By the Editors of AVweb.
The regulation itself is extremely simple:
In plain English, what this means is that if I want to manufacture a replacement part (say, an alternator) for a type certificated aircraft (say, a Piper Super Cub), then I need first to obtain a Parts Manufacturer Approval (PMA) from the FAA to do so. Otherwise, I'm manufacturing an unapproved ("bogus") part in violation of FAR 21.303(a) and the FAA should throw the book at me and shut me down.
In addition, I need to obtain an FAA Supplemental Type Certificate (STC) that says my alternator is approved for retrofit installation on a Piper Super Cub. Without such an STC, an owner who installed one of my alternators on his Super Cub would render his airplane technically unairworthy.
However, Bill Bainbridge's company doesn't build alternators for Piper Super Cubs. The B&C model L-40 alternator in question is a small lightweight unit that B&C builds for the homebuilt aircraft market. B&C's sales literature as well as the installation manual it ships with these units are prominently marked as follows:
As it happens, the little L-40 alternator does work very well on Super Cubs, and has become quite popular among Alaskan bush pilots, strictly through word-of-mouth (since B&C doesn't advertise or promote the product for use on Super Cubs). But, since the alternator is not FAA-approved for use on certificated aircraft, wouldn't you think it would be a violation of the FARs for an aircraft owner to install one on a Super Cub?
Form 337 basics
Well, not necessarily. It turns out that an aircraft owner can install virtually anything on an aircraft, provided that an FAA Airworthiness Inspector approves the installation by signing off on it using FAA Form 337 in a process known colloquially as a "one-time field approval." So, for example, if John Doe wanted to replace the cabin door of his Cessna 172 with a double-pane aluminum storm door that he bought at Sears, that would be perfectly legal provided Mr. Doe could convince an FAA Airworthiness Inspector to sign off on it.
Obviously, if Doe installed that Sears storm door on his airplane and went flying without first obtaining a one-time approval, the 172 would be unairworthy and he'd be in violation of the FARs as owner/operator and pilot. (On the other hand, it's also pretty obvious that the FAA would not go after Sears because they failed to obtain a PMA or STC on their storm doors!)
But with a signed Form 337, the installation is perfectly okay. In essence, such a one-time approval makes the "unapproved part" into an "approved part" for purposes of that one installation on that one specific aircraft.
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. In some cases, extensive engineering data is required. In others, flight testing is needed. (I suspect both would be necessary to get the Sears storm door installation approved for Doe's Cessna 172.) One way or the other, the inspector must be satisfied that the installation is safe before he approves it.
This one-time approval procedure is essential, because it's simply not practical to require that all parts installed on an aircraft be pre-approved (by type certificate, STC, and/or PMA). In this case, John Doe might have very good reasons for wanting to put that Sears storm door on his airplane, and it might be a perfectly safe thing to do. But it would be hardly practical for Doe to persuade Sears to apply to the FAA for a PMA and STC. After all, Sears doesn't build those storm doors for use on certificated airplanes. So a one-time approval is Doe's only practical way to go, and that's exactly why the FAA established the Form 337 procedure decades ago.
Now suppose John Doe has a friend named Frank Jones, who also owns and flies a Cessna 172. Jones sees the nifty Sears storm door installation on Doe's airplane and says, "gee, I'd like to put one of those on my 172, too!" Doe tells him that his installation was approved by an FAA inspector named Smith in the local FAA Flight Standards District Office. Jones calls up Inspector Smith at the FSDO and says, "remember that Sears storm door installation you approved for John Doe's Cessna 172? I fly the same model airplane as John does, and I'd like to do the same thing." Jones' burden is now far less than Doe's, because Inspector Smith has already convinced himself that the installation is safe. So once Smith verifies that Jones' airplane and the Sears storm door he wants to install are both indeed the same as Doe's, the inspector is quite likely to sign off Jones' Form 337 without much fuss or testing. In FAA parlance, Inspector Smith is using the previous one-time approval for Doe's aircraft as "approved data" to justify the new one-time approval for Jones' aircraft. Doing so is specifically provided for in the Airworthiness Inspector's Handbook (Volume 2, Chapter 1, page 1-1, paragraph 5-9).
Even if Jones lived a few thousand miles away and needed to work with a different FSDO, he could provide his local FAA Airworthiness Inspector a copy of Doe's Form 337 to use as "approved data" and the inspector could (and most likely would) approve Jones' installation on that basis. And even if Doe didn't provide Jones with a copy of his Form 337, Jones could obtain one simply by contacting the FAA Records Branch in Oklahoma City, since all Form 337s are on file with the FAA and considered to be public documents.
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.