Field Approvals

Apr 1, 2017

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Do you really need the FAA’s blessing to modify your aircraft?

Does this require a field approval?

A Bonanza owner wanted to power his portable GPS and his iPad simultaneously in flight, so he asked his avionics shop to install an extra cigar lighter socket on the panel. He was told that doing so would require preparing an FAA Form 337 and obtaining a Field Approval from the local FSDO, and he was warned that this might be time-consuming and costly. Later, the owner made the same request of his A&P mechanic, who said “no problem,” installed the socket and signed it off with a simple logbook entry. 

Who was right, the avionics shop or the A&P?

A Cessna 210 owner had his A&P install a gear mirror on the underside of the left wing so he could visually check that his landing gear was down and locked. Several years later, during an annual inspection, an IA told the owner that there was no Form 337 for the mirror and that it would have to be removed before the IA could sign off the inspection as airworthy. The IA indicated that the only alternative would be to ask the local FSDO to send out an airworthiness safety inspector (ASI) to inspect the mirror installation and see if the ASI was willing to grant a field approval. 

What about this?

Who was right, the A&P who originally installed the mirror, or the IA who insisted it had to come off.

Clearly there’s a lot of confusion about field approvals. It’s not just confusing to aircraft owners; lots of mechanics are confused by it, too. Let’s see if we can clear some of the fog.

Major or minor?

Any time you modify a certificated aircraft in any fashion whatsoever, you’ve committed what is referred to in the FARs as an “alteration” to the aircraft’s type design. The regs recognize two kinds of alternations: major and minor. Major alterations are the ones that are significant enough to pique the FAA’s interest. Minor alternations are the ones they don’t want to be bothered about.

FAR 1.1 defines a major alteration as one that might appreciably affect weight or CG limits, structural strength, performance, powerplant operation, flight characteristics, or “other qualities affecting airworthiness”—or that is not done according to “accepted practices” or cannot be done by “elementary operations.” Any other alteration is considered minor.

Now that definition leaves a lot to the imagination. What is “appreciable”? What are “other qualities affecting airworthiness”? What are “accepted practices” and “elementary operations”? The meaning of those fuzzy phrases is pretty much in the eye of the beholder.

To bring this major-vs.-minor distinction into sharper focus, the FAA provides a laundry list of things they consider to be major alterations in FAR Part 43 Appendix A. There are also some non-regulatory Advisory Circulars that can help mechanics decide whether an alteration is major or minor. (For example, installation of shoulder harnesses or angle-of-attack indicators is considered minor, while installation of an IFR-certified GPS is considered major.)

Who decides?

It’s important to understand that responsibility for determining whether an alteration is major or minor lies with the mechanic who performs the alteration. It’s the mechanic’s job to evaluate the alteration—using the FARs, advisory circulars, and any other relevant FAA guidance—and determine whether the alteration is major or minor. (A mechanic who asks the FSDO whether an alteration is major or minor is simply passing the buck, not doing his job, and not doing you any favors.)

If the mechanic determines that the alteration is minor, he can perform it using methods, techniques and practices that he considers to be acceptable, and then sign off the work with a simple logbook entry. The FAA never gets into the act (and doesn’t want to).

On the other hand, if the mechanic determines that the alteration is major (i.e., “above his pay grade” to approve), then he’s obligated to perform the alteration in strict accordance with “approved data.” The FAA defines “data” as meaning the drawings, specifications, methods, techniques and practices used to repair or alter something. “Approved data” means that the data has been signed off as approved by some authorized representative of the FAA.

The most common source of approved data for performing major alterations is a supplemental type certificate (STC). Another possible source is an airworthiness directive (AD). But if you want to perform a major alteration for which no STC or AD exists, then your mechanic is going to have to make up the data himself and then find some authorized FAA representative to approve it. That’s most commonly an ASI from the local FSDO, although in certain circumstances the data may need to be evaluated and approved by an independent FAA-designated engineering representative (DER) that you hire. 


This “Job Aid” is the FAA’s bible for field approvals.

If your A&P thinks you might need a field approval for a modification you’re considering, his first step should be to consult the FAA’s online “Job Aid” that FSDO ASIs are required to use in evaluating field approval applications. The easiest way to find it is to Google “major alteration job aid.” (Most A&Ps never heard of this, so you might need to educate yours.) This pivotal 90+ page document provides ASIs with a long table of possible kinds of alterations, with each kind coded as “EVL” or “ENG” or “STC.” If your proposed alteration is marked “EVL” in the Job Aid, you’re in luck: the ASI has the discretion to evaluate your data and issue you a field approval. If it’s marked “ENG” then you’ll need to hire a DER to perform an engineering evaluation of your data before the ASI can issue a field approval. If it’s marked “STC” then a field approval is not possible, and you’d have to apply for an STC to get your data approved.

Assuming the Job Aid classifies your proposed alteration as “EVL,” the next step is to prepare an FAA Form 337 with the data for your proposed alteration attached. To maximize the chances of your data being approved, it’s wise to structure it to conform with the 16-point checklist found in FAA Advisory Circular 43-210A titled “Standardized Procedures for Obtaining Approval of Data Used in the Performance of Major Repairs and Major Alterations.” (Unless your A&P is experienced seeking field approvals, you might have to point this one out to him, too.) 

16-point checklist from AC 43-210A.

The completed Form 337 with your 16-point data package is then submitted to the local FSDO, where it will be assigned to an ASI for evaluation and possible approval. If you’re A&P is an IA, he will have a particular Principal Maintenance Inspector (PMI) at the FSDO with whom he already has a working relationship, and that can help.

The ASI will evaluate your data package in the fashion set forth in the Job Aid. He will first ensure that what you’re proposing to do is truly a major alteration that requires FAA approval. He’ll then ensure that it’s within his authority to approve (i.e., it’s an EVL and not an ENG or STC). Finally, he’ll determine if it complies with applicable FAA airworthiness standards.

Here’s what a field approval looks like.

ASIs are not aeronautical engineers. Therefore, if your proposed alteration needs an engineering evaluation (e.g., if it’s categorized as “ENG” in the Job Aid), you’ll probably need to hire a DER to perform that engineering work and sign an FAA Form 8110-3 “Statement of Compliance with Airworthiness Standards.” You’d then attach this Form to your data package and submit it to the FSDO for field approval.

If your data package passes muster, the ASI will return it to you with Block 3 of the Form 337 (“FAA Use Only”) stamped and signed, and you will then be the proud recipient of a field approval and can make the mod to your aircraft with impunity. Otherwise, he will return your package unapproved and explain the reasons he could not approve it, giving you the chance to amend your package and resubmit it.

If this sounds like a lot of work, it is! That’s why you really don’t want to get the FSDO involved if you can possibly avoid it. The best way to avoid it is to find an A&P who is comfortable treating your mod as a minor alteration and signing it off with a simple logbook entry.

So…who was right?

This brings us back full-circle to where we began: The Bonanza owner who wanted to install a second cigar lighter socket, and the Cessna 210 owner with the gear mirror. Were either of these major alterations? Did either require an FAA field approval?

To my way of thinking (as an A&P/IA), the Bonanza owner’s extra cigar lighter socket was clearly a minor alteration. It does not meet any of the criteria of the FAR 1.1 definition of a major alteration. Part 43 Appendix A says that for an electrical system alteration to be considered major, it must be a “change to the basic design of the electrical system.” In my opinion, converting from a generator to an alternator or adding a second alternator or an additional electrical bus would constitute a change to the basic design of the electrical system, but simply adding a second cigar lighter and a protective fuse or circuit breaker would not. I think the avionics shop was wrong and the Bonanza owner’s A&P was right.

The Cessna 210 owner’s gear mirror is a bit tougher. It does hang out in the breeze and so I guess one could argue that it “might appreciably affect…flight characteristics” (thereby meeting the FAR 1.1 definition of a major alteration), although I find that highly dubious. I think the A&P who installed it as a minor alteration was acting reasonably and within his discretion, and the IA who insisted the mirror be removed (or a field approval sought) had his butt cheeks clenched a bit too tightly. But admittedly this one is a bit muddier than the cigar lighter.

Of course, it really doesn’t matter what I think. It’s the opinion of the installing mechanic that counts.

You bought a plane to fly it, not stress over maintenance.

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