I have just started a simple flight simulator and have implemented Roll and pitch. In the beginning, testing went very well; however, after about 15-20 seconds of constantly moving the thumbsticks in a random or circular motion, my model's scale begins to grow. At first I thought the model was moving closer to the camera, but i set break points when it was happening and can confirm the translation of my orientation matrix remains 0,0,0. Is this a result of Gimbal Lock?

Does anyone see an obvious error in my code below?

    public override void Draw( Matrix view, Matrix projection )
        {
            Matrix[] transforms = new Matrix[Model.Bones.Count];
            Model.CopyAbsoluteBoneTransformsTo( transforms );

            Matrix translateMatrix = Matrix.Identity
                * Matrix.CreateFromAxisAngle( _orientation.Right, MathHelper.ToRadians( pitch ) )
                * Matrix.CreateFromAxisAngle( _orientation.Down, MathHelper.ToRadians( roll ) );

            _orientation *= translateMatrix;

            foreach ( ModelMesh mesh in Model.Meshes )
            {
                foreach ( BasicEffect effect in mesh.Effects )
                {
                    effect.World = _orientation * transforms[mesh.ParentBone.Index];
                    effect.View = view;
                    effect.Projection = projection;
                    effect.EnableDefaultLighting();
                }
                mesh.Draw();
            }
        }

        public void Update( GamePadState gpState )
        {
            roll = 5 * gpState.ThumbSticks.Left.X;
            pitch = 5 * gpState.ThumbSticks.Left.Y;
        }