Avionics News March 2017 - 22
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on these six criterion, 14 CFR 21.113 would mandate that we
apply for an STC.
§ 43.13 (b) states that each person altering an aircraft, engine,
propeller or appliance "shall do that work in such a manner and
use materials of such a quality, that the condition of the aircraft,
airframe, aircraft engine, propeller, or appliance worked on will
be at least equal to its original or properly altered condition (with
regard to aerodynamic function, structural strength, resistance
to vibration and deterioration, and other qualities affecting
So when we talk about a major alteration and the need for
approved data, either via a field approval or DER, we are
answering the question "I don't know," it might appreciably
affect a critical system and, therefore, prudence requires a second
set of eyes; the administrator's approval.
The regulations provide great latitude when we clearly know
what we are doing and what the effect of what we are doing is
on the aircraft. If we can clearly show that what we are doing
is mainstream and has no effect on the aircraft's seven criterion
(major change has six, alterations have seven - including the
powerplant installation), then by regulation the alteration is minor
and we can use any of dozens of sources of "acceptable data."
This can be a challenge for retrofit on older legacy aircraft.
The aircraft systems were not designed for the newer electronics
being retrofitted. As a result, what might be a minor alteration to a
2010 aircraft may be a major alteration to an aircraft produced in
the 1950s without the same electrical system. This is one reason
it is difficult for anyone to make blanket statements about major
and minor alterations; we don't have standard aircraft.
And finally, there is always that other catch-all - the
Well, yes, the administrator does have the authority to publish
guidance or policy that, regardless of the technical quality of the
modification, dictates that it must be treated at a higher level. We
see this today with ADS-B Out system installation, and a few
years ago (before it was sunset) where GPS systems were to be
treated as major alterations, and before that, Lorans. But those
are the exceptions.
With a few exceptions:
* If you can confirm that your proposed modification
has an appreciable effect on the six criterion of a major
change in type design, then you must apply for an STC.
* If you can confirm there is no appreciable effect on the
seven characteristics of a major alteration, then it is a
* If you don't know, it is a major alteration and you need
a second set of eyes looking at the data (approved data).
* Of course, if the administrator has published a policy on
this particular modification, follow that guidance.
It isn't much more complicated than that.
Contained within the repair station resources tab on the AEA
website is a decision tree for alterations to type certificated
products. This is a checklist the association encourages every
member to use. It helps guide the applicant through the regulatory
analysis that is required for every modification. There also is
an online training course, Architecture of an Alteration, that
discusses how to evaluate an alteration. q
THE VIEW FROM WASHINGTON
The advantages remain
The limitations notwithstanding, the appeal of ADS-B
In and the portable systems pilots covet remain well
grounded. With the text weather products available from
ADS-B In - Terminal Area Forecasts, meteorological
observations in METARs, pilot reports, AirMets, Sigmets
and, of course, radar images - pilots enjoy far better
access to recent, even current conditions than via a visit
with Flight Service.
As long as users understand the limitations of the
technology and constraints of the delivery system,
they should fly with the comfort of knowing updated
information is only a few minutes away.
With a picture still worth a thousand words, pilots
enjoying ADS-B In access via portable devices can be
confident they're seeing all there is to see. q
FIS-B uses weather radar images collected from the
radar stations and processed in six-minute cycles it takes
to make six sweeps, each at an ever-higher angle above
the ground. Next, those images are combined into the
composite NEXRAD images we're used to seeing, then
processed by computers before being distributed through
the ADS-B ground station network. That network consists
of some 500-plus ground stations, with more still to come.
So six minutes to make the six sweeps that go into
the composite radar image, then a couple minutes more
processing those images into a form for distribution to
the ground station network, and then, finally, the FIS-B
packages are broadcast through the ground stations.
By that time, the composite radar image is several
minutes old and waiting for the next update. It's never
younger than eight to nine minutes.
A fast-moving weather can be miles away from where
datalink images show it to be - hence the admonition
against trying to use weather datalink radar images for
real-time weather avoidance.
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THE PROBLEMS WITH PORTABLES