Getting my airplane to stay level?

Every time I add something new I seem to encounter a different issue!

Now, I have my airplane takeoff, and it flies for a while and then begins falling back to Earth! Sometimes the throttle just stops working, even when I'm pressing down on the key. This particularly intrigues me, as the part in the script I'm working on doesn't even affect the throttle! It's really annoying!

     void FixedUpdate () 
     {
         //Engine
         if(Input.GetButton ("Throttle")) //Throttle is increased
             Throttle ();
             
     
         
         //Triggers        
         if(rigidbody.velocity.magnitude > TakeOffSpeed)
             TakeOff();
         
         if(rigidbody.velocity.magnitude > TakeOffSpeed + 10)
             Balance ();
         
     
      void TakeOff()
     {
             rigidbody.AddForce (0,50,0,ForceMode.Acceleration);//Lift
         rigidbody.AddRelativeTorque (5,0,0,ForceMode.Force);//Nose Up
                 
         
     }
     
     void Balance()
     {
         
         rigidbody.AddForce (0,-40.19F,0,ForceMode.Acceleration);//Equalizes Lift
       rigidbody.AddRelativeTorque(-5,0,0,ForceMode.Force);//Balances Nose
         
     }

So, when the velocity of my aircraft is greater than my variable, "TakeOffSpeed", it executes TakeOff(). This adds the lift and makes the plane pitch up a tiny bit. So if I left it at that my plane would just continue to float up into the air.

When the velocity reaches 10 more than my variable, it calls the function Balance() which, well, balances the aircraft. It adds a force pushing down which makes the velocity of the lift 9.81. To maintain a steady and completely flat flight, the lift must be equal to gravity, which now it is.

It just doesn't work though! It flies up into the air and then begins to drop back down! If I fly vertically up or down or something (as this is a fighter jet) it just messes up. I've even tried turning Gravity to false when it reaches a certain speed and then changing the lift. Nothing!

I know this is either really complicated or really simple and I'm just too stupid to see it, but a solution would be greatly appreciated, as I have been working on this script for a long time!

Thanks