I think that my statement of
you wouldn't have had the thrust available to maintain the speed and commence even a 5 degree bank angle turn
may be correct and I'll explain why below. It would be interesting if someone with access to a sim climbed up to max at a weight of 70T and then slowed to the top of the amber bar and tried a few turns and reported back to us. Anyone?
Cosmo, you said
I don't fly with 40 degs bank at high altitude, max 15 degs as
pr. Boeing recommendation.
I didn't think you would, I just wanted to know if you were mislead by the statement in the FCOM that says the lower amber band provides you with 40 degrees bank prior to buffet because as you know, that is an aerodynamic consideration and is only true if the thrust is available to do the turn, and at max you won't have the thrust available to both maintain speed and the 40 degree bank at the top of the amber band.
If we look at what Boeing use to determine the max altitude in any given situation and then apply it to your photo it might help strengthen my argument.
1/ The altitude for 1.3 g margin to initial buffet.
In the picture this speed is 17 or 18 kts slower than the speed you are cruising at and is not limiting you, it is an aerodynamic consideration and not a thrust consideration.
2/The service ceiling ( altitude for 100 fpm climb rate at max climb thrust)
This is a thrust consideration and may be limiting you but maybe not.
3/ Max cruise thrust limit ( altitude for 0 fpm climb capability at cruise thrust)
This is a thrust consideration and is sometimes lower than the service ceiling limit and may have been limiting you.
4/ normal accel to initial buffet at optimum altitude + 2000 feet.
An aerodynamic consideration and was not limiting you, if it was the selected speed would have had you sitting on the bottom of the upper amber band.
5/bank angle to max cruise thrust at opt alt + 2000 feet, ISA+15
Another thrust consideration and what I think was most likely the limiting consideration. Your opt was 359, so why wasn't the max 379? Because when a bank angle limit of 15 degrees was applied to 379 it failed to meet the requirement ( maybe the thrust limit was reached at 10 degrees) so the calculation s were done to where the aircraft could comply and your max was calculated at 373 where a 15 degree bank could be sustained with the thrust available. If you slowed to a speed where drag was less, the bank angle capability would have increased, but as you slow beyond that the ability to sustain the minimum required bank angle would have diminished. I maintain that at 240kts it would have been less than the 15 degrees required.
What we really need is someone with sim access to go and do it and report back to us.
Another way of looking at it:
Yesterday in the cruise I determined the max for three different speeds
.76
.77
.79 ( LRC)
The max altitudes were
FL385
FL 386
FL382
So if I had a great desire to fly at FL 386 I would have to cruise at 0.77, if I then changed my speed to 0.76 I would be 100ft higher than the max altitude calculated for the new speed. Which limit am I not complying with? Who knows but at my new speed I no longer have the margin of one of the 5 limits written above. If I used 0.77 to climb up there and then accelerated to 0.79 I would be 400ft above where one of those limits kicked in.