Avionics News March 2017 - 13
than a major alteration." So according to the regulations, if I
perform a top-down analysis of my modification and no higher
level of certification is warranted, the regulations very clearly state
that the modification must be a minor alteration.
Note: While most countries' regulatory system contain a
two-tiered modification regulation (major/minor change
in type design (Part 21)), the FAA system for in-service
aircraft contains a three-tiered modification rule (major
change in type design (Part 21) and major/minor
alteration (Parts 1 and 43)).
In general, the evaluation of major
and minor is hugely simple. It doesn't
warrant the effort put into it, and
more importantly, it doesn't warrant
the 'cost' associated with it.
So for the analysis, we start at the top (highest level of
certitude). We answer simply, do I require an STC or not? 14 CFR
21.113 contains the requirement when an applicant must apply for
supplemental type certificate.
"§ 21.113 (b) If a person does not hold the TC for a
product and alters that product by introducing a major
change in type design that does not require an application
for a new TC under § 21.19, that person must apply to the
appropriate aircraft certification office for an STC."
So the logical question is: What is a major change in type
Unlike alterations, the regulations define a minor change and
define a major change as anything not minor. With the exception
of some additional environmental criteria addressing noise, §
21.93 (a) defines a minor change as "one that has no appreciable
effect on the weight, balance, structural strength, reliability,
operational characteristics, or other characteristics affecting the
airworthiness of the product." The regulations further define a
major change as: "All other changes are major changes." It is
important that we remember this definition when we look at major
alterations and the fundamental purpose of the procedures of a
Sure, look at those broad, undefined topics: six characteristics
plus a measure of "appreciable." That leaves so much to
interpretation. Not really.
In 2002, the FAA's Aircraft Certification Division (AIR-110)
published guidance in the form of FAA Order 8110.46, which
clearly defined the scope of the criterion used in the regulations.
This document was the basis of all the regulatory guidance
published on the subject in the last 15 years, including the field
approval job aid referenced today. But it was a lost document
and, therefore, a lost history. However, the scope of the criterion
defined in the order is still valid and usable today. In addition,
appreciable was defined in the Civil Aeronautics Regulations
back in the 1950s. History is a critical element of knowing and
understanding the regulations, especially regulations with origin
of nearly 90 years ago. Without history, you lead through obscure
policy and guidance without foundation.
So this defines the top tier.
Now, let's define the bottom tier: minor alteration. As stated
earlier, a minor alteration is an alteration other than a major
alteration. The regulations define a major alteration as "an
alteration not listed in the aircraft, aircraft engine, or propeller
specifications - (1) That might appreciably effect 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."
So by regulation, a minor alteration is one that we can confirm:
1. Is listed in the aircraft, aircraft engine, or propeller
2. Has no appreciable effect on weight, balance, structural
strength, performance, powerplant operation, flight
characteristics, or other qualities affecting airworthiness.
3. Is done according to accepted practices or cannot be done
by elementary operations.
4. Is not listed in Part 43 Appendix A section 43xA.a major
With this, we have the top and bottom defined. Now what about
the middle: a major alteration?
What makes a major alteration special? We know that Part
43 section 43.9 and appendix B require special recordkeeping
for a major alteration, but that doesn't define what makes it
special to me. What makes it special for every person performing
maintenance, a mechanic, air carrier or repair station, is the
requirement for "approved data." And what is the purpose of the
administrator's approval of the data? Simply to "assure" that the
effect of the alteration does not have an "appreciable effect on
the weight, balance, structural strength, reliability, operational
characteristics, or other characteristics affecting the airworthiness
of the product." If the alteration was to have an appreciable effect
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