Parts Primer

How to tell a good part from a bad one.

March 2005

In the mechanic's list of career priorities, second only to a well-stocked toolbox, is the desire for a good, dependable, source of replacement parts. A good mechanic needs to know what kind of parts are out there and how to tell a good part from a bad one. What follows is an overview on what kind of parts can go on an aircraft.

Parts! Like anything else in aviation we have a rule that covers it. The rule is found in Part 21, section 21.303 Replacement and modification of parts. It starts off by sternly requiring us to use parts manufacture approval (PMA) replacement parts on a type certificated product. In the next paragraph of the rule it mellows out and grants owners/operators four exemptions to the PMA requirement. These exemptions are

  1. A part produced under a type or production certificate such as a Cessna or Boeing produced part or,
  2. An owner or operator produced part to maintain or altering their own product or,
  3. Parts produced under a Technical Standard Order (TSO) such as radios, life vests and rafts, and GPS or,
  4. Standard aviation parts such as fasteners, washers, or safety wire.

Since the biggest areas of confusion are "owner produced parts" and "standard parts" I will deal with those guys last. First let's talk PMA and TSO parts.


The parts manufacture approval or PMA is a part that is designed to be a direct replacement part. It won't be legal on any other aircraft or system except what the part numbers say that's what the part is for. Furthermore, Part 21.303 requires the PMA manufacturer to have an incoming inspection, design drawings, and quality control system to ensure that each replacement part will work as good as the original TC holder's part.

There are three things I want you to remember about a PMA part.

First, a PMA part is considered a "remove and replace" item so it is rare that they are used for alterations.

Second, when you install a PMA part 99 percent of the time you only need a logbook entry filled out in accordance with FAR 43.9. So a FAA Form 337 is usually NOT required!

Third, PMA parts are identified in accordance with FAR 45.15 with the letters FAA-PMA, the name, trademark or symbol of the PMA holder, the part number and the name and model designation of each type certificate product on which the part is eligible for installation. If the PMA part is small, then the rule allows the PMA manufacturer to tag the part with the required information.


The rule for Technical Service Order or TSO is found in Part 21, section 21.601. A TSO part is a part that is generic in nature so it can be installed on many different kinds of aircraft. TSO parts can take the form of radios, flight and engine instruments, lights, tires, batteries, seat belts, ELT, life vests, life rafts, etc. The list of TSO items is endless. But what is a TSO? A TSO part is a part that meets a minimum FAA performance standard. When the applicant is issued a TSO he has both an FAA design and production approval. TSO parts must be identified in accordance with section 21.603 with the letters TSO and the appropriate number after it.

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