If I have Designated Engineering Representative (DER) approved data do I still need a field approval?
Data approved by DERs may, or may not, be sufficient by itself to support a major alteration. Alterations involving flight manual supplements, airworthiness limitations, ground and flight test plans, ground and flight tests, compliance inspections, modifications to critical structure or life limited parts, instructions for continued airworthiness, special conditions, and equivalent level of safety findings may require data that a DER is not normally authorized to approve.
If the alteration requires approval of data beyond the DER's authority, then additional approval, such as an FAA field approval or Supplemental Type Certificate, is required.
Who determines if I have all the data I need to get a field approval or perform a major alteration?
If you are performing the alteration work, then you are responsible for certifying that the alteration is accomplished in accordance with all applicable sections of 14 CFR and FAA-approved data.
If you are an installer, and you are not sure you possess FAA-approved data adequate to completely support an alteration, then you should contact your Flight Standards District Office for assistance.
If you are a DER supporting an alteration by furnishing approved data, then you should indicate whether the data does, or does not, constitute all the data necessary to substantiate compliance of the alteration with all applicable airworthiness regulations.