How to calculate the tilt angle of an object?

Hi,

I have the following problem: I want to draw a horizon-line on the UI for my plane that shows the current tilt compared to the ground plane. For that I need the tilt angle of my plane.

First I tried to calculate the angle between this.transform.right and Vector3.right but as the plane moves around all axis (and the Vector3.right always stays the same) that only works when the plane is flying in one direction:

         float lineAngle = Vector3.Angle(this.transform.right,Vector3.right);
         Vector3 polar = Vector3.Cross(this.transform.right, Vector3.right);
         if (polar.z < 0) lineAngle = -lineAngle;
 
         horizont.transform.eulerAngles = new Vector3(0, 0,lineAngle);

How can I do that correctly?