Avionics News July 2013 - 17

FREQUENTLY ASKED QUESTIONS
United States

elect to obtain an STC for equipment installation
concurrent with obtaining the TSOA/LODA. This has
proven to be an effective method to aid installations.

Follow-on Installations to AML-STCs

AC 20-138C 2-3.a. All STC applicants are encouraged
to use an AML for aircraft makes/models (including
14 CFR Parts 23, 25, 27 and 29 aircraft) with similar
characteristics and compatible equipment configurations
to streamline the approval of subsequent installations.
Applicants and ACOs need to contact the applicable
directorate for specific policy and guidance information
on obtaining an STC-AML.

The following information is from Federal Aviation
Administration regulations and advisory materials.

QUESTION:

Can you do a follow-on installation to an AML-STC, and
are they by default major alterations?

ANSWER:

Yes, you can do a follow-on installation to an AMLSTC just like any other STC. And no, there is no implied
level of alteration simply because the STC holder chose to
utilize the AML process.
Many people assume that because the initial application
was via an STC, then therefore, all follow-on installations
must be a major alteration. This assumption is flawed. The
following is an explanation of the flawed logic.
Assumption 1: The STC was required by regulation;
therefore, any follow-on installation must be a major
alteration.
§ 21.113 Requirement for supplemental type certificate.
(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.
Assuming the original alteration was determined to be a
“major change in type design” (§ 21.93) and that an STC
was required by § 21.113, then any follow-on alteration
would also be major change in type design and require
an application for STC. As such, this alteration would not
be a major alteration and would not be eligible for a field
approval; the regulation would mandate an STC, not a
field approval.
Assumption 2: STCs are only issued for a major change
in type design.
Unfortunately, there are conflicting guidance
documents that address this. While there is an FAA order
telling the Aircraft Certification community that they
may only issue STCs for major changes in type design,
there are numerous other advisory circulars and guidance
documents to the contrary.
AC 20-138C 2.2.b. It is recommended that the
positioning and navigation equipment manufacturer

The advisory material on AML-STCs themselves
supports that not all STCs are major changes in type design.
The AML STC was redesigned to replace field approvals
in modern avionics systems. A field approval for a major
alteration, by regulation, must be a lesser level alteration
than a major change in type design.
From Chapter 2 of AC 23-24:
“Until recently, hardware costs and certification
costs made it impractical to build new technology
avionics systems for Part 23 use. As new technology
is introduced, hardware costs continue to come down.
In contrast, the avionics installation approval process
has increasingly become more rigorous for applicants
seeking installation approval of new systems in Part 23
and CAR 3 airplanes.
“Applicants sought the approval path of least resistance,
which has increased the number of follow-on field
approvals for installation of avionics systems in small
airplanes. Field approvals are typically based on a single
STC with little or no follow-on ACO involvement,
which may only be suitable for certain types of
installations. For more complex avionics installations,
a follow-on field approval may not properly address
interface considerations to existing equipment.”
The bottom line is AML-STCs were first introduced to
replace and standardize field approval, and by their nature,
cannot be a major change in type design. In addition,
AML-STCs are encouraged for all avionics installations
regardless of the level of alteration. In conclusion,
there is no implied level of alteration associated with a
previously issued AML-STC, and each and every followon installation should be evaluated as any other alteration
to determine which category of change the alteration fits
into, utilize the appropriate level of data and record the
alteration to the appropriate regulatory standards.
Continued on following page

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Avionics News July 2013

Table of Contents for the Digital Edition of Avionics News July 2013