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Aircraft Maintenance: Major vs minor alterations, part 2Aircraft Maintenance: Major vs minor alterations, part 2

Last time, we covered the importance of determining if an alteration to an aircraft is major or minor. This time, we’ll get deeper and discuss how to make that determination (and how to build a case for any modification you are considering).

Jeff Simon

Acceptable vs. approved data

The data required to get an alteration approved is the most important difference between a major and minor alteration. Minor alterations require data that is “acceptable” to the FAA. Acceptable data is data that the installer can refer to from FAA guidance materials, such as Advisory Circular 43.13-1B, Acceptable Methods, Techniques and Practices. However, data from the manufacturer can be used or even interpreted from. This means that if you can correctly classify your alteration as minor, your mechanic will have a reasonable amount of leeway in the project.

For example, let’s say that you wanted to install a mount for a removable, portable navigation or entertainment device in your panel. The aircraft manufacturer has already demonstrated the methods for supporting panel-mounted equipment by the design of the panel for the existing equipment. If the installer uses the same mounting methods to the same structure of the aircraft, he or she could logically conclude that they are using acceptable data provided by the manufacturer when they designed the original. The point is that, for any minor alteration, the installer should be able to demonstrate logical reasoning and a clear path from existing data to the work that they have done.

Approved data for a major modification is much different. This is data that is explicitly approved by the FAA, a designated engineering representative (DER), or the aircraft manufacturer. No interpretation allowed, period.

Determining major vs. minor

It is the responsibility of the installer to determine whether an alteration is minor or major based on the federal aviation regulations. This makes choosing a good shop to do the alteration very important. There are a lot of excellent mechanics out there who understand the regulations, can help you navigate the process, and can get your alteration done in a safe and legal manner. Unfortunately, there are also quite a few shops out there who follow the “No STC = No Alteration” philosophy. This approach has no basis in the regulations, so steer clear of anyone not willing to evaluate your proposed alteration on its own merits. 

The primary regulatory guidance for determining a major vs. a minor alteration can be found in 14 CFR 21.93  and 14 CFR Part 1.1.

According to 14 CFR 21.93, “A ‘minor change’ is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are ‘major changes’ (except as provided in paragraph (b) of this section).”

Definitions provided in 14 CFR Part 1.1 state, “Major alteration means an alteration not listed in the aircraft, aircraft engine, or propeller specifications—

(1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or

(2) That is not done according to accepted practices or cannot be done by elementary operations.”

And “minor alteration means an alteration other than a major alteration.”

Sounds simple, right?  Unfortunately it’s not. First of all, there’s the term “appreciably.” What exactly is an “appreciable affect”?  Is it 1 pounds or 10 pounds? Then there are the other questions: “What exactly does the FAA mean by something that affects weight, balance, structural strength, performance, etc.?”

To get the answers, we can turn to the FAA guidance materials for their airworthiness inspectors. There have been many revisions to these guidelines over the years. However, the only guidance that specifically provides these critical definitions is FAA Handbook 8110.46, Major Alterations that Require Supplemental Type Certificates. That handbook has since been rescinded and replaced by newer guidelines. However, the FAA did not include these key definitions in the updates. According to Ric Peri, vice president at the Aviation Electronics Association, the FAA still considers the 8110 definitions as a good point of reference.

So, let’s take something as simple as the line “no appreciable effect on the weight, balance…” from 14 CFR 21.93 and look at it according to 8110.46.

“Typical alterations that may appreciably affect weight and balance include, but are not limited to:

(1) Changes that increase the certificated maximum weight limits (increases in the maximum gross weight, maximum take-off, or landing weights).

(2) Changes in the certificated center of gravity range limits (for example decreasing the forward limit or increasing the aft limit).

(3) Changes that increase the operational limits (maximum speed limits such as VA, VFE, VNE; minimum speed limitations such as stall speed; increases in service ceiling, and so forth).”

You’ll notice that the definition has absolutely nothing to do with adding a few pounds to the aircraft. It’s all about changing the certified limits of the aircraft. So, just because the alteration adds 5 pounds to the aircraft does not mean that it is a major alteration. However, if an alteration exceeds the certified maximum weight or balance limits, stop here because it is a major mod.

The definition of an appreciable affect to structural strength has a similar definition in 8110.46:

“Typical alterations that may appreciably affect Structural Strength include:

(1) Changes to primary structures (structure that carries flight, ground, or pressure loads as defined in AC 25.571-1, Damage Tolerance and Fatigue Evaluation of Structure). *

(2) Substituting an engine, propeller, rotor or airframe primary structural materials (such as replacing a reciprocating engine with a turbine engine or increasing horsepower output by 10% or more).”

So, here the FAA is telling us that it is alterations to the primary structure of the aircraft that make an alteration a major one. Unless you are modifying or attaching something to a primary structure, you are not having an appreciable affect on the structural strength of the aircraft.

There are similar definitions in 8110.46 for performance, powerplant operation, flight characteristics, or “other qualities affecting airworthiness.” So, read these definitions carefully in your evaluation of whether an alteration is a major or minor one.

The bottom line is that if you are changing how a system works, the basic structural integrity, or the certified operational characteristics of the aircraft, you are making a major alteration. If not, it’s generally a minor alteration. As long as your mechanic can document reasonable logic to justify the classification as minor, you may be surprised at the ways you can improve your airplane without getting on the wrong side of the FAA. And that, I believe, we can all agree is a good thing!

Social FlightJeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 10,000 aviation events. Free apps available for iPhone, iPad and Android, and on the Web at

Jeff Simon

Jeff Simon

Jeff Simon is an A&P, IA, pilot, and aircraft owner. He has spent the last 17 years promoting owner-assisted aircraft maintenance and recently certified the FlexAlert Multifunction Cockpit Annunciator. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 20,000 aviation events, $100 Hamburgers, and educational aviation videos
Topics: Maintenance, Ownership, Gear

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