You say Boeing have built in a margin on this but I have never read that anywhere. Can you provide a reference?
I can.
It's in the note in the brackets below:
The FMC predicted maximum altitude is the lowest of:
• thrust limited altitude - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb. (Reference the
Long Range Cruise Maximum Operating Altitude table in the PI chapter of the QRH).
Service ceiling is determined at the speed for minimum drag = minimum thrust required. Minimum thrust required = Max Range Cruise. MRC in the FMC is ECON cost index 0 (no wind). Reference:
Cruise Speed Determination
Entry of zero for cost index results in maximum range cruise.
So how big is that margin? How much better climb rate do you get at mach .75 compared to mach .79? To be honest I don't know, because I never tried to climb above the FMC predicted max altitude.
But it is certainly enough to maintain level flight, even with the odd speed variations due to changing wind and temp. Even more so when you increase your thrust available by approx. 5% N1 by selecting CON on the N1 page, as recommended by Boeing.
I ask again,
where is the problem?
Boeing also provides this note in the FCTM, by the way:
On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet.
Anyway, to sum this all up. It seems that most people get scared by "Having the low and high speed buffet hooks touching", as one poster wrote on page 3 (though they really won't, but will give you quite a few knots of space). But as you, Framer, correctly recognized, a much bigger problem is the thrust available. It's a problem, because there is most like not enough available to "pull you out" of the flaps up amber band, should you inadvertently get in there. The upper amber band, on the other hand is not so much of a problem, as long as you are not in a 30 deg bank (which you shouldn't be anyway according to Boeing recommendations).
Since the upper band is not a problem for your thrust available and really doesn't affect the type of maneuvering you are doing, it would be safe to fly in the band temporarily. Hence your total margin would be from top of flaps up maneuvering band, practically to MMO. This means that at max FMC predicted altitude, you have maybe 8-9 knots on each side of your bug speed (when flying just below the upper amber band) of to absorb wind variations. Is this a problem on normal day for you? Or let me ask you another way, how many knots margin do you need on a normal smooth day?
Yes efficiency is another matter. I reiterate, I do not blindly climb to max altitude. But there can be a number of operational reason why it can be a benefit - "Maintain level 340, sorry level 360 is occupied same destination as you, can you accept level 380? I have an aircraft 80 nm ahead of you level 380, he reported it is smooth", "able level 380"
