Originally Posted by
vilas
Hi pineteam
Let some more guesses come in. The point I am making is the amount of thought that goes in designing an aeroplane. So it shouldn't be brushed aside merely on personal opinion.
OK, I’ll play.
In CLB thrust is fixed. Therefore changing from NAV to HDG will have no effect on Aircraft trajectory. The engineers know the thrust setting in CLB is always CLB
Conversely, In DES thrust is variable from IDLE to SPD/MACH. My guess is the engineers are trying to minimize the variable state of thrust in DES by using a mode reversion to force thrust into the SPD mode.
This is the same reason when changing at the FCU altitude during ALT*. Thrust is variable during ALT*, therefore a mode reversion to SPD is forced to set a fixed thrust setting.