A field approval is one of the means used by the FAA to approve technical data used to accomplish a major repair or major alteration. It is an approval by the Administrator, through an authorized Aviation Safety Inspector (airworthiness), of technical data and/or installations used to accomplish a major repair or major alteration. Technical data so approved becomes “technical data approved by the Administrator.” This type of approval may be accomplished for one-time approval. (FAA Order 8900.1, Vol. 4, Chapter 9.)
Note: The above link opens the Flight Standards Information Management System (FSIMS) home page. To locate FAA Order 8900.1 Field Approvals sections, select the "8900.1 CONTENTS" link from the list on the left side of the page, then select Vol. 4, and Chapter 9.