Originally Posted by
extricate
..
Anyone has any idea why oftentimes in CRZ page, RECMD alt recommends, say FL 350 and LSK 1R shows STEP TO FL350 and below it, it shows AT XXXXz and XXXnm?
Can't we just climb to FL350 now since it's RECMD? Why is there a specific point?
You can climb now, but the STEP TO point is more accurate because it takes more into account.
This is for the 78. I expect 777 is much the same.
RECMD only considers current position and out to 250 to 500nm ahead, depending on step size. It does not consider fuel for climb to the new altitude. It considers levels down to 9,000' below and up to MAX. The trigger for a change is low and may only require a gain of a few kg. Therefore RECMD can hunt around.
STEP TO only offers Climbs (because Boeing thought a STEP TO descent would be silly and ignored), accounts for Climb (and Descent to original level at end of cruise) and considers the winds out beyond 500nm.
So climb based on STEP TO rather than RECMD is more efficient. As STEP TO will never offer a descent, you need to consider RECMD when a consistently lower RECMD level is presented. This could occur when there are markedly better winds at lower levels.