Gone With the Wind: Field approval

Gone With The Wind? The field approval process Stephen P. Prentice The process has essentially remained the same . . . some areas required more attention to detail than others and that is going to require more time and effort to complete...


The list of items requiring an application for a Supplemental Type Certificate (STC) approval goes on for some 20 pages. Not much is left out. In order to remain in the running for a field approval you have to design your alteration or repair so that it avoids falling into the STC category. All technicians thinking about field approval of a project should carefully review the criteria that would require a STC rather than a field approval.
There have probably been less than a dozen instances (I can think of two or three) where alterations were improperly approved in the field and should have had STC oversight by an ACO.

Because of these several alleged failures at the FSDO level the certification people may well have reacted and thought they were losing their grip on certification. The bottom line, now your FSDO will probably send all but the most simple alterations or repairs to the ACO. Let's hope they don't however, and continue the sensible approach that has existed to date. If the ACO system prevails many fear you will wait forever to get any approval.

For those that are still interested in field approvals, here is a summary of the procedure you have to follow now. It does not differ substantially from the past procedure except for the additional detailed paperwork.

Request for a Field Approval
You have to file a Request for a Field Approval before you commence any work. Starting the work without first taking this step may well waste your time. This formal request may be set out starting with a form 337 or other form or letter statement (original and two copies). You may use any other forms that are commonly used by manufacturers or operators.

The request and attached supporting data must be reviewed before any work can proceed. Include with the request,

  • Any proposed flight manual supplements needed.
  • Form 8110-3.
  • A complete description of the proposed alteration or repair. Include similar STC data or other similar 337 approvals. Keep in mind that former 337 approvals, approved as a one-time alteration or repair are acceptable data that may be used as a basis for developing your approved data for the alteration or repair. This data can help the approval of your request considerably.
  • Any and all methods, sketches, drawings, stress analysis, photographs, electrical load analysis, etc., to ensure that all applicable design standards have been considered. Ground and flight test requirements, if any, to meet requirements to substantiate the alteration or repair.
  • Instructions For Continued Airworthiness (ICA's). Since 1998 major alterations must have instructions for continued airworthiness written for them. However, you can incorporate ICA's for an existing field approved major alteration by way of a revision process and use them for your job.

The purpose of the ICA's is to provide instructions on how to maintain aircraft that are altered and appliances that are installed in accord with a field approved major alteration.

Review and evaluation
The next significant hurdle you have to get over is an internal review of your proposed work, usually by the field approval review committee at your FSDO. The key element here is whether or not the proposed alteration or repair affects the airworthiness of the aircraft. Yes, your field approval go ahead will most likely come from a committee.

The primary thrust of this inquiry is to ensure that no previous alterations or repairs will impact your proposed changes and thus affect the airworthiness of the aircraft.

The critical decision at this point is whether or not the proposed alteration is suitable for a field approval in the first place, or whether an STC is required and thus a trip to the ACO.

However, there are two other alternate methods of approval outside the STC route.

Alternate methods for approval

  1. DER-DAR approval of data
    If you know a DER or a DAR, he or she can be a great help. Many retired FAA inspectors get their DER or DAR designation at the time they retire and thus can continue to keep their expertise available and provide a source of income to supplement their retirement. They are excellent choices to assist you and won't, from my experience, break your bank.

    Here's the little secret that many forget. Where DER approved data is provided and it addresses all of the requirements of CFR parts 21 and 43 the approval process is complete! The aircraft can be conformity checked and returned to service by you without a further finding or decision by an ASI or some committee at the FSDO. The DER need only sign Block 3 of form 337. Nothing else need be done except that you, the technician, finish the work and approve the aircraft for return to service. The paperwork can always come back however, and this procedure is no guarantee against further requirements by your FSDO.

  2. Coordination.
    If the job is of questionable complexity and the FSDO wants some assistance from the ACO they can ask for a Coordinated Field Approval where the ACO simply looks over the FSDO work and assists where necessary. This is a joint effort at approval and will no doubt extend the time for an approval.

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