Avionics News January 2015 - 13

An excellent quick reference for policies that drive a higher
level of certification is the Field Approval Job Aid - a nice
tabulated resource that addresses most of the common
alterations and many of the new technologies.

Part 21 and Part 43 are opposites in their definitions
of modifications. In Part 21, the agency defines a "minor
change in type design" and then defaults all others as
major. In Part 43, the agency defines a "major alteration"
and then defaults all others to minor.
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. (§ 21.93)
Section 21.93 further states that, with a few exceptions,
"All other changes are 'major changes.'"
There are two definitions here that need to be reviewed:
appreciable and product. Since there is no regulatory
definition for "appreciable," we look to the MerriamWebster Dictionary, which defines appreciable as "capable
of being perceived or measured."
Part 21 defines "product" as meaning an aircraft,
aircraft engine or propeller. (§ 21.1 (b) (5))
So the first step in the evaluation is determining if your
modification has a measurable effect on the aircraft, aircraft
engine or propeller. If your analysis determines you do
have a measurable effect, then you must apply for an STC.
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. (§ 21.113 (b))
So with the exception of a new type certificate, the
in-service modifier who introduces a modification that
is confirmed to have a measurable effect on the aircraft,

aircraft engine or propeller, the regulations mandate the
application for an STC.
Part 1 defines a major alteration with three criterion:
* Not listed in the aircraft, aircraft engine or propeller
specifications.
* Might appreciably affect weight, balance, structural
strength, performance, powerplant operation,
flight characteristics or other qualities affecting
airworthiness.
* That is not done according to accepted practices or
cannot be done by elementary operations.
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. (§ 1.1)
I think the two areas of most concern are the first and
third criterion. If the modification is listed in the aircraft
specifications, it is not a major alteration. Then there is the
long-standing debate on what are "accepted practices and
elementary operations." There is not any clear guidance
on what is considered accepted practices or elementary
operations.
Let's look at "acceptable practices." We certainly
have acceptable defined in a number of places; logically
following any appropriate, applicable practice, what is
Continued on page 55
avionics news

january

2015

Avionics News January 2015

Table of Contents for the Digital Edition of Avionics News January 2015