thanks for your answers, two things are clear to me now:
1) Amph. Assault: All air units can retreat at the same time, if they wish.
2) Normal Battle: Air units, if they retreat, have to do this also at the same time.
Unfortunately, one ambiguity remains:
Sankt Hallvard says:
You have to retreat ALL units or press attack, amphibious assault is the only exception.
That sounds ok and would be supported by LHTR rules. Either retreat everything
But then trihero:
If you withdraw fighters, you withdraw all of them at the same time. The fighters retreat separately from other units and can always retreat if they're the attackers,
Does this mean, that in a normal land battle, I can retreat all fighters at once, even if my land
units continue the assault?