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STC: Major or Minor Alteration?

Posted 30 days ago ago by jhadmin

Is a Supplemental Type Certificate (STC) a major or minor alteration? This is one of those questions that can easily disappear down the aviation rabbit hole. However, with some basic knowledge on aircraft alterations and certifications, it can also number among the easiest to answer.

A majority (pun intended) of people believe all STCs are major alterations simply due to its name. Another group believes that if a part comes with STC paperwork, it always requires completing a Form 337.

Fortunately for us, from a maintenance point of view, STCs and aircraft alterations are as close to an apple-to-orange comparison as we have in aviation.


To begin with, STCs are born and raised as an airworthiness standard under “Part 21: Certification Procedures for Products and Articles.” Whereas aircraft alterations, a performance standard, are found under “Part 43: Maintenance, Preventative Maintenance, Rebuilding, and Alteration.”

These are two separate regulations that control two separate regulatory processes.

Nowhere in Part 21 does it mention major/minor alteration. And while there is a single mention of “STC” in Part 43.2, it’s in specific context to records of overhaul and rebuilding. So, why the widespread belief all STCs are major alterations?

Perhaps it’s because of limited personal experience or just a misunderstanding of the regulations. Or maybe another explanation is it’s generational learning that’s to blame, where methods and practices are passed down as fact (like the misconception that placing batteries on concrete floors causes them to discharge).  

While I probably can’t convince everyone that putting a battery on concrete is no longer an issue, I can provide a better understanding on how STCs interact with aircraft alterations.


Let’s start with the easy side: aircraft alterations. Part 1 defines a major alteration as:

            “An alteration not listed in the aircraft, aircraft engine, or propeller specifications—

that might appreciably affect weight, balance, 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.”

This definition is further supported by Part 43 Appendix A(a) which contains an itemized list of parts and alterations that are considered major alterations. And, as seen in the definition above, there is no reference to STCs.

Conversely, a minor alteration is defined as an “alteration other than a major alteration.” It’s all pretty straight forward.

On the STC side, it’s not as simple. Neither Part 1 nor Part 21 define “Supplemental Type Certificate” And most attempts to delve into existing guidance for a proper definition push us closer to that rabbit hole than closer to the answer we are looking for: namely, whether an STC is a major or minor alteration.

So, let’s look at an STC from a different angle.  

STC 411

As an owner/operator or mechanic, we usually encounter an STC in one of two ways: when we purchase an article or kit for installation, or when we physically modify our aircraft from its original type design. Though these two conditions appear similar, they each go through a different process.

For example, when a person wants to offer/sell an article or product for use on a type-certificated aircraft, they must obtain two approvals: a design approval and a production approval. The latter provides the regulatory method to produce/manufacture the item listed in the design approval.

Part 21 provides design or production approval procedures as separate certifications like a TC or a PC, or as a combined design and production approval like a PMA or TSOA.

And as luck would have it, “design approval” is defined in Part 21:

“Design approval means a type certificate (including amended and supplemental type certificates) or the approved design under a PMA, TSO authorization, letter of TSO design approval, or other approved design.”

Bingo! Simply put, an STC is a specific type of design approval. Nothing more.

In fact, there are actually several variants of STCs. But due to limited space, I’ll stick to two types: a standard STC and an AML-STC (AML stands for Approved Model List.)  

The difference between these types is defined in Order 8110.4 and, more recently, in AC 20-180:

“A standard STC is an approval method for a change in type design that is limited to a single type certificate (TC). An AML-STC is a multi-model approval method that allows a set of compliance data (i.e., type design data and substantiating data) to be designated as “baseline data” that is applicable to various aircraft models.”

In some cases, vendors will use AML-STCs to streamline their design approval certification tests and costs. An AML-STC is like a blue-light special when it comes to design approvals: one approval for multiple aircraft or type certificates.

But before we continue, let’s turn to the other STC condition we mentioned.


In the case of field modifying an aircraft, an initial determination will be made by a mechanic as to whether the modification is a major or minor alteration.

If it’s considered a minor alteration, only acceptable data and a logbook entry by a mechanic is required. But if it’s a major alteration, then we will need FAA-approved data, a Form 337, and a mechanic with IA privileges to complete the project. However, the process doesn’t stop there.

Once the mechanic submits the proposed alteration data to the FAA, they will assess the complexity of the alteration and make a separate determination on whether this major alteration is considered a major or minor change to the aircraft type design.  A major/minor change to type design is not the same thing as a major/minor alteration.

Even the Feds concur in an attempt to clarify that point in FAA Order 8300.16:

“The use of the terms ‘major’ and ‘minor’ are sometimes inappropriately applied or misunderstood. A major change in type design can be approved only by an ACO as an amended type certificate (ATC) or Supplemental Type Certificate (STC). A major alteration requires the use of FAA-approved technical data.”

In other words, if the FAA determines our major alteration is a major change to type design, we are now required to use an STC approval process instead of using the FAA Field Approval process.

So whether defined as a design approval or as a major change to type design, an STC plays no direct role in the determination of a major or minor alteration. That determination falls solely to a mechanic under Part 43, or in some cases, to a pilot installing replacement parts under Part 43 Appendix A(c).


For example, say you want to install an aftermarket emergency floatation system on your Airbus 350B2. Prior to selecting which kit to install, a quick review of the FARs finds this landing gear alteration fits not only the Part 1 definition of a major alteration but is also listed in Part 43 Appendix A(a).

Note we didn’t use any of the float system documentation to determine it’s a major alteration. This is key.

And in order to perform a major alternation what do we need? Approved data. Here’s where the float documentation comes into play. Wouldn’t you know it, the first page of the float system data package is its FAA design approval, better known as an STC, which meets the requirements of FAA approved data for a major alteration.

Then why do I receive, or are required to buy, an STC for a part that does not meet the definition of a major alteration? Good question.

Remember, an STC is only a design approval which can be used in various ways by vendors. To confuse matters worse, sometimes STC data is used for the design portion of an FAA-PMA replacement part.

However, an STC is also considered intellectual property to a point, and therefore receives some regulatory protection. Part 91.403(d) states:

            “A person must not alter an aircraft based on a supplemental type certificate unless the owner or operator of the aircraft is the holder of the supplemental type certificate, or has written permission from the holder.”

But keep in mind, not all non-STC parts and installations should be considered minor alterations either.


As hinted above, you will see STCs listed together with PMAs and TSOs covering the same type parts or installations. So, which approval to use? Well, it depends.

You may find that installing a PMA or TSO part in one aircraft is a major alteration, but in another similar aircraft, it’s not. Sometimes it’s not the actual part that triggers the requirement. A number of relatively benign or simple part installations can, in fact, be major alterations especially when the aircraft is modified as part of the installation.

For example, specific tasks like cutting a hole in a cowling or disabling a warning system in order to complete the installation, would qualify as major alterations per 43 Appendix A(a)(vii) and (xii), respectively, even when the actual part installation does not qualify.  

In the end, an STC can be either a major alteration or a minor alteration. And we didn’t even slip down the bunny hole! Now, for those who don’t believe in rabbit holes, give FAA AC 23-27 a read sometime. But just be careful you don’t fall through the looking glass and end up back in the psychedelic ’60s.


About the author: After 32 years maintaining helicopters in various capacities, Scott concluded a full time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line tinkering on aircraft for beer money. He can be contacted at tekaviationllc@gmail.com.

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