Step climbs

[Old Smokey]

xfeed,

I assume that you mean B777-200 when you refer to 772. The default Step Climb in the standard B777 FMC (there are other customer options) is ICAO, i.e. 4000 feet. It is pilot selectable to other values as thousands of feet, typically 2000 feet in RVSM airspace, or 1000 feet in countries like Australia where ATC is quite flexible in allowing non-standard levels if traffic permits. The annoying bit is that 'odd-ball' steps such as 1600 feet are not selectable, very useful in China and some other countries where metric altitudes are still used with 'odd-ball' steps between standard levels.

The standard B777 FMC displays 3 significant levels.

First is the pure aerodynamic optimum level for the speed schedule chosen in still air. This increments upwards at about 100 feet every 7 to 8 minutes. Nice to have, BUT it displays still air values, which has little relevance in a windy world.

Second is the Max Alt, this considers Thrust limited level for the speed schedule chosen, Maximum level for buffet boundaries, or the maximum permitted level for the aircraft, whichever is the most limiting. Again, this increments upwards at 100 foot intervals.

Third, and most important for cost / fuel effective operations is the recommended level. This increments upwards at the set Step Interval as described earlier. If a good range of en-route winds and temperatures have been input above and below the nominal level, the recommended level will advise that for the best operation, and advise 'Step To' points in terms of Distance and Time. If a range of winds above and below the nominal level have not been input, it will simply recommend the standard level closest to the still air optimum level. When 'ducking under' Jetstream Headwinds, it's not uncommon to see something like Optimum Level 357, Maximum Level 374, and Recommended Level 290. Alternatively, when a positive wind gradient exists (increasing tailwind with increasing level), it's not uncommon to see something like Optimum Level 357, Maximum Level 374, and Recommended Level 370, that is, above Optimum Level, but at or below Maximum Level. (A lot of pilots will never see this phenomonom as it's common practice for many to simply insert wind at one level only).

A shortfall in the B777 system is also that whilst it allows several levels and winds to be input, only one 'reference' level with it's associated temperature is possible. It assumes standard lapse rates for the other levels, and this can be detrimental where the tropopause is well above standard (36089 ft), and much higher temperatures than actual are assumed at higher levels, delaying advice of the recommendation to step to a higher level.

If no significant wind gradient exists, the perfect means of achieving the desired Time / Fuel profile is to 'Cruise Climb' at the optimum level. Airspace is becoming too congested for this mostly, but as recently as last week I obtained 4000 foot block levels from Anchorage Centre (admittedly in a remote part of the world). Again, if no significant wind gradient exists, and block levels are not available, the pilot or the FMC must determine the most appropriate step point. For simplicity, this is typically done midway between the points where the present level, and the next projected level's performances are equal. The reality is that it should be approximately 2/3 of the way between. If cruise alone is considered, by all means step at the point where the 2 levels are equal, BUT a climb is required, and extensive analysis of the OVERALL fuel burn indicates waiting a little longer until the aircraft is lighter, and the incremental step climb fuel is therefore less, yielding the best OVERALL Time / Fuel for the sector.

That's my 2 cents worth.

Old Smokey