add continious force to plane when it is flying

Hello and Hi,
i am flying the plane and if
i want to give the constant force to plane
from backside to transform it .
what should i do…

i cant use transform.translate to forward 
because my planes direction may change any time.
and forward means to the z direction.

please give me the hint...

my plane is flying perfectly at initially and if i am rotating
 it to the new direction then its really
 a kayos...

       its feeling like floating... 
 just because its going to the the z direction though
 my tip of plane is pointing to any other direction.....


please please ....help me..
i think i should use something different...

To move a plane forward you can use transform.forward. This will point forward from whater side of your game object is pointed at positive ‘z’ when the rotation is (0,0,0). From your comments, I assume you are using translation to move your plane. Try something like:

transform.Translate(transform.forward * speed * Time.deltaTime);