737NG optimum vs. maximum altitude

[Walter Sobchak]

Hallo,

Why are the optimum alt and max alt sometimes indicating the same altitude on the B737NG FMC? I know the FMC checks for three altitudes(max certified, thrust limited and maneuver limited) but we weren´t max certified limited.
My understanding was opt alt is always a bit lower than max alt(max certified being the exception), but then again I´m not a pro on those things.

Thanks in advance,

the Walter:confused:

[Shaka Zulu]

Well if you just stop and think about it just for a moment, you'll realise what you're saying.
At your aircrafts weight at that time, FMC Opt Alt=Max Alt.
The aircraft flies quite happily through FL410 which is Max Alt, but the cabin pressurisation isn't build for it. That's why Max Certified does quite regularly coincide with Opt Alt.

[Old Smokey]

There's another consideration, and this isn't just specific to the B737. If you've entered the full range of winds at all Flight Levels (I always do), a positive wind gradient, i.e. increasing tailwind / decreasing tailwind with increasing altitude will push your optimum GNMPK above the optimum ANMPK, oftentimes, all of the way to maximum. Similarly, an unfavourable wind gradient will recommend a level somewhat lower than the Optimum Altitude, which (on most of the Boeing family FMCs) is for still air.

On very long range flights I always enter the full range of winds as early as possible in the flight so that the FMS / FMC can more intelligently compute the optimum step climb points. It's not at all uncommon to see the distance to S/C change by anything up to 600-700 miles as a result.

[mutt]

Old Smokey,

What you have said is correct provided that the FMC is flying using Cost Index values. Otherwise the Optimum Altitude will be the altitude that gives the greatest NAM/1000 kgs.

The Maximum Altitude will be the Altitude where the aircraft can achieve the required buffet margin with rate of climb or is thrust limited.

The data is identical to the optiumum altitude/altitude capability charts in the Operations Manual.

For the given conditions is quite possible for both values to be the same.

Mutt.

[Old Smokey]

Mutt,

Thanks for the qualification. It's so long (literally years) since I've flown non Cost Index that it didn't come to mind.

Thanks again,

Smokey