In aviation, however, safety and reliability are such overriding concerns that all aircraft parts must carry some type of verifiable FAA approval, and usually we must keep the approval documentation for all the parts currently installed on the airplane in the aircraft maintenance records. Even "non-functioning" parts, such as upholstery and carpeting, require approval for fire resistance before we can install them legally in an aircraft.
Unapproved parts are a clear and present danger in aviation. They are attractive to some buyers because they cost less, or are available when an approved part may not be. However, these parts have not been manufactured or tested according to minimum FAA standards. That means they may not perform as well or as long as the same FAA-approved part and that should be a serious safety concern for everyone.
Some sources of unapproved parts are the military, which often sells parts as surplus; parts being illegally "recycled" after they have reached or exceeded their approved life limits; parts intentionally made by unapproved manufacturers and sold to unsuspecting buyers; and auto parts and non-aviation quality hardware installed on aircraft.
Shops that specialize in vintage and antique aircraft commonly find auto parts and hardware installed on aircraft. Even if the item functions adequately, it's not legal and the aircraft cannot be approved for return to service after an annual inspection.
When an aircraft technician installs a part on an FAA-certified product, he or she must meet five standards, or requirements. One, the part must have FAA approval and that approval must be documented and kept in the aircraft maintenance records. Two, the part must be the proper part for that installation. Three, the part must be installed correctly. Four, the part must be function-tested to ensure that it works properly. And five, the maintenance records must be completed, showing the work accomplished, the function test performed, and that a properly certificated person or facility approved the work for return to service. The FAA parts approval process is one of the prime contributors to both the high level of safety and the high cost of aviation.
The FAA has several ways that a part may receive approval for installation on an aircraft. It may be approved by the original manufacturer; it may be approved under a Parts Manufacturers Approval (PMA) by an after-market manufacturer; it may be approved as part of an Supplemental Type Certificate (STC) by an after-market manufacturer; it may be approved by meeting the requirements of a Technical Standard Order (TSO) by an after-market manufacturer; it may be approved by an FAA Field Approval (FAA Form 337); or it may have a "blanket" approval by virtue of meeting some accepted industry standard, such as a "MIL spec" or MS.
The FAA approves parts installed by the original manufacturer when it builds the aircraft under the manufacturer's Type and/or Production Certificate, and these parts are eligible for both original and replacement installation. When we order a replacement part from the original manufacturer, it comes with certification documentation that verifies it's FAA-approved. The documentation might also include a part number and, sometimes, a serial number.
An after-market parts manufacturer can apply to the FAA for a PMA to manufacture and sell parts for FAA-certificated products. To apply for a PMA, the manufacturer must tell the FAA specifically on what products the part is to be installed. It must give the FAA the name and address of the facility that will build the parts, and the drawings and specifications that show the part's configuration, dimensions, materials, the tests that will define its structural strength, and test reports and computations necessary to show that the part meets FAA regulations. Once the FAA grants a PMA, the part must carry a number and possibly a serial number, as well as the proper documentation to verify that it's FAA-approved under a PMA and is legal to be installed on the appropriate product.
Generally, a PMA part replaces a factory original. To upgrade or enhance an airplane with products and kits not available from the original manufacturer or PMA source, such as a larger engine, floats, skis, and the like, we need to seek an after-market firm that has earned an STC approval. An STC alters the aircraft's original type certificate and is therefore a major alteration to the aircraft.
A person wishing to apply for an STC must show the FAA that the altered product meets all applicable airworthiness requirements. If the aircraft was originally certificated under FAR Part 23, it must continue to meet all of the requirements of Part 23 after the STC is installed.
Parts included in the STC will be FAA-approved as part of that STC and can be used under that approval only. If the parts manufacturer wants the part to be approved for use other than with the STC, it must receive a PMA for the part.
Some aeronautical products are approved under a TSO. Common TSO parts include seat belts, avionics, and emergency locator transmitters (ELTs). A TSO is a minimum set of FAA performance standards for a product, and to earn it a manufacturer builds its product to meet them and verifies this by testing the product. The TSO means the part itself is FAA approved, but it implies no FAA approval for installing the part on any specific aircraft. Installing a TSO'd part in a specific process is done under another process.
A part or component may be approved under an FAA Field Approval (FAA Form 337). I own a 1946 Taylorcraft with wood wing spars. When rebuilding the aircraft I found that the old spars didn't meet my standards, so I made new spars and submitted the proper documentation to the FAA for approval to install them. The FAA gave the spars a field approval on a Form 337, which is part of the aircraft's permanent records, along with a logbook entry for the installation. This process can be used for a wide variety of parts and is especially useful on older aircraft where replacement parts are no longer available.
A wide variety of parts generally classified as hardware are approved by what the FAA refers to as "established industry or U.S. specifications." Most items of aircraft hardware are identified by their specification number or trade name. Threaded fasteners and rivets are usually identified by AN (Air Force-Navy), NAS (National Aircraft Standard), or MS (Military Standard, or MIL spec) numbers.
Unapproved parts are a serious safety concern, and they present a problem that's difficult to resolve. I've attended unapproved parts programs where parts were displayed that appear to be acceptable in every way - but are really bogus. It's extremely difficult, if not impossible, for an aircraft technician to identify an expertly manufactured, marked, and documented bogus part. One partial solution is to purchase parts only from reputable sources.
The FAA is currently considering requiring FAA certification for parts suppliers. This would increase the cost of parts somewhat, but it would also increase the level of safety by making it more difficult for unapproved parts to make their way into the system. The requirement for FAA approval of aircraft parts and the system to accomplish that does increase the cost of flying, but it increases the safety of flying even more.
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