Parts Primer

March 1, 2005
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.

Here are a few things that you must pay attention to when you install a TSO part. First, the vast majority of TSO parts are used for alterations, not remove and replace items. Second, use of a TSO part may require a logbook entry and a Form 337. So the first thing you need to determine is, if the installation of the TSO part or parts constitutes a major alteration. For example, what about a complete TSO radio package with coupled autopilot installed in a King Air B-200? Is it major or minor? In this case it is a major alteration. But slipping a couple of TSO life vests in a seat pocket of a Piper Arrow before a trip to the Bahamas is not because nothing has been altered.

Type or production parts

This is easy to explain. You open up the aircraft's parts manual and order the part you need. The part comes directly from the manufacturer or another manufacturer who has been authorized by the TC or Production Approval Holder for direct ship authority. The part is usually identified by manufacturer part number, or tagged, or has an 8130-3 tag.

A small problem might come into focus when you order a part using a part number out of the manufacturer's parts manual. When the part arrives, it looks, feels, and even smells the same as the one you are replacing, but it has a different part number on the invoice. You call up the manufacturer and you get the story that the part number has been superceded but the part manual has not been revised and go ahead and install the part.

This is not a good idea! Try explaining to the NTSB or FAA if the aircraft goes in and the part you installed last Tuesday is high on their list of probable causes and the part's guy is suffering from memory failure.

Be smart! Get the revised part number change in writing from the manufacturer! An e-mail or a fax from the manufacturer explaining the part number revision is fine. Be sure that the e-mail or fax includes the person's name, position in the company, and phone number. At least you will have something in your pocket if the aircraft has a problem instead of being the only one holding the wet paper bag of responsibility.

Owner produced parts

Despite the title, "owner produced parts" the owner does not have to make the part himself. However to be considered a producer of the part the owner must have participated in controlling the design, manufacture, or quality of the part such as:

  1. provide the manufacturer with the design or performance data from which to make the part, or
  2. provide the manufacturer with the materials to make the part or,
  3. provide the manufacturer with fabrication processes or assembly methods to make the part or,
  4. provide the quality control procedures to make the part or,
  5. personally supervise the manufacturer of the part.

One of the most important things to remember about owner produced parts is an owner or his agent can not automatically make a part by reverse engineering it. You need an approved data trail. Either make the part using original design drawings, have a DER make some new drawings, or make your own drawings of the part and have the data field approved by the local FSDO.

Most pilots and owners are blessed with so much mechanical hands-on talent that you would have to times it by 100 to fill up a Corvette's trunk. Realizing this, the FAA allows the owner to contract out for the manufacture of the part to his mechanic or anyone else and still have a part that is considered "owner produced" as long as the owner participated in one of the five functions listed above.

The question always comes up if a mechanic manufactured parts for an owner is he considered in violation of section 21.303(b) (2)? The answer would be no if it was found that the owner participated in controlling the design, manufacture, or quality of the part. The mechanic would be considered the agent for the producer and would not be in violation of section 21.303(a).

On the other hand if the owner did not play a part in controlling the design, manufacture, or quality of the part the mechanic runs a good chance of being in violation of section 21.303 (b)(2). To stay out of harms way the mechanic should never make a logbook entry saying that he "made" a part under his certificate number. However, the mechanic can say on the work order that he helped manufacture an "owner produced part" under 21.303 (b) (2). To avoid from making friends with the local FSDO, the owner or operator should be encouraged to make a logbook entry that is similar to section 43.9 maintenance entry that states that: The part is identified as an owner produced part under section 21.303 (b)(2). The part was manufactured in accordance with approved data. The owner/operator's participation in the manufacture of the part is identified such as quality control or supervision. The owner should declare that the part is airworthy and sign and date the entry.

As the mechanic who is going to install this owner produced part you must ensure that it meets form, fit, and function and ensure that it meets its approved type design. Then install the part on the aircraft and make an operational check if applicable and make the required section 43.9 entry.

OK, where does the responsibility lie for this owner produced part? The short and sweet answer is the owner is responsible for the part meeting type design and is in a condition for safe operation. The mechanic is responsible for the installation, that it's airworthy, and a record of installation is made. Once the part is on the aircraft it is treated just like any other aircraft part.

Standard parts

Think nuts, bolts, safety wire, etc. that conform to an industry specification. If you want the real definition according to AC 20-62D a standard part is a part manufactured in complete compliance with an established U.S. government or industry-accepted specification which includes design, manufacturing, and uniform identification requirements. The specification must include all information necessary to produce and conform the part. The specification must be published so that any party may manufacture the part. Examples include, but are not limited to, National Aerospace Standards (NAS), Air Force-Navy Aeronautical Standard (AN), Society of Automotive Engineers (SAE), SAE Aerospace Standard (AS), Military Standard (MS), etc.

On June 5, 1997, the FAA Aircraft Engineering Division published in Volume 62 of the Federal Register an expanded interpretation for standard parts. FAA determined that certain kinds of electrical and electronic parts built to an industry standard fit within the limits of Section 21.303(b)(4) such as resistors, capacitors, diodes, transistors, and nonprogrammable integrated circuits such as amplifiers, bridges, switches, and gates. The Notice of Interpretation is five pages long and if you are interested please go to the Federal Register web site and check out Volume 62, June 5, 1997, titled Notice of interpretation.

Surplus parts

A surplus part is a product, assembly, part, or material that has been released as surplus by the military, manufacturer, owner/operator, repair facility, or any other part supplier. These products should show traceability to an FAA approved manufacturing procedure. The biggest problem with surplus parts is that in many cases the internal condition of the part is unknown. Surplus composite fabric, sealant, paints, glues, and parts with internal O-rings, seals, and gaskets are usually put on the surplus market by the ton, just so the military overhaul facility can pass an IG inspection without being gigged for overdue shelf life items. Since the first Gulf War the military has tightened up its inventory control, so real "bargains" are hard to find and if you do, they are even harder to document . . .

Overhauled parts

According to section 43.2 an overhauled part has been disassembled, cleaned, inspected, repaired, tested, and reassembled to meet the manufacturer's service limits. Well this is a fine definition but every manufacturer's service limits define a finite area that sits between brand new and worn out. It is quite possible a part could be legally signed off as "overhauled" even if it is just .001 inch inside the maximum limit. So it would stand to reason that overhauled parts will not match new or rebuilt TBO times. This is something to think about when you order your next "overhauled" part.

Rebuilt parts

Again according to section 43.2 a rebuilt part is similar to an overhauled part except it must meet new part standards. However, the regulations allow the certificated person to use approved oversized or undersized parts such as bearings or bushings to meet new dimensions.

Zero time

This refers to section 91.421 Rebuilt engine maintenance records. This rule allows the owner or operator to use a new maintenance record, without previous operating history for an aircraft engine rebuilt by the manufacturer or an agency approved by the manufacturer. So the rebuilt engine from the manufacturer starts life all over again as a zero time engine.

Remanufactured parts

This is an excellent example of the term we call "advertising." There is no such word in FARs. In the real world the word "remanufactured" usually means "rebuilt" but then again you can never be sure because you have no FAA regulation to back up the claim. Get something in writing that tells you what limits the part meets before you buy.

Parts advertised as like new, as is, and parts from crashed aircraft: are questionable parts, most likely without paper, and should be avoided. Remember the old saw; you get what you pay for.

Closing comment

Every now and again in a mechanic's career you get lucky and run across a weird looking part that tickles the suspicious nature of your mind. Most likely you are looking at a SUPS or suspected or unapproved part. If you are not sure what it is, report it using FAA Form 8120.11 or by calling the FAA Hotline toll free at 1-800-255-1111.

Bill O'Brien is the national resource specialist for the Federal Aviation Administration in Washington, D.C.

About the Author

Bill O'Brien