BodyVelocity; Making an aircraft go forward and upwards/downwards?

Hey, I'm working on an aircraft but I need help making it continue to move forward while going down/up; I have the up/down controlled by key but when it's pressed it just stops. Here's what I have.

Plane.Thrust.velocity = (((Plane.CFrame*CFrame.fromAxisAngle(Vector3.new(YSpeed, 0, 0), math.pi/2)).lookVector) * Plane.CFrame.lookVector * CurrentSpeed)

If I remove the Plane.CFrame.lookVector piece then it does go up/down but it stops moving forwards. Any suggestions? Thanks