Comment: It was within 15' from the

It was within 15' from the ground, but how far was it from your grandfather when he fired the shots?

In your original post you mentioned that the plane was over a neighboring field. How close did it come to your grandfather's field, or was overspray the primary concern?

The Texas law which you give mentions the permission of force to divert trespass. "Force" alone, though, is not the eqivalent of "deadly force". A bullet has the potential to be deadly--five or ten feet of difference and the pilot could have been dead. I find it hard to believe that your grandfather could have hit a fast moving plane without aiming for it, but I'm willing to be convinced otherwise.