In 2009, the FAA's regulations concerning aircraft parts approval were changed. The new regulations explain that an article that is reasonably likely to be installed on a type certificated product (aircraft, propeller or engine) may not be produced unless it fits into one of six categories:
(1) It is produced under a type certificate;
(2) It is produced under an FAA production approval;
(3) It is a standard part (such as a nut or bolt) manufactured in compliance with a government or established industry specification;
(4) It is a commercial part as defined in § 21.1 of this part;
(5) It is produced by an owner or operator for maintaining or altering that owner or operator's product; or
(6) It is fabricated by an appropriately rated certificate holder with a quality system, and then consumed in a repair or alteration.
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#3 can possibly allow it if the aircraft manufacturer has developed it as an industry standard for said aircraft type during certification. This is not likely but certainly possible if the aircraft manufacturer and a commercial hardware company (Zed screw company) had close ownership relations. Simply in the aircraft manuals state where locally obtained Zed screws can be used on say the Concord wings.
Some time back a fixed ELT using D cell Batteries said "locally obtain Duracell D cell batteries" and fit. From memory the use by date on the battery was reduced as part of the approval. All 100% legal with a till slip from local supermarket. (However the bulk packs of these exact approved batteries were a different diameter and did not fit).
Battery only as an example that it is possible.
ELT SYSTEM E-01
- Uses standard Duracell® batteries
- Smallest, lightest TSO-C91a ELT available
- Easy to install. Requires no aircraft power
- Fire resistant, high impact plastic case
- Waterproof to 60 ft.
- Easily removed from aircraft for service
- Coherent beacon designed for satellite detection
- Two Year Warranty