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Summer 2011

Author: Sergio Martins
This article appears in Issue 2: the Summer 2011 edition of the Aircraft IT Operations eJournal. For your own free subscription to the eJournal - click on 'SUBSCRIBE FOR FREE' for full details.

Getting the right data transmission for the job

Sergio Martins, President, LinkSMART outlines a 
guideline for an efficient Data Link service experience

Airlines have long explored the potential for aircraft and flight crews exchanging data in real time with ground based staff and systems (Data Link) to enhance operational efficiency and safety. ARINC’s development of ACARS (aircraft communications addressing and reporting system) AEEC 618 (airlines electronic engineering committee) protocol by the end of the 1970s, followed by SITA’s (Systemes Internationale de Telecommunications Aeronautique) launch of its VHF AIRCOM service, based on the same communications protocol, served to position ACARS as the industry’s standard for real time data exchange between aircraft and flight crews, and operators’ ground based staff and systems.

This article aims to provide readers with an historic overview of how Data Link technology has evolved over the last four decades, within the wider context of the air transport industry’s activities, while also providing aircraft operators with a set of technical, commercial and strategic recommendations, developed to help airlines experience full efficiency in their Data Link service programs now and into the future.

No matter how much effort airlines dedicate to the planning stages of flight operations, a new reality prevails after an aircraft has left the gate - changing meteorological conditions over the route, varying performance of aircraft systems, unpredictable air traffic control restrictions and/or opportunities can all affect the flight. Operators can only achieve an optimum level of actual operation against the original flight plan if they have the means to continuously monitor flight operations, and enable their ground resources to interact with aircraft systems and flight crews as required; in real time and in a way that facilitates the efficient handling of unexpected events. That’s the real strength of Data Link, which no other existing technology can beat.

The early days

In the early days of Data Link services, transmission of simple messages such as the traditional ‘OOOI’ (movement reports identified by an internationally adopted acronym for ‘Out of the Gate / Take-Off / On the Ground / In the Gate’ events) and meteorological information, paved the way for the improvement of a wide range of airlines’ internal processes, resulting from the unique combination of real time and error free characteristics of the ACARS service.

Just focusing on typical airline activities such as operations control, aircraft assignment, crew rotation, fuel management and passenger services, it’s easy to understand how important it was for ground personnel and processes to be able to be promptly, automatically and accurately notified, each time an aircraft departed or landed anywhere; and that was made possible with the introduction of ACARS protocol.

The evolution

In time, the development of additional applications for air-ground real time data exchange was the natural result of a joint design and implementation effort by airlines, service providers and avionics manufacturers. Below is a brief summary of some flight safety and efficiency related enhancements that airlines enjoyed, as Data Link technology evolved into more sophisticated functions:
  • Real Time weather monitoring

Flight Briefings are prepared by flight dispatchers a couple of hours before departure. That means, especially with long haul operations, weather conditions may significantly change between flight briefing preparation and the actual flight. Fast and easy to use mechanisms have been developed for flight crews to obtain updated weather information via Data Link as they approached critical flight phases – approach, landing, re-clearance points, ETOPS (extended twin engine operations) entry point, etc.

  • Flight Plan Reanalysis

Also as a result of the time lapse between preparation of the flight plan and the actual flight, a number of conditions (final take-off weight, en-route winds, aircraft systems’ performance, etc.) may necessitate operational changes with a huge potential impact on fuel requirements for flight completion. Flight plan systems available in the industry were rapidly upgraded to allow flight plan reanalysis, while Data Link systems were adjusted to accommodate real time ground to air transmission of re-calculated flight plans, which could be accepted by flight crews and automatically loaded into flight management systems. 

  • Remote Troubleshooting

Flight events associated with aircraft airworthiness can occur in flight which could impact on flight crews’ ability to efficiently handle the aircraft either in real time or after landing. The sooner airlines’ ground personnel have access to the details of such events and their impact on aircraft airworthiness, the sooner corrective actions can be implemented through flight crew coordination with ground staff.

Obviously, the ability to have ground staff notified of such events in order to be able to agree with the flight crew on what to do next (either in-flight or on the ground), greatly contributes to airlines’ ability to improve their level of safety (getting the right things done at the right time) and efficiency (corrective actions anticipated to avoid unnecessary delays).

The intrinsic value of air to ground real time data communications 

As far as the AOC (airline operational communication) Data Link is concerned (airlines’ use of Data Link for operational purposes, which has been further improved with the introduction of HF and Satellite based services) it is important to highlight that service providers are required to either deploy (via VHF/HF ground stations) or make use of (via satellite services) a complex set of network components, in order to provide airline users with a consistent and reliable end to end service. As a consequence, the cost of Data Link service is considerably higher than traditional ground to ground communications, which also explains why Data Link costs are only justifiable for the transfer of perishable data required to support time critical decisions by either flight crews or ground personnel.

To illustrate this point, I’d like to consider two different scenarios and identify the adequacy of real time air to ground communications in each case.

  • In-flight delays

Flights are quite often subject to in-flight delays or diversions as a result of air traffic control restrictions, meteorological conditions, and the like. Let’s examine just a few activities which might potentially be affected as a result of such events:

  • Subsequent aircraft operations may be commensurately delayed or there might be alternative ways for the airline to re-assign aircraft in such a way as to avoid or minimize further delay.
  • Passengers might potentially miss their connecting flights and need booking onto other flights.
  • Crew members may face schedule disruptions, making them unavailable for flights to which they had previously been assigned.
  • Slots may be missed at busy airports, causing further restrictions to the airline’s scheduled operations.
  • Fuel requirements for the continuation of the flight may substantially change, in which case flight crew will need a flight plan reanalysis for fuel requirement purposes.        

Solution providers have developed a number of applications for the efficient handling of the above situations, based upon quite sophisticated algorithms. The potential for airlines to deal with such events have been greatly improved, but that potential for improvement may only be realized when accurate information is made available in real time and on which the application can base recalculations, thus allowing airline staff to proactively deal with the prevailing event. The above examples are typical situations where Data Link proves its unique value by enabling airlines to apply the functionalities provided by all such operation optimizer tools. 

  • Aircraft Systems Diagnosis/Prognosis

Airlines are encouraged to continuously monitor the performance of their aircraft systems in order to both determine possible causes of systems malfunction and proactively to detect existing trends for systems’ failures. Those monitoring activities are typically conducted on a regular basis, using the massive aircraft data easily available as part of an airline’s ground based engineering activities. In these circumstances, real time transmission of such data, within the scope of the process they are meant to feed, adds absolutely no value for the airline, as no real time data processing will be performed. In such cases, the use of Data Link would be unnecessarily expensive and inefficient.   


It is, then, easy to understand why, as the daily handling of modern aircraft evolved into a data centric science, the airline community virtually adopted ACARS as a default method for accessing an increasing volume of flight data. Data Link programs became, over time, considerably more expensive and increasingly inefficient, as a result of this lack of focus in the adoption of ACARS protocol as a ‘de facto’ means of accessing aircraft data.

Real time access to in-flight data is (and will always be) a unique asset for airlines, in the areas of:

a)   Operational Efficiency

Real time access to flight status (via movement messages, in-flight position reports, delay/diversion reports, etc.) has been proven critical for the handling of airlines’ ground based processes – aircraft rotation, crew assignment, slot management, passenger handling, fuel control and all other activities which may benefit from the availability of real time, error free information associated with flight status.

b)   Flight Safety

Data Link’s strength in the area of flight safety is simply immeasurable. Aircraft are flying machines (and subject to malfunctioning) operating within a dynamic (sometimes, almost unpredictable) environment. As ever more sophisticated aircraft are built, flight operations are conducted under ever more strict conditions, thus requiring operators to improve their ability to monitor operations and provide flight crews with state-of-the-art real time support, with special emphasis on particularly critical operations, such as ETOPS.

Air Traffic Services - A new piece within the Data Link puzzle

By the early 1980s, the international aeronautical community was becoming concerned with and confounded about how to address the increasingly evident reality that airspace was not unlimited. Squeezed between low altitudes, where high fuel consumption makes aircraft operation economically prohibitive and operational ceilings (the maximum aircraft altitude, given by aerodynamic variables), air transport activity, especially in higher traffic density areas such as Europe and the United States and more open airspaces such as oceanic routes, was expected to experience serious limitations, given the capabilities of existing technologies and the air traffic control model. All this was aggravated by a gradual deregulation of air transport activity, which would make it less tolerant to airlines’ business inefficiency.

In response to this growing concern, in 1983, ICAO (International Civil Aviation Organization) created the FANS Committee (Future Air Navigation System) working group, charged with developing new concepts and technologies to support air transport activity, based on which a new air traffic management model could be devised to cope with anticipated future traffic demands.

That was the basis of what would be defined in 1993 as the CNS/ATM (Communications, Navigation, Surveillance / Air Traffic Management) a new air traffic control environment, specifically designed to accommodate future traffic volume.

This new environment assumed the use of Data Link as a more efficient alternative than voice for routine communication between pilots and air traffic controllers, and the adoption of GNSS (Global Navigation Satellite Systems) satellite positioning, the best known of which was GPS (Global Positioning System), as the one system used by aircraft to determine their position within the airspace. 

Two new Air Traffic Control procedures were devised, as the basis for the new airspace management model:

CPDLC (Controller-Pilot Data Link Communication)

Clearances issued by Air Traffic Controllers by means of data communications (Data Link)

ADS (Automatic Dependent Surveillance)

Automated transmission (via Data Link) of aircraft position reports (obtained through GNSS) to air traffic control centers.


Additionally, the CNS/ATM concept assumed the development of a huge air-ground, ground-ground, high performance, bit oriented telecommunications network, the ATN (aeronautical telecommunications network), as the communications platform amongst the various components of the new system – aircraft, air traffic control centers, airlines, etc. – which would replace the existing air-ground (ACARS) and ground-ground Aeronautical Fixed Telecommunication Network (AFTN) communications platform.

In parallel to that, and amongst a number of different concepts proposed as replacements for the traditional character oriented ACARS communications, the very high frequency digital link (VDL) mode 2 (VDLM2) has been adopted by ICAO as the recommended air-ground communication medium to support ADS procedures over continental areas. VDLM2 was in fact an evolution of VHF communications, whereby data could be digitized and transmitted over the ATN network. Nevertheless, due the extreme complexity of the ATN network concept, aircraft manufacturers (Boeing and Airbus), as well as air traffic control workstation developers were able to fully support CPDLC and ADS bit oriented applications well before the telecommunications industry was able to turn the ATN network concept into reality.

This has now been made possible thanks to the development by Boeing and Airbus of a suite of airborne equipment, known as FANS-1/A (‘1’ for Boeing and ‘A’ for Airbus). Those systems enabled the use of the existing character oriented ACARS network for the transport of bit oriented messages, through the use of an encapsulation algorithm.

The main consequence of this for the international air transport industry was that airlines were able to acquire FANS-1/A aircraft supporting CPDLC and ADS procedures a number of years prior to the ATN network’s actual development, which in turn justified investments in VDLM2 avionics.

Nowadays, while ATN/VDLM2 is the target setup for the Link2000+ program by EUROCONTROL over the European Airspace, an accommodation scheme is in place to allow aircraft equipped with FANS-1/A (and the evolution: FANS-2/B) operating oceanic routes into the Link2000+ environment. At the same time, the industry is working hard to establish a single standard that is able to support ATS data link communications in both continental and oceanic sectors.

In a nutshell, we could say that airlines should aim for full compliance with ATS Data Link mandates but with an eye on their individual requirements and possibilities, which are dependent on a number of factors such as existing onboard systems, geographic area of operation and long term fleet renewal plans. The combination of such elements may lead airlines to go towards full ATS Data Link compliance either via an ATN approach for fleets operating strictly over continental airspaces fully served by VDLM2 coverage or via a FANS approach for fleets operating over oceanic airspaces, where satellite communications will be of huge value.

A critical need for Data Link Air efficiency

As the use of Data Link for the use by air traffic services materializes, it needs to be understood that ATS Data Link applications are, by nature, traffic intensive. Therefore, it is imperative that airlines review their practices associated with the use of Data Link for operational AOC (air operator certificate) purposes, in order to avoid an expensive and unnecessary boom in their Data Link traffic, which would, at the same time, raise their service costs to unbearable levels and significantly undermine the operational efficiency of their Data Link experience.

As previously highlighted, the airline community has historically tended to adopt ACARS as a default means to access increasing volumes of aircraft data, although more suitable ground-ground wireless communication technologies have been around for a while (WiFi, 3G and so forth). That led several airlines worldwide to incur unnecessary costs, receive meaningless media advice, and undergo intermittent switches between service providers; plus a number of other events which caused their Data Link programs to become unnecessarily expensive and inefficient.

ACARS protocol (the basis of Data Link technology) was designed to enable air-ground real time exchange of low volumes of time critical data, meaning information with potential impact on immediate decision making processes, either when airborne or on the ground. However, at the frenetic pace of data processing technology development, the efficient handling of a modern aircraft has become a truly data centric science and the air transport industry surfed the wave by developing a wide range of data centric applications. ACARS protocol was not meant for that and Data Link’s strength has always been associated with real time transfer of low volume, perishable data to support decision making processes, now.

Air-ground, real time transmission of error free data will always be an irreplaceable asset for airlines committed to efficient, safe and cost effective operations; but those same air-ground communications will be always more expensive than a wide range of available ground-ground communication technologies. As well as commitment, airlines seeking to make wise decisions and consistent investments within the scope of their long term plans will need knowledge and expertise about what level of data transmission is consistent with each job for which the data is used.

The time has come for airlines to consistently adopt a combination of the available means of data exchange between aircraft and ground systems, based upon their specific requirements with regards to operational efficiency, cost effectiveness and industry compliance.

A recommended roadmap

Decades of experience within the international Data Link scenario have lead us to develop a consistent and fully integrated long term roadmap for airlines wishing to fully realize the potential of their Data Link programs. Such a roadmap should be driven by two main objectives:

  • To achieve full AOC Data Link cost effectiveness;
  • To comply with relevant ATS Data Link mandates, based upon the airline’s individual requirements and possibilities.

In order to address the above requirements as part of a fully consistent, integrated and long term compliant approach, we suggest a combination of higher management decisions and staff training initiatives, both targeting the following main elements of Data Link efficiency.

  • Data generation ownership. As aircraft should not be treated as living machines with the authority to determine which data should be transferred and when, it is the airline’s responsibility to configure their aircraft in accordance with their individual requirements of real time access to aircraft data.
  • Full service coverage, within the terms of operational procedures: intermittent service means no service at all.
  • Message processing proficiency, as an aircraft generates raw data which need to be converted into meaningful information for end user consumption.
  • End user engagement, which may only be achieved by means of staff training. A with all technologies, Data Link’s true value may only be materialized at end user level.
  • Compliance with relevant ATS Data Link mandates, as an airline’s profitability may be highly impacted by its ability to efficiently approach regulatory requirements whilst remaining fully consistent with its individual operational needs.
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