The Question: To field approve or to STC?

April 1, 2002
The Question: To field approve or to STC?
By Joe Hertzler
Recently I was studying the field approvals of FAA Form 337s, what I consider to be the hottest issue between the FAA and industry, when it hit me - my Inspection Authorization may not be what I thought it was all these years. Years ago life was simple, aircraft maintenance, as it does today, included major repair and major alteration, and supporting data was most often field approved. Back then, when we wanted to modify an aircraft by replacing the old "fix it every 100 hours" rotating beacon with a new strobe, the approved data was the installation instructions from the strobe manufacturer. I remember taking the instructions with me to the Flight Standards District Office (FSDO) with a 337 to get a field approval (thinking that I needed one) when I was told that the instructions from Whelen were approved data. The reference the FAA used at that time was the Advisory Circular used as a Study Guide for an IA, which has since been superceded by FAA-G-8082-11. The old guide stated that an appliance manufacturer's installation instructions were an acceptable form of approved data. When we wanted to duplicate modifications on several aircraft of the same make and model the FAA would issue an FAA Field Approval. From that point on we could perform the same modification and attach the original 337 to the new one as supporting data without an additional field approval. Times have changed Now when we want to make a modification, the FSDO is often reluctant or refuses to field approve our data. In many cases, the 337 form must go through the Aircraft Certification Office (ACO) for them to review the data and give the FSDO a "warm fuzzy." The FSDO then signs the field approval. This wave of new found reluctance definitely has its roots in the Lear and Citation cargo field approval fiasco. A few years ago, several Part 25 Learjet and Citation aircraft were field approved by the FAA for conversion to carry cargo. Some FSDO offices had been issuing field approvals for cargo conversions for Part 23 aircraft for which there weren't any specific cargo compartment requirements. These Learjets and Citations were just barely Part 25 aircraft, and it appears the FSDO treated them as Part 23 aircraft. The resulting field-approved installations apparently did not meet all of the applicable classification requirements of Part 25.857. Additionally, there is a rule in Part 135 (Ref. 135.87) that also addressed carrying cargo under Part 135, which these aircraft were. The whole situation created quite a stir. The result was/is a fear factor within the FSDO ranks. From that point on, almost anything presented for field approval was being sent to Aircraft Certification for evaluation before the FSDO would give field approval. Now, put yourself in the shoes of the ACO. These guys are used to handling Aircraft Certification (hence the name). The process of issuing Supplemental Type Certificates (STCs) is detailed and complex and takes into account all of the regulations pertaining to the aircraft's type design and current configuration. To facilitate the process they issue a one-time STC that isn't approved for duplication. That makes the process simpler and faster and allows the ACO to really get their arms around the modification. The result -the field approval process essentially disappears. The time it takes to get a one-time STC is only a little less than that of a multiple STC. And now if I want to get a one-time STC I need to hire a DER (Designated Engineering Representative) and submit "approved" data, among other things, before they will issue the STC. And to boot, the ACO told me just the other day, "If you are going for a one-time STC it will be the last on our list of priorities." This is crazy, I started out just asking for a field approval, the FSDO sent it to the ACO for a "warm fuzzy" and now I've been directed to a DER and am wrapped up into the STC process. In fact, if I hire a DER to approve the drawings and the installation information regarding the modification I'm performing, I don't need any help from the FSDO. As an IA (or repair station) I can sign block 8 of the FAA form 337 when the data is already approved. Or can I? We recently gathered all the information required, obtained all of the required approvals and FAA Forms 8110-3's and created what we thought was all that we needed as a data package for the aircraft to support the installation. We approved the aircraft for return to service, completed an FAA Form 337 in duplicate (as required by 14 CFR Part 43 Appendix B) and submitted the copy to the FSDO. We had moved on to our next project when the FSDO called to tell us that we needed to obtain a field approval for the 337 Form we had submitted. What? Why does this not calculate? If that is the case, who is in violation of an FAR if I don't? What FAR? 14 CFR Part 43.9 and Appendix B clearly describe the requirements for major repair and major alteration approval for return to service. The regulations say that in order to sign block 7 of the FAA Form 337 the data must be FAA approved. We hired the DERs with all of the appropriate complexities and authorities and made all of the corrections and improvements required by them (FAA designees) to obtain the approving document (8110-3). And we approved the aircraft for return to service. That means we inspected the installation and found it to match the "approved" data and we signed for the installation. Every check box had been marked off and the aircraft was properly approved for return to service and airworthy as far as we were concerned. If we simply say OK and begin sending all 337s to the FSDO for evaluation we are like the monkey chasing the weasel around the mulberry bush. The chase will never end and standardization of the FAA and its interpretation of each regulation will continue to be a pipe dream. The fact is, in such a situation there is no regulatory basis for a field approval. The FAA is really only concerned that the IA or the repair station signing block 7 on the FAA Form 337 does not know enough to cover all of the possible installation complexities. The reason the FSDO wants to look it over is to make sure it is complete. But this is not the intended purpose of the field approval process. A field approval is intended to approve the data contained in block 8 and attached sheets. Dealing with the process Here are a few comments and recommendations for dealing with the process of modifications and obtaining approval.
  • The field approval process as we once knew it is gone. We will never again see a field approval "for duplication."
  • The most effective way to obtain approved data is to hire an independent engineer with FAA designation (DER) to approve your data and submit the 337 with the supporting 8110-3.
  • Push back a little on the FSDO that will not recognize the signature of a DER and work through the differences. Let the FAA know that you can cover all of the disciplines yourself and then show them that you can.
  • Apply for and obtain an STC whenever you make modifications that will likely be duplicated on identical aircraft. This will mean breaking the modification down into pieces that can be evaluated and approved separately. The first one will take some time and money, but every installation after that will be a breeze in comparison.


Joe Hertzler is the president of AVTRAK Inc., an Aurora, CO-based company. Joe is an Airframe and Powerplant Mechanic with Inspection Authorization and also a Private Pilot.