HF and VHF waiver
The aviation industry has always been distinguished by a conservative approach to any innovations. From the moment a new feature is developed to its implementation, 10-15 years can easily pass. And the cost of the project can be much higher than in any other industry. In such conditions, a company striving to become a pioneer takes a big risk: having invested a lot of money, you may not get the desired result at all.
But sometimes innovation is a necessary measure that has to be applied when traditional and well-proven tools stop doing their job. This is what happened with ACARS (Airborne Communications Addressing and Reporting System), a digital communication system used to transmit simple text messages between aircraft and ground stations. In today's article, we will briefly describe how this system works and how it needs radical changes.
How ACARS works
In fact, ACARS is a living reminder of how, in the first half of the 18th century, the best minds on the planet struggled to invent a form of communication that would instantly transmit important information over long distances. It was the telegraph: it could be used at any time and in bad weather. This invention reduced the cost of delivering text messages by more than 30 times and gave a powerful impetus to the further development of such technologies.
By the beginning of the 19th century, thanks to the efforts of Guglielmo Marconi and Alexander Popov, humanity had acquired the first wireless communication technology. In addition to the traditional telegraph, devices appeared that allowed sending telegrams over a radio channel – radio teletypes (RTTY). Half a century later, the telegraph began to give way to computers with more modern communication methods.
But radioteletype did not die, but received a second life in the form of the ACARS system, which began to be implemented on civil aviation aircraft in 1978. It allowed receiving and sending short text messages using the HF and VHF ranges at a speed of 2,400 baud. The frequency of 131.550 MHz is used as the main ACARS frequency worldwide. The signal has AM modulation and is encoded with two tones – 1,200 Hz and 2,400 Hz.
Airlines began to actively use this system to quickly deliver information on board an aircraft – for example, if a flight plan needed to be adjusted, it could be sent directly to the aircraft in flight, and the pilots would receive it as text printed on the plane's tiny thermal printer. Yes, %USERNAME%, there is a specially dedicated printer in the pilot's cockpit for this purpose:
Of course, this is not a one-way communication, but the response capabilities will be somewhat limited by the phrases pre-programmed in the FMC (Flight Management Computer). Usually these are options like ACCEPT or REJECT. Pilots can also request a change of course or flight level from the controller and receive a prompt response, which is very useful in conditions of high radio congestion. It should be mentioned here that ACARS never claimed to completely replace voice radio communication, but only allowed it to be supplemented in some situations.
ACARS can be used to send any text message up to 216 characters long:
Modifications
Many may think: “The system works and works, there is no need to touch it.” But planes do not always fly where there is a corresponding repeater. The range of VHF communication, even in ideal flight conditions at flight level, often does not exceed 300-350 km. But what to do if the plane is over the ocean, where there are no such repeaters? There is only one answer – satellite communication. ACARS traffic often passes through the Inmarsat group and lands at two main providers: ARINC And SITAOf course, such a service costs money, and quite a lot of it.
So by default the transponder tries to use cheaper VHF channels and only if they are unavailable does it switch to satellite communications. In the famous MH370 disappearance incident, ACARS data allowed us to expand the picture of what was happening based on the received messages from the aircraft via Inmarsat satellites (provider SITA).
Pilots often use ACARS, but not as intensively as the aircraft's automatic systems do. The fact is that most of the ACARS traffic is now occupied by service messages about the technical condition of the aircraft as a whole and some units in particular. The system can broadcast data on engine parameters that are sent directly to their manufacturer.
Sometimes it looks like a bunch of numbers, without any explanation. Airlines can activate a subscription to this data to speed up the maintenance process. For example, if a sensor breaks in flight, the airline will know about it before the plane lands. And it can plan a replacement in advance, sending spare parts or a corresponding order to the destination airport.
On some aircraft, ACARS transmits the engine condition trend monitoring system status (ECTM):
#CFBPAGE 00001 MDC REPORT: ENGINE TREND WRITE OPTION: ACARS AUTO FILENAME: AC10309B.ETD TIME: 15:24 DATE: 30Jun2024 MDC APPLICATION PN: 832-6207-420 MDC TABLES PN: 810-0042-115If an airline subscribes to ETCM, the monitoring system will periodically record flight parameters and corresponding engine data. The analysis helps to identify negative trends before obvious failures occur. This allows for early planning of maintenance and spare parts orders.
It's not just engines that can provide extended information. Even a burnt-out light bulb in a toilet will be included in the automatic ACARS report and sent to the airwaves, not to mention more serious incidents:
ACARS mode: 2 Aircraft reg: N709BR [Canadair CRJ200]
Message label: 1M [Estimated Time of Arrival report] Block id: 8 Msg no: M36A
Flight id: OO6144 [] [Skywest Airlines/Delta Connection/United Express]
Message content:-
00015209700 RETURNING TO OMA.,OVB COOL FAIL.,NO PRESSURIZATION.,
----------------------------------------------------------[ 04/08/2010 11:21 ]-Aircraft are becoming more complex every year, with more and more systems being installed that can generate service data and load the ACARS messaging system. Aircraft are now generating 75% more data than before, and this volume will only increase. This naturally leads to an increase in airlines' costs for messaging, and this is what has become the catalyst for the upcoming changes to the system.
What next?
Until 2018, the transition to more modern communication systems was a difficult scenario to implement. Everything changed with the approach of Elon Musk, who decided to make satellite Internet broadband and cheap. Currently, there are ~6,100 satellites flying around our planet, allowing data exchange at a speed of about 100 Mbps. Compared to the tiny bandwidth of other systems, where the available channel width is less than 1 Mbps and for which you have to pay $30 per minute, communication via Starlink looks like a miracle of technical progress.
The fact voiced above made many airlines think that it is possible to save a lot if ACARS messages are sent not via radio channels, but via satellite communication in the form of IP packets. Now the process of gradual implementation of AoIP technology (ACARS over IP) has already begun.
Standard ACARS 618 messages are encapsulated in IP packets and transmitted via the Internet to the service provider, and then to the ATC and airlines. To ensure security, a VPN is used in most cases. This simultaneously relieves traditional communication channels and copes with the ever-increasing volume of data generated by aircraft.
It is clear that such a modification will require capital expenditures for the acquisition of equipment compatible with old systems and for concluding new contracts with communication service providers. So, apparently, the same situation will arise as with IPv6. IPv6 already accounts for 41% of global traffic (data for 2024), but IPv4 still continues to maintain its leading position and is not going to give up. It seems that AoIP will be implemented by a limited number of airlines, but HF/VHF radio channels will remain the main and critical transport for ACARS messages for now.
What do you think about the implementation of AoIP? We are waiting for you in the comments.
