<s>
In	O
aviation	O
,	O
ACARS	B-Algorithm
(	O
;	O
an	O
acronym	O
for	O
Aircraft	B-Algorithm
Communications	I-Algorithm
Addressing	I-Algorithm
and	I-Algorithm
Reporting	I-Algorithm
System	I-Algorithm
)	O
is	O
a	O
digital	O
datalink	O
system	O
for	O
transmission	O
of	O
short	O
messages	O
between	O
aircraft	O
and	O
ground	O
stations	O
via	O
airband	B-Algorithm
radio	O
or	O
satellite	B-Application
.	O
</s>
<s>
The	O
protocol	O
was	O
designed	O
by	O
ARINC	O
and	O
deployed	O
in	O
1978	O
,	O
using	O
the	O
Telex	B-Application
format	O
.	O
</s>
<s>
More	O
ACARS	B-Algorithm
radio	O
stations	O
were	O
added	O
subsequently	O
by	O
SITA	O
.	O
</s>
<s>
Prior	O
to	O
the	O
introduction	O
of	O
datalink	O
in	O
aviation	O
,	O
all	O
communication	O
between	O
the	O
aircraft	O
and	O
ground	O
personnel	O
was	O
performed	O
by	O
the	O
flight	O
crew	O
using	O
voice	O
communication	O
,	O
using	O
either	O
VHF	B-Algorithm
or	O
HF	B-Algorithm
voice	O
radios	O
.	O
</s>
<s>
In	O
an	O
effort	O
to	O
reduce	O
crew	O
workload	O
and	O
improve	O
data	O
integrity	O
,	O
the	O
engineering	O
department	O
at	O
ARINC	O
introduced	O
the	O
ACARS	B-Algorithm
system	O
in	O
July	O
1978	O
,	O
as	O
an	O
automated	O
time	O
clock	O
system	O
.	O
</s>
<s>
The	O
original	O
avionics	O
standard	O
was	O
ARINC	O
597	O
,	O
which	O
defined	O
an	O
ACARS	B-Algorithm
Management	O
Unit	O
consisting	O
of	O
discrete	O
inputs	O
for	O
the	O
doors	O
,	O
parking	O
brake	O
and	O
weight	O
on	O
wheels	O
sensors	B-Application
to	O
automatically	O
determine	O
the	O
flight	O
phase	O
and	O
generate	O
and	O
send	O
as	O
telex	B-Application
messages	O
.	O
</s>
<s>
It	O
also	O
contained	O
a	O
MSK	B-Device
modem	I-Device
,	O
which	O
was	O
used	O
to	O
transmit	O
the	O
reports	O
over	O
existing	O
VHF	B-Algorithm
voice	O
radios	O
.	O
</s>
<s>
Global	O
standards	O
for	O
ACARS	B-Algorithm
were	O
prepared	O
by	O
the	O
Airlines	O
Electronic	O
Engineering	O
Committee	O
(	O
AEEC	O
)	O
.	O
</s>
<s>
The	O
first	O
day	O
of	O
ACARS	B-Algorithm
operations	O
saw	O
about	O
4,000	O
transactions	O
,	O
but	O
it	O
did	O
not	O
experience	O
widespread	O
use	O
by	O
the	O
major	O
airlines	O
until	O
the	O
1980s	O
.	O
</s>
<s>
Early	O
ACARS	B-Algorithm
systems	O
were	O
extended	O
over	O
the	O
years	O
to	O
support	O
aircraft	O
with	O
digital	O
data	B-General_Concept
bus	I-General_Concept
interfaces	O
,	O
flight	O
management	O
systems	O
,	O
and	O
printers	O
.	O
</s>
<s>
ACARS	B-Algorithm
as	O
a	O
term	O
refers	O
to	O
the	O
complete	O
air	O
and	O
ground	O
system	O
,	O
consisting	O
of	O
equipment	O
on	O
board	O
,	O
equipment	O
on	O
the	O
ground	O
,	O
and	O
a	O
service	O
provider	O
.	O
</s>
<s>
On-board	O
ACARS	B-Algorithm
equipment	O
consists	O
of	O
end	B-Protocol
systems	I-Protocol
with	O
a	O
router	B-Protocol
,	O
which	O
routes	O
messages	O
through	O
the	O
air-ground	O
subnetwork	O
.	O
</s>
<s>
Generally	O
,	O
ground	O
ACARS	B-Algorithm
units	O
are	O
either	O
government	O
agencies	O
such	O
as	O
the	O
Federal	O
Aviation	O
Administration	O
,	O
an	O
airline	O
operations	O
headquarters	O
,	O
or	O
,	O
for	O
small	O
airlines	O
or	O
general	O
aviation	O
,	O
a	O
third-party	O
subscription	O
service	O
.	O
</s>
<s>
The	O
ACARS	B-Algorithm
equipment	O
on	O
the	O
aircraft	O
is	O
linked	O
to	O
that	O
on	O
the	O
ground	O
by	O
the	O
DSP	O
.	O
</s>
<s>
Because	O
the	O
ACARS	B-Algorithm
network	O
is	O
modeled	O
after	O
the	O
point-to-point	O
telex	B-Application
network	O
,	O
all	O
messages	O
come	O
to	O
a	O
central	O
processing	O
location	O
to	O
be	O
routed	O
.	O
</s>
<s>
ACARS	B-Algorithm
messages	O
may	O
be	O
of	O
three	O
broad	O
types	O
:	O
</s>
<s>
Air	B-Application
traffic	I-Application
control	I-Application
messages	O
are	O
used	O
to	O
request	O
or	O
provide	O
clearances	O
.	O
</s>
<s>
A	O
major	O
function	O
of	O
ACARS	B-Algorithm
is	O
to	O
automatically	O
detect	O
and	O
report	O
the	O
start	O
of	O
each	O
major	O
flight	O
phase	O
,	O
called	O
OOOI	O
events	O
in	O
the	O
industry	O
(	O
out	O
of	O
the	O
gate	O
,	O
off	O
the	O
ground	O
,	O
on	O
the	O
ground	O
,	O
and	O
into	O
the	O
gate	O
)	O
.	O
</s>
<s>
These	O
OOOI	O
events	O
are	O
detected	O
using	O
input	O
from	O
aircraft	O
sensors	B-Application
mounted	O
on	O
doors	O
,	O
parking	O
brakes	O
,	O
and	O
struts	O
.	O
</s>
<s>
At	O
the	O
start	O
of	O
each	O
flight	O
phase	O
,	O
an	O
ACARS	B-Algorithm
message	O
is	O
transmitted	O
to	O
the	O
ground	O
describing	O
the	O
flight	O
phase	O
,	O
the	O
time	O
at	O
which	O
it	O
occurred	O
,	O
and	O
other	O
related	O
information	O
such	O
as	O
the	O
amount	O
of	O
fuel	O
on	O
board	O
or	O
the	O
flight	O
origin	O
and	O
destination	O
.	O
</s>
<s>
ACARS	B-Algorithm
interfaces	O
with	O
flight	O
management	O
systems	O
(	O
FMS	O
)	O
,	O
acting	O
as	O
the	O
communication	O
system	O
for	O
flight	O
plans	O
and	O
weather	O
information	O
to	O
be	O
sent	O
from	O
the	O
ground	O
to	O
the	O
FMS	O
.	O
</s>
<s>
ACARS	B-Algorithm
is	O
used	O
to	O
send	O
information	O
from	O
the	O
aircraft	O
to	O
ground	O
stations	O
about	O
the	O
conditions	O
of	O
various	O
aircraft	O
systems	O
and	O
sensors	B-Application
in	O
real-time	O
.	O
</s>
<s>
Automated	O
ping	B-Protocol
messages	O
are	O
used	O
to	O
test	O
an	O
aircraft	O
's	O
connection	O
with	O
the	O
communication	O
station	O
.	O
</s>
<s>
In	O
the	O
event	O
that	O
the	O
aircraft	O
ACARS	B-Algorithm
unit	O
has	O
been	O
silent	O
for	O
longer	O
than	O
a	O
preset	O
time	O
interval	O
,	O
the	O
ground	O
station	O
can	O
ping	B-Protocol
the	O
aircraft	O
(	O
directly	O
or	O
via	O
satellite	B-Application
)	O
.	O
</s>
<s>
A	O
ping	B-Protocol
response	O
indicates	O
a	O
healthy	O
ACARS	B-Algorithm
communication	O
.	O
</s>
<s>
ACARS	B-Algorithm
interfaces	O
with	O
interactive	O
display	O
units	O
in	O
the	O
cockpit	O
,	O
which	O
flight	O
crews	O
can	O
use	O
to	O
send	O
and	O
receive	O
technical	O
messages	O
and	O
reports	O
to	O
or	O
from	O
ground	O
stations	O
,	O
such	O
as	O
a	O
request	O
for	O
weather	O
information	O
or	O
clearances	O
or	O
the	O
status	O
of	O
connecting	O
flights	O
.	O
</s>
<s>
The	O
response	O
from	O
the	O
ground	O
station	O
is	O
received	O
on	O
the	O
aircraft	O
via	O
ACARS	B-Algorithm
as	O
well	O
.	O
</s>
<s>
Each	O
airline	O
customizes	O
ACARS	B-Algorithm
to	O
this	O
role	O
to	O
suit	O
its	O
needs	O
.	O
</s>
<s>
ACARS	B-Algorithm
messages	O
may	O
be	O
sent	O
using	O
a	O
choice	O
of	O
communication	O
methods	O
,	O
such	O
as	O
VHF	B-Algorithm
or	O
HF	B-Algorithm
,	O
either	O
direct	O
to	O
ground	O
or	O
via	O
satellite	B-Application
,	O
using	O
minimum-shift	B-Device
keying	I-Device
(	O
MSK	O
)	O
modulation	O
.	O
</s>
<s>
ACARS	B-Algorithm
can	O
send	O
messages	O
over	O
VHF	B-Algorithm
if	O
a	O
VHF	B-Algorithm
ground	O
station	O
network	O
exists	O
in	O
the	O
current	O
area	O
of	O
the	O
aircraft	O
.	O
</s>
<s>
VHF	B-Algorithm
communication	O
is	O
line-of-sight	O
propagation	O
and	O
the	O
typical	O
range	O
is	O
up	O
to	O
200	O
nautical	O
miles	O
at	O
high	O
altitudes	O
.	O
</s>
<s>
Where	O
VHF	B-Algorithm
is	O
absent	O
,	O
an	O
HF	B-Algorithm
network	O
or	O
satellite	B-Application
communication	O
may	O
be	O
used	O
if	O
available	O
.	O
</s>
<s>
Satellite	B-Application
coverage	O
may	O
be	O
limited	O
at	O
high	O
latitudes	O
(	O
trans-polar	O
flights	O
)	O
.	O
</s>
<s>
+	O
A	O
typical	O
ACARS	B-Algorithm
VHF	B-Algorithm
transmission	O
.	O
</s>
<s>
In	O
the	O
wake	O
of	O
the	O
crash	O
of	O
Air	O
France	O
Flight	O
447	O
in	O
2009	O
,	O
there	O
was	O
discussion	O
about	O
making	O
ACARS	B-Algorithm
an	O
"	O
online-black-box	O
"	O
to	O
reduce	O
the	O
effects	O
of	O
the	O
loss	O
of	O
a	O
flight	B-Device
recorder	I-Device
.	O
</s>
<s>
However	O
no	O
changes	O
were	O
made	O
to	O
the	O
ACARS	B-Algorithm
system	O
.	O
</s>
<s>
In	O
March	O
2014	O
,	O
ACARS	B-Algorithm
messages	O
and	O
Doppler	O
analysis	O
of	O
ACARS	B-Algorithm
satellite	B-Application
communication	O
data	O
played	O
a	O
very	O
significant	O
role	O
in	O
efforts	O
to	O
trace	O
Malaysia	O
Airlines	O
Flight	O
370	O
to	O
an	O
approximate	O
location	O
.	O
</s>
<s>
While	O
the	O
primary	O
ACARS	B-Algorithm
system	O
on	O
board	O
MH370	O
had	O
been	O
switched	O
off	O
,	O
a	O
second	O
ACARS	B-Algorithm
system	O
called	O
Classic	O
Aero	O
was	O
active	O
as	O
long	O
as	O
the	O
plane	O
was	O
powered	O
up	O
,	O
and	O
kept	O
trying	O
to	O
establish	O
a	O
connection	O
to	O
an	O
Inmarsat	O
satellite	B-Application
every	O
hour	O
.	O
</s>
<s>
The	O
ACARS	B-Algorithm
unit	O
on	O
the	O
Airbus	O
A320	O
of	O
EgyptAir	O
Flight	O
804	O
sent	O
ACARS	B-Algorithm
messages	O
indicating	O
the	O
presence	O
of	O
smoke	O
in	O
toilets	O
and	O
the	O
avionics	O
bay	O
prior	O
to	O
the	O
aircraft	O
's	O
crash	O
into	O
the	O
Mediterranean	O
Sea	O
on	O
May	O
19	O
,	O
2016	O
,	O
which	O
killed	O
all	O
66	O
persons	O
on	O
board	O
.	O
</s>
<s>
On	O
24	O
February	O
2021	O
the	O
ACARS	B-Algorithm
unit	O
of	O
a	O
South	O
African	O
Airways	O
flight	O
from	O
Johannesburg	O
's	O
OR	O
Tambo	O
International	O
Airport	O
to	O
Brussels	O
sent	O
an	O
ACARS	B-Algorithm
message	O
about	O
an	O
“	O
alpha	O
floor	O
event	O
”	O
,	O
which	O
was	O
activated	O
when	O
the	O
Airbus	O
A340-600	O
'	O
s	O
envelope	O
protection	O
system	O
activated	O
to	O
override	O
the	O
pilots	O
to	O
prevent	O
the	O
plane	O
from	O
stalling	O
on	O
take-off	O
.	O
</s>
<s>
On	O
19	O
February	O
2023	O
there	O
were	O
numerous	O
ACARS	B-Algorithm
reports	O
of	O
a	O
large	O
white	O
balloon	O
near	O
Hawaii	O
in	O
the	O
aircraft	O
lanes	O
.	O
</s>
<s>
In	O
2002	O
,	O
ACARS	B-Algorithm
was	O
added	O
to	O
the	O
NOAA	O
Observing	O
System	O
Architecture	O
.	O
</s>
<s>
Thus	O
commercial	O
aircraft	O
can	O
act	O
as	O
weather	O
data	O
providers	O
for	O
weather	O
agencies	O
to	O
use	O
in	O
their	O
forecast	O
models	O
,	O
sending	O
meteorological	O
observations	O
like	O
winds	O
and	O
temperatures	O
over	O
the	O
ACARS	B-Algorithm
network	O
.	O
</s>
