When using the jetpack you seem to collect fall damage as you fly so you will take fall damage when landing 3 blocks below your max flight height. 7 above and you die from it. Nothing, feather falling, trying to landing softly, etc makes a difference.

Jetpack has no on/off option.

Jetpack does not have hover mode.

GalacticraftCore-1.7-3.0.12.446
GalaxySpace-1.0.8+STABLE