Ok, yes, I was thinking about the idea that maybe it was a procedure of Lufthansa.
I wanna present the following example:
- You must overfly point A and B.
- Between these two points, you have 20 miles.
- The arrival tells you, that point A must be overflown at FL250 and point B at FL220.
- After point B you must continue the descend.
So, you have to descend 3000 feet in 20 miles.
Before A, you are flying in ALT HOLD mode maintaining FL250.
Once you pass point A, you set the new altitude in the MCP alt window (FL220) and FLCH is engaged, but you can see, that the green arc of the ND is placed before point B.
In other words, if you continue with FLCH mode, you are going to reach FL220 a few miles before point B.
What do you do?
1- Continue with FLCH until FL220 and then you continue to point B in ALT HOLD mode.
2- You change to VS mode and modify the vertical speed, in order to place the green arc over point B.