Not long after purchasing my first VHF receiver, and programming in all of the Irish aviation frequencies, I came across a couple of places near the top of the airband which had regular, short data bursts, and I was keen to figure out what these were.

A little research introduced me to the world of ACARS, or 'Aircraft Communication Addressing and Reporting System'. 

ACARS is a digital datalink system used for the transmission of messages between aircraft and ground stations, using telex formats. It is, essentially, text messages exchanged between air traffic control (ATC) or the airline, and their airplanes.

The system was designed by ARINC in 1978 and was adapted by most airlines in the late 1980s. The aim was to reduce the crew workload by saving on the amount of voice transmissions the pilots would have to make. 

By the early 1990s, a datalink interface was put in place in the cockpit, between the ACARS unit and the Flight Management System (FMS, the on-board computer which controls navigation on modern airliners). The FMS holds the flight plan, and guides the aircraft along this path. With the introduction of an interface between the two systems, flight plans and wind information were now able to be sent from the ground to the FMS, via the ACARS unit. This allowed the airline to update the FMS while the aircraft was in flight, and granted the crew access to new weather information, alternate flight plans, etc. This was a huge development in the airline industry. 

              

                 ATIS sent via ACARS to the cockpit's datalink printer. Photo courtesy of Kent Wien. 

                                                        (Click images to view in larger size)

ACARS is primarily used by the airlines to keep track of their fleet. The systems (ACARS & FMS) are also connected to a datalink printer in the cockpit, which can be controlled by the airline's ground base to automatically print a message they send. 

The system is commonly used by pilots to notify their destination of any delays, maintenance issues, or special requirements on arrival such as wheelchair passengers. 

Air Traffic Control (ATC) also use ACARS to provide aircraft with clearances. 

A network of 'Remote Ground Stations' or RGSs, ensure that aircraft can communicate with their airline operations base, or ATC, from almost anywhere in the world. Communications are commonly conducted via VHF, but when the aircraft is outside of VHF range, there are two other options; 'SATCOM', which is ACARS sent via a satellite link that provides worldwide coverage with the exception of the poles, and 'HF datalink', a relatively new system which was completed in 2001, and provides coverage all over the world, including the polar regions. HF datalink has allowed for new polar routes, and is a much cheaper option than SATCOM. 

Whichever method the messages are sent via, they are all are directed to the RGS. Here in Ireland, there are RGSs at Shannon (at Ballygirreen), and at Dublin and Cork airport. 

          ACARS and FMS on view, American Airlines Boeing 757-223. Photo courtesy of Kent Wien. 

The ACARS network is modelled after the point-to-point telex network, and from the RGS, all messages are sent to a central processing location via landlines. The central processing location is the datalink service provider's (DSP) main computer system. ARINC, who developed ACARS, are one of two main DSPs in the world, the other being SITA, who operate much of the European network. The DSP uses information contained in their routing table to forward the message to the airlines or ATC. The downlink time from the pilot hitting the 'send' key to the message being received at the other end generally takes between 6 and 15 seconds, which is really quite impressive for the long route it is sent on. 

The uplink, a message from the ground (airline or ATC) to the aircraft, follows the same process, but in the opposite direction. Most airlines use a system called 'SITA Aircom Server' to enable them to receive the downlinks, send uplinks and track their aircraft.

In the pre-computer days, messages received at the processing location were punched to paper tape, and physically carried to the machine connected to the final destination.

Below are two different types of ACARS unit.

The first is one I own, used by American Airlines on the 767, A300, and 727. Letters are entered by flicking the toggle switches.

And the second unit, seen below, is fitted on American Airlines' 757s, and comes with a keypad included, which I'm sure makes typing much easier. Photo courtesy of Kent Wien. 

    

While most messages follow a standard format, it is possible for the pilots to enter anything they wish, and that's what makes decoding ACARS so interesting. I've seen all kinds of unusual messages over the years, from regular requests for sports results, to detailed reports of unusual technical issues with the aircraft. 

There are also a series of automatic messages which obtain data from sensors on board the aircraft, and send the information to the airline's Operations HQ. 

Should oil temperature exceed normal operating conditions, or unusual vibrations occur in an engine, a message will automatically be sent through ACARS to the airline's maintenance department.  

Prior to departure, sensors on the doors and parking brakes automatically report changes to their condition, such as "doors closed" or "brakes off", which allows the airline to know exactly when their aircraft have departed the gate. This is also how all time records are kept on the airplane's mechanical history.

Many ACARS systems also automatically report the fuel on board, and I have been informed that with some digging in the 737 FMS, some interesting information was discovered to be logged, including 'hard landing' reports, which are automatically sent to the airline. Wonder how many pilots are aware of that..?

Searching this topic online brings up quite a lot of information on how to decode ACARS, but I was looking for something a little more informative for this article, so I quizzed my buddy Kent who flies the 757 and 767 for American Airlines. He was more than happy to provide me with some "inside information" that you won't find online. 

He explained to me that when the crew board the aircraft, they load the departure airport, arrival airport, fuel on board, APU hour meter reading and flight number into the system. They request the ATIS (airport information such as weather, the current runway in use, and anything else of importance that the pilot may need to know). They can then request a clearance from ATC with the push of just one button.

They proceed to load the FMS, requesting route information, and takeoff data. The captain also pulls up the weather at the destination to include in his PA.

On climbout through 10,000 feet, two flight progress reports are printed out, and these show the waypoints the aircraft will be crossing, the estimated time, and the expected fuel at those points. Kent explains that the pilots then write in their actual time and fuel next to the planned numbers to track the fuel burn.

Kent lists many pros to the unit; the ability to pick up clearances and the ATIS without having to write it down, possibly saving a mistake; the option to pull up the weather at diversion airports during transatlantic flights; being able to make contact with Operations at any time from anywhere in the world; having the chance to let the maintenance engineers know if something was broken in-flight, and he mentions other conveniences too such as being able to send 'free text' messages to dispatch, and request football scores and other information that can be relayed to passengers in the PA.

At American Airlines, they send a position report at least every 90 minutes. With this report, they include turbulence level and current fuel figures. Not all ACARS units will send the fuel information automatically.

45 minutes prior to landing, the crew request a changeover, telling the destination what time they will be arriving, and this automatically updates the arrival time on the monitors in the airport. When the changeover is received, a printout of the connecting gates is automatically sent to the cockpit. The pilots pass this printout onto the flight attendants so they can relay the information to the passengers. 

When the aircraft lands, the times that the brakes were applied and the doors were opened are again automatically sent to the company for their records. 

Kent also mentions that when they reach the gate where the aircraft will park, the crew will submit a notice of which pilot made the landing. They'll also send the APU hour meter reading, and the arrival fuel to the company via the ACARS keypad.

The system has certainly made life much easier for pilots.

Personally, I think ACARS is one of the most interesting digital modes that can be decoded.

 

What's particularly appealing about this mode is that anyone with access to an airband receiver and a computer can decode it, for free. It's very easy; just hook up the receiver to a relatively decent antenna (I spent years using an old RTÉ antenna on the roof which had partially fallen down in a storm; not very useful for TV reception anymore, but it worked perfectly well in its new application!), connect your receiver to your computer using a simple audio cable, download some software and away you go!

A couple of tips: aircraft transmissions on VHF are in AM. Make sure to leave the squelch off, the data bursts are often less than one second in duration and you need to catch the entire burst.  The most common frequencies for Ireland are 131.725, 131.825 and 131.525MHz, but there is also activity on 136.900, 136.750, 131.850 and 136.925MHz. I generally allow the receiver to sit between whichever two frequencies are busiest at a given time. 

 

There are many different software types available online, for any operating system. Most are free. My personal favourite has to be AirNav Suite 4, which you can pay for, or opt to frequently download the free trials. I also strongly recommend 'WACARS', which is free and easy to use. 

The most interesting feature of ACARS to me are the route-plotting options.  When an aircraft sends a position report (which contains their current position, altitude, the series of waypoints their route will take them through, and the time they will reach various points along that route), the software plots the co-ordinates onto a map. It's quite easy to have several hundred aircraft within VHF range plotted on the map at any given time.  

Many of these programs will also pull in a picture of the actual aircraft, by cross-searching the tail registration on an online database. 

If you're still not convinced about decoding ACARS, visit http://www.acarsd.org/ to check out some live messages appearing from someone else's set-up! 

 

Ruthann EI8JY

Article written for MREN newsletter, January 2009.

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