alternatively, you could use a 'get in' event handler on the chopper in question...
from igor drukov's tutorial -
"getin" and "getout" (to vehicles only) :
_this select 0 : the vehicle the EH is attached to ;
_this select 1 : in/from what position (driver, gunner, commander, cargo) ;
_this select 2 : who's just got in/out ;
just in case you want to be specific about the player being in cargo, and not just in the chopper willy nilly.
heheh. i said 'willy nilly'
archaic language is such fun.