The FAA Order covering the application for PMA is Order 8110.42 revision D. This document is worded as instructions to the FAA reviewing personnel. An accompanying Advisory Circular (AC) 21.303-4 is intended to address the applicant. 8110.42C addressed both the applicant and the reviewer. Per the order, application for a PMA can be made per the following ways: Identicality in which the applicant attempts to convince the FAA that the PMA part is identical to the OAH (Original Approval Holder) part. Identicality by Licensure is accomplished by providing evidence to the FAA that the applicant has licensed the part data from the OAH. This evidence is usually in the form of an Assist Letter provided to the applicant by the OAH. PMA may also be granted based upon prior approval of an STC . As an example: If an STC were granted to alter an existing aircraft design then that approval would also apply to the parts needed to make that modification. A PMA would be required, however, to manufacture the parts. The last method to obtain a PMA is Test & Computation. This approach consist of one or a combination of both of the following methods: General Analysis and Comparative Analysis. General analysis compares the proposed part to the functional requirements of that part when installed. Comparative Analysis compares the function of the proposed part to the OAH part. As an example: If a PMA application for flight control cables were to show that the PMA part exceeds the pull strength requirements of the aircraft system it is meant for, that is general analysis. To show that it exceeds that of the OAH part is comparative analysis. The modern trend is to use a variety of techniques in combination in order to obtain approval of complicated parts - relying on the techniques that are most accurate and best able to provide the proof of airworthiness desired. The cognizant regional FAA Aircraft Certification Office (ACO) determines if the applicant has shown compliance with all relevant airworthiness regulations and is thus entitled to design approval.