greater the inaccuracy of total bag weights, especially on long haul wide body flights. Thus the increased erroneous figures will further skew the actual weight of the aircraft. I fly non-fly by wire aircraft. Under non-adverse weather conditions, I have pretty much learned how to determine if my aircraft is loaded (CG wise) as advertised on the load sheet when the rotation is initiated, and then just before the aircraft becomes airborne (weight wise). A lot happens very quickly at this point, especially when the end of the runway is coming up pretty quickly. Hopefully very few times in your careers, but out of curiosity how many times has the end of the runway appeared sooner before your jet came off the ground? Then how many of you after the jet was off the ground and climbing away from terra firma, in silence of course, for a minute or two to regain composure have looked at each other after you cleared the cow in the pasture at the end of the runway and said “S**t! The W&B is all F**ed up!”
Pilots may use different methods when it comes to flex on top of derate. Perhaps add a few degrees to the max flex temperature, assume the jet weighs more than as advertised on the load sheet, or screw the flex along with the derate altogether.
In the heat of scheduled service, we don’t have the luxury to see what’s going on around our jet, and have to take it for granted that the aircraft is being loaded, and fueled properly, and the paper work is accurate as well.
In cruise it’s a bit easier to better approximate the weight of the aircraft by TAT, body angle, power settings to maintain the selected cruise Mach. This will make the approach and landing hopefully less exciting than the takeoff.