AOPA Turbine Pilot Magazine - July 2022 - T-16

Controller-pilot datalink communications
(CPDLC) holds the promise to
drastically reduce misunderstood clearances,
while also reducing frequency
congestion, cutting down on departure
delays, and saving time and fuel en route.
Two pilots reading " DESCEND TO AND
MAINTAIN 2400 " on a display are not
going to be confused as to the clearance.
If a descent is sent by CPDLC directly
to N434PA there is no chance N304PA
thinks it was intended for them.
While at its essence CPDLC couldn't
be more simple-direct, text-based
communication between aircraft and
ATC-the implementation has been
problematic, and the emergence of multiple
CPDLC systems has only increased
confusion. Looking at the four types of
datalink systems currently in use through
a consumer electronics model may help
break down the complexities of the various
CPDLC hardware, software, and
connection requirements.
WHAT ARE THE PIECES?
Consider sending a message to a friend
from a tablet. We'll first have to pick from
several possible options of apps to use,
some of which may be constrained by the
friend's own use of given apps or the intercompatibility
of the apps. For example, if
we open the Gmail app on the tablet to
send an email to a friend, they don't have
to be a Gmail user to receive the message.
In contrast, a Facebook Messenger communication
can only be sent and opened
within its narrow ecosystem. Similarly,
one of the two main protocols used in
CPDLC is in broader use, while one is
limited to a specific system running in
defined geographical areas.
With the app open and message ready
to send, we next need to get the message
off the tablet. Depending on the tablet's
hardware it may only have the ability to
send messages over Wi-Fi, with associated
limits on where we can be physically
while maintaining two-way communications.
By spending a little more money on
the tablet we can add a cellular radio in
addition to Wi-Fi-now we're much less
limited geographically and can send messages
from the road traveling between our
house and a coffee shop, not just when in
T-16
AOPA PILOT / July 2022
ACARS as a service
can be loosely
compared to using
internet provider AOL
in the late 1990s.
close proximity to each. However we send
the message, at the next step a " toll-taker "
comes into play. Ultimately someone is
footing the bill to connect the device to
the internet via an internet service provider
(ISP). Maybe we pay directly to a
cellular provider for monthly access, or
maybe the coffee shop whose Wi-Fi we're
using is putting a penny of our latte's cost
toward its internet bill, but this step ultimately
isn't free.
Depending on the final destination of
the message, that might be the only " toll "
collected-if we're sending an email to
a friend, for example. In contrast, some
apps or services add their own expense.
Uploading not a message, but a photograph
to an online storage host, for
example, may have an annual expense
associated with the service, in addition to
the cost of connecting to the internet.
HOW DID THIS ALL BEGIN?
For more than 40 years aircraft have had
the ability to send and receive textual messages
through the Aircraft Communication
Addressing and Reporting System
(ACARS). Originally implemented as a
system to automate the routine communications
needed during every airline flight
(e.g., takeoff time, ETA, landing fuel quantity),
ACARS developed into a full-featured
service presently utilized widely by both
general aviation and airline traffic.
ACARS: WHAT DOES IT DO?
Staying with the consumer electronics
model, ACARS as a service can be
loosely compared to using internet provider
America Online (AOL) in late 1990s.
While some interaction with the outside
world occurs, the interaction is often
filtered through a third-party service
provider and largely occurs in a " walled
garden " environment. For a single subscription
fee that covers both network
access to the system (paying the " ISP " )
and the services provided, the pilot can
perform flight planning on an app or
computer before flight and then have the
flight plan pushed directly into the avionics
over the air; send and receive emails
in flight; request weather information
including METARs, TAFs, and textual
ATIS (d-ATIS); and even order catering
to be ready at the next airport.
The first ACARS feature that enters
CPDLC territory (albeit in the most limited
of ways) is pre-departure clearance
(PDC). An official IFR clearance containing
all the elements that would normally
be received by voice, a PDC replaces the
need to call clearance delivery. PDC does
not come directly to the aircraft from
ATC, however-it is routed through a
third-party provider. Because of this, a
pilot doesn't even need ACARS capability
in the flight deck to receive a PDC. Flight
planning services such as ForeFlight and
FltPlan offer PDC routing to aircraft without
ACARS via email or text message to a
phone or tablet.
There are limitations associated with
PDC. It is only available for flights departing
from roughly 70 of the busiest airports
in the United States. Further, the system
limits any aircraft's receipt of a PDC from
a given airport to one per 12- to 18-hour
period. A pilot making two pick-ups at
Teterboro Airport on the same day would
need to pick up the second outbound
clearance over voice. Finally, if a revision
to the flight plan is made after delivery, for
example for a weather reroute, the PDC
system cannot accommodate the change;
the pilot will need to receive the revision
via voice communications.
Slightly different from a PDC is a
departure clearance issued over ACARS,
or ACARS-DCL. Currently in use in
Europe, ACARS-DCL also is transmitted
over an ACARS connection; however
it is not routed through the third-party
provider, but rather transmitted directly
from ATC to the aircraft. This distinction
is not of practical importance to the
AOPA Turbine Pilot Magazine - July 2022

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