I imagine there's a few ways you could do this but here's one way using an unlock action.
Player you want to have the action would be named West1 in the mission, and add this to the chopper's init:
nul = this addAction ["Unlock", "unlock.sqf", "", 2, true, true, "", "_this == West1 && locked _target"];
nul = this addEventHandler ["GetOut", {if (_this select 2 == West1) then {(_this select 0) lock true};}];
this lock true;
then include the annoyingly brief file unlock.sqf:
(_this select 0) lock false;
Cheers