Larry:

A 337 form is nothing more than a statement that an aircraft has been repaired or modified in accordance with "FAA approved" data.

The form states who did the work, who inspected it, and what "FAA approved data" was used to accomplish the work.

The FAA approved data can take the form of the aircraft manufacturer's approval (Structural repair manual, maintenance manual, service bulletin etc.), Supplemental Type Certificate, or DER approved data.

Field approval means that the local FAA inspector is accepting responsibility for the approval of the data used in repair or modification. The FAA inspector's power is unlimited and if in there opinion, the mod is O.K., they can approve it.

Field approvals were common in the past and many "major" modifications have been approved on 337's, in some cases without any engineering data or flight testing.

Due to past field approvals which shouldn't of happened, the FAA now frowns on field approvals and requires all major mods and repairs to go through FAA engineering or to at least be DER approved. Field approvals still happen, but most inspectors will pass the responsibilty to FAA engineering or a DER.

In most modifications or repair, the first step is to determine whether the mod is major or minor. This alone is a major job since the way the FAR's are written, any thing you do on the aircraft could be defined as major. The rule of thumb I use here is if the modification (if done improperly) could affect flight safety, or safety of passengers, then its a major mod and FAA approval is required. But again the deffinitions of major/minor vary in interpretation and when in doubt you should run it by the FAA.

Second step is to get with your IA and discuss the modification/repair. Have him discuss the mod with his FAA inspector to determine what will be required.

If existing FAA approved data does not exist, you may want to go to a local DER. In some cases, the cost may be prohibitive, but you don't loose anything by asking. With regards to structural mods and repairs (which is what I'm familiar with) A DER can approve most mods except for those which can affect flight characteristics (anything hanging outside the aircraft). In those cases you will need flight testing which will put you into an STC.

In my 20 plus years of doing DER and STC work I have seen both extremes. I have seen some Major-Major mods done with field approvals and no engineering data and then I've also had one case (an instrument panel and avionics replacement) where one particular FAA inspector would not accept a 337 form which had DER approval (8110-3 forms) for both structural and electrical aspects. We had no choice but to get the guy a one-time STC for the installation. It cost the aircraft owner alot of time and money. Had he asked the right questions before starting the mod, he may have saved himself alot of trouble.

I hope I have helped and not confused the issue more.

Regards,

Ray Torres