can someone tell me what really the diffrence is between opt and max altitude on the vnav cruiz page, and say the opt says fl330 and the max says fl390 can i fly say fl350?Happy FlyingAnthony Hurst

Optimum is the most fuel efficient altitude to fly at, taking into account aircraft weight, cost index and temperature.Maximum altitude is the aircrafts "ceiling" (defined as the altitude at which a 100ft/min rate of climb can be achieved... I think), under the current weight/cost index/temperature.You should pick the closest hemispherical level above your optimum, then as the optimum goes up during flight, you climb again when optimum if halfway between you and the next available level above you. In reality we often climb earlier, to secure our level, so no one else takes it and blocks us, in this case you can climb up to your max alt if necessary, but its not a comfortable thing to do.

The other thing HPSOV doesn't mention is that your FMC (in the real airplane, I doubt that PIC767 does) will take into account a "g" limit (usually 1.2g to 1.3g) to account for turbulence etc and the 1g high speed/low speed stall limits.Kevin in CYOW

Yes, as Kevin stated about the 1g high/low speed stall limits. If you operate too far above optimum alt, you'll be setting yourself up for a visit to coffin corner.