A&C, Can you explain the airframe engine maintenance issues verse fuel burn more?
All the figures are fed into a computer(by the cost department), the maintanace hours cycle, the major inspection cycle etc and balanced against time in the air and fuel burnt from this is produced a COST INDEX, this figure is then fed into the FMGS or FMS before ech flight to calculate the optimum FL and mach number. Wind and weight are also entered into the computer to enable this calculation. (Then ATC say that level isnt available usually)