You wrote the you were even below the optimum and only just managed to maintain altitude. I assume that is was more a coincidence that you were below optimum... hence had you been higher you would have had to descent too?
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.
Not really a coincidence, I can't remember how the FL was chosen on that day but I do know that it would not have been much above optimum initially, and then it would have burned down to the situation of being below optimum. Interesting that you mention efficiency because flying at Max is quite inefficient compared to being within cooee of Optimum but that's a different conversation I guess.
Aha, there we have the misunderstanding!! You are confusing FMC max altitude with "service ceiling". This is absolutely incorrect. Flying at max FMC altitude is not flying at service ceiling. Because Boeing already build in a margin for you:
Ok, one of us has a misunderstanding here, I don't think it is me but I will happily admit that it is if we can get to the bottom of it and I am wrong.
The FMC compares three limits in finding the Max altitude
1/Maximum Certified Altitude (Structural).
2/ Thrust Limited Altitude
3/Manoeuvre Limited Altitude
We all know the Structural limit. The thrust limited altitude is based on a residual rate of climb of 100fpm. You say Boeing have built in a margin on this but I have never read that anywhere. Can you provide a reference?
The Manoeuvre Limited Altitude is an Aerodynamic consideration and isn't related to the other two.
Most of the time in an NG you will be operating restricted by number 2.
When limited by the thrust the lower amber band provides no guidance as to whether or not straight and level flight can be maintained at that speed, and it does not change with temperature. If you are at ISA + it is even more misleading. So again, if you are at your maximum altitude and the temperature increases, can you maintain your required 100fpm? Unless you can describe the margin that Boeing have built in then I say no.
The ability to maintain the 100fpm is not the big issue here. It just illustrates how limited your options are when operating at Max. But either way we should determine which of us has the misunderstanding so that we can carry on with increased knowledge
