If all your men are in the same group, give the group a name :
mygroup=group this
in the init field of the leader.
Then, in the condition field of your trigger :
"_x in chopper" foreach units mygroup
That way, the trigger will activate if the whole group has boarded the chopper, but regardless of the potential casualties.
*edit*
wooooops, this doesn't work, for some reason. Back in a few minutes with a REAL answer.