Damage Doubler Filler Doubler riveted in place Filler riveted in place Damaged area cut  to a smooth rectangle  with corner radil Insertion Doubler Skin Insertion Patch Skin Skin Skin Doubler 1/4 inch deep dent Insertion patch method Cover patch method P E E Figure 4-177.  Repair patch for a non-pressurized area. Figure 4-176.  Typical flush patch repair.  Usually, repairs to open skin can be made in the conventional  manner using standard rivets, but in repairing closed skin,  some type of special fastener must be used. The exact type  to be used depends on the type of repair being made and the  recommendations of the aircraft manufacturer. Damage to the aircraft skin in a non-pressurized area  can be repaired by a flush patch if a smooth skin surface  is required or by an external patch in noncritical areas.  [Figure 4-177]  The first step is to remove the damage. Cut  the damage to a round, oval, or rectangular shape. Round  all corners of a rectangular patch to a minimum radius of  0.5-inch. The minimum edge distance used is 2 times the  diameter and the rivet spacing is typically between 4-6 times  the diameter. The size of the doubler depends on the edge  distance and rivet spacing. The doubler material is of the same  material as the damaged skin, but of one thickness greater  than the damaged skin. The size of the doubler depends on  the edge distance and rivet spacing. The insert is made of  the same material and thickness as the damaged skin. The  size and type of rivets should be the same as rivets used for  similar joints on the aircraft. The SRM indicates what size  and type of rivets to use. This section describes typical repairs of the major structural  parts of an airplane. When repairing a damaged component  or part, consult the applicable section of the manufacturer’s  Typical Repairs for Aircraft Structures  Design of a Patch for a Non-pressurized Area 4-98