As a DER, I am frequently quizzed on this often misunderstood process.
Not everybody understands the difference between a "field approval" vs. "337 with DER Approved Data". So.... here is an effort to explain.
A&P/IA's are trained that they must use FAA approved data when executing the FAA form 337 to document major repairs and alterations for the aircraft's return-to-service. For many years, it was customary to go to an FAA FSDO safety inspector for "field approval" of data when it was not readily available. This can still be done. However, the overall decline of experienced FSDO inspectors has resulted in fewer and fewer field approvals being done. These days, DER's can bridge this gap.
The "field approval" term refers to the case of obtaining data approval from the FSDO safety inspector. Alternatively, an equivalent means of approving an alteration is through "DER support of a 337", which means the data approval is from a DER. Either way, you end up with FAA approved data.
Field Approvals can be one way to obtain FAA Approved data, but DER's can also provide FAA approved data. A common misunderstanding of the process is the erroneous belief that all 337's are "field approvals". Not true. In reality, a "field approval" is not needed when using DER(s) - as long as a DER approves all of the applicable regulations associated with the alteration or repair.
i.e. DER approved data = FAA approved data
With a drawing and DER approval in hand, any IA can execute a 337 and reference the drawing in block 8 of the 337 as the "approved data". Once completed, the IA will forward the 337 paperwork directly to the FAA to be filed in OKC. There is no need to call a FSDO inspector, except in certain circumstances. Ask your IA abut this. It is routine.
The following is an excerpt from FAA AC 23-21, Change 1...
One of the methods of obtaining approved data for a particular alteration is to utilize a DER to provide necessary approved data. This methodology is described in FAA Order 8110.45, “Use of Data Approved by Designated Engineering Representatives to Support Major Alteration,” dated 8/30/02, and Order 8110.37C, “Designated Engineering Representative (DER) Guidance Handbook,” dated 9/30/98. Both Orders can be found at the FAA website http://www.faa.gov. When you identify a requirement for data during an alteration, you may contact a DER with the authority, as listed in Order 8110.37C, to generate the report and submit approved data. The use of a DER to develop FAA approved data can save time because it is an efficient means to ensure completeness of the required data. [If 100 percent of the necessary data is DER approved data, no further FAA approval is required.—Change 1] When contacting a DER, only DERs specifically authorized by their managing ACO are permitted to approve data for major repairs and alterations.