Parts Manufacturer Approval (PMA)

In aircraft maintenance, Parts Manufacturer Approval (PMA) is an approval granted by the United States Federal Aviation Administration (FAA) to a manufacturer of replacement aircraft parts.  PMA is a combined design and production approval for modification and replacement equipment.  It allows a manufacturer to produce and sell these articles for installation on type-certified products.  FAA Orders 8110.42 and 8120.2 prescribe the approval procedures for FAA personnel and guides the applicants in the FAA approval process.  The FAA approval process can be found at .

FAA Logo

PMA-holding manufacturers are permitted to make replacement parts for aircraft, even though they may not have been the original manufacturer of the aircraft.  It is generally illegal in the United States to manufacture replacement or modification aircraft parts without a PMA.  The actual FAA regulation is extremely simple:


FAA Regulation 21.303 Replacement and modification of parts.
(a) Except as provided in paragraph (b) of this section, no person may produce a modification or replacement part for sale for installation on a type certificated product unless it is produced pursuant to a Parts Manufacturer Approval issued under this subpart.


Translated into plain English, this FAA regulation on replacement parts dictates that if I want to manufacture a replacement part, say a replacement auxiliary power unit (APU),  for a type certified aircraft (for instance, a Cessna), then I first need  to obtain a Parts Manufacturer Approval (PMA) from the FAA to do so.  Otherwise, I’m manufacturing an unapproved part, which would be in violation of FAR 21.303(a) and the FAA could take enforcement action against me,  and just stop any further activity.  In addition, I need to obtain an FAA Supplemental Type Certificate (STC) that says , for example,  my APU is approved for retrofit installation on a Cessna.  Without this STC,  an owner who installed an APU on his Cessna would render the  airplane technically un-airworthy.

But wait, don’t despair!  If all the above gives you a sinking feeling in the pit of your stomach, there is help available.  It turns out that an aircraft owner can install virtually anything on an aircraft, provided that an FAA Airworthiness Inspector approves the installation of a replacement part by signing off on it using FAA Form 337 in a process known loosely as a “one-time field approval.”  With a signed Form 337, the installation of an aftermarket non-OEM replacement part is just fine. This Form 337 one-time approval converts an “unapproved part” into an “approved part” for purposes of that one installation on one specific aircraft.

PMA in aircraft maintenance

This procedure is documented extensively in Volume 2 of the Airworthiness Inspector’s Handbook (FAA Order 8300.10).  The Handbook provides extensive guidance to FAA inspectors concerning what testing and documentation they should require before granting such a one-time approval.  Sometimes, extensive engineering data is required. Other times, flight testing is needed.  One way or the other, the inspector must be satisfied that the after-market installation is safe before the FAA Airworthiness Inspector approves it. The FAA’s one-time field approval procedure has been in place for decades, and has provided a proven method of enabling aircraft owners to improve the safety and utility of their aircraft at affordable cost.  Because each installation must be individually approved by an FAA Airworthiness Inspector, and because the Airworthiness Inspector’s Handbook provides detailed guidelines to ensure that the necessary testing and documentation be provided to ensure that the installation is safe, the procedure has been well accepted by both the FAA and the aviation community.

Saphir 5 Auxiliary Power Unit - PMA in aircraft maintenance

Saphir 5 Auxiliary Power Unit. Credit: Wikipedia, Varga

In addition, thanks to the Internet, it’s now not only easier to find parts for your plane, it’s also simpler to make sure you’re not getting something the FAA hasn’t approved. Inventory Locator Service from ILSmart ( has cross-referenced its database of more than five billion  parts with the FAA’s Part Manufacturer Approval (PMA) system (, which lists all the approvals for parts made for certified airplanes. The result, with this database on the Internet,  is more exposure for replacement parts makers and better selection for parts buyers. But what happens if one of those parts falls from grace with FAA? Enter the FAA’s Unapproved Parts Notices  (UPN) online service ( When the FAA finds parts that don’t measure up, they broadcast it to anyone who is on their e-mail list. Fortunately, the occurrence of bad or bogus parts is rare (about 15 notices a year) so the service is not annoying. Logging on (or signing off) is easy.

The entire PMA process is extensively covered in the “PMA Guidebook,”  available at . The trade association representing the PMA industry is the Modification and Replacement Parts Association (MARPA –  MARPA works closely with the FAA, promoting PMA in aircraft maintenance.


by Steve  Adams


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