When submitting an FAA Form 337 with non-previously-approved data, which item should be left blank?

When submitting an FAA Form 337 that includes non-previously-approved data, leaving the section regarding "Approval For Return to Service" blank is appropriate. This is because the approval indicates that the work has been inspected and found to be in accordance with the approved data and standards. Since the data being submitted has not been previously approved, there cannot be a validation or endorsement by an authorized person assuring that the work meets the necessary regulatory standards.

The other sections of the form serve different purposes. For instance, the description of work accomplished, conformity statement, and work performed all provide crucial information about the job done. However, since the work cannot be approved without prior data approval, you leave the return to service section blank, effectively indicating that the work is pending further review or approval before it can be certified as airworthy.