The FAA is taking a revised step-by-step approach to implementing datalink communications in US airspace


The US Federal Aviation Administration's collaborative effort with users and industry to develop controller-to-pilot datalink is on track for critical initial trials at the Miami Air Route Traffic Control Center next June.

But the benefits of using controller-pilot datalink communications (CPDLC) to supplement voice communications with digital data messages will not be realised until its use spreads across the USA. Much remains to be accomplished before frequencies are unclogged, allowing speedier clearances and greater controller efficiency.

The US aviation agency's CPDLC development remains faithful to the aeronautical telecommunications network (ATN) standard defined by the International Civil Aviation Organisation (ICAO), which would allow aircraft to transfer seamlessly from one part of the world to another. But the FAA has changed its CPDLC evolution to roadmap on how to get from point A to point B.

As the introduction of datalink will have "profound implications" for controllers and pilots, and will "fundamentally change the way they communicate", the US Department of Transportation's Office of the Inspector General (DoT IG) audited the CPDLC programme in 1999. It determined that "implementing CPDLC is a complex, long-term development effort that will require the FAA and industry to address a wide range of issues." The report concluded: "Given the complexity of the effort and uncertainty regarding controller and pilot issues, revisions to cost and schedule are likely. But the challenges and risks...are not insurmountable and can be addressed through careful planning and effective risk mitigation."

The FAA's goal has always been to implement CPDLC in US en route airspace by 2005. To mitigate risk, the FAA had planned to field CPDLC in four phases or "builds" - I,IA, II and III - with more messages and capabilities supported for each build. But, the FAA now says only Build I and IA are firmly planned. Build II is undefined and Build III "does not exist".

CPDLC - known as e-mail in the sky - is integral to the FAA's over-arching Free Flight airspace system modernisation. CPDLC is part of Free Flight Phase 2, which will run from 2003 to 2005 and involve the wider deployment of air traffic management tools being fielded during Phase 1, as well as develop new capabilities such as CPDLC. The FAA's original datalink plans called for CPDLC implementation in US airspace in 1996, but disagreement between the FAA and industry has delayed the programme.

Today, VHF analogue two-way radio is the only way controllers and pilots communicate during flight in domestic airspace, passing routine flight information, warnings and weather updates. While this system has served aviation well, problems continue with missed clearances and communications errors and misunderstandings between controllers and pilots. When a controller must share a single radio channel with up to 25 aircraft, competition for the channel can also lead to substantial delays.

The limitations of the communication network will increasingly constrain the controller's ability to optimise air traffic flow as volume grows. These constraints will contribute to the "aviation gridlock" that is projected to occur in the USA in 2005 unless action is taken.

Aircraft limits

"Controllers can only talk with so many aircraft in a given period of time using today's system. This limits the number of aircraft they can manage safely causing delays. These delays translate directly into economic losses for aircraft operators and for the flying public, and have an impact on the nation's economic well being in general," says a high-level FAA official.

"The ability of airlines-to operate on schedule and within budget is dependent upon efficient communications between pilots and controllers. Anything that hinders these communications can cause delays. CPDLC offers a way to reduce delays and have a positive economic impact across the industry," he adds.

As a result, the FAA and the airline industry have long urged the fielding of a second digital channel of communications, which they expect to be faster and more reliable than voice communications. The FAA says simulations show that en route controllers can monitor and control more aircraft in a safer and more efficient manner using digital data messages to augment voice communications. But the DoT IG report asks whether CPDLC will confuse, stress and increase workload for controllers and pilots, the latter being distracted from flight duties such as monitoring the instrument panel.

The FAA and the airline industry turned to CPDLC after abandoning attempts to use the existing airline datalink, the Airborne Communications Addressing and Reporting Systems (ACARS). CPDLC uses VHR Data Link Mode 2 (VDL-2), deemed to provide faster and more reliable data exchange between aircraft and controllers than ACARS. Recognising the difficulty and cost in developing ATN-compliant systems, the FAA entered into a consortium in 1995 with ATN Systems (ATNSI), a privately held corporation owned by 14 US air carriers, to develop CPDLC software. A year ago, the FAA awarded Computer Sciences a contract worth $86 million to implement the next generation digital air-to-ground communication system.


In June 2002, Miami ARTCC will be the first site in the USA to receive CPDLC. A major participant in Build I activities is American Airlines, which will test CPDLC using Boeing 767-300ERs and 737-800s equipped with Rockwell Collins CPDLC avionics. The CPDLC Build IA project is to become operational at Miami in June 2003 and nationally in phases beginning six months later at the country's other 19 ARTCCs. VDL-2 will continue to be used in Build IA, as the avionics will be designed to handle the larger message set. More air carriers are set to take part in Build 1A as CPDLC gains acceptance.

According to Michael Hawthorne, the FAA's CPDLC programme manager, Build I will only offer four initial services: transfer of communication, initial contact, altimeter setting and information free text menu capability. Build IA adds five services - it increases the message set to accommodate assignment of speed, heading and altitude, as well as a route clearance function and a capability to handle pilot-initiated altitude change requests.

The FAA's CPDLC roadmap has changed in recent years, the original plan calling for four steps, but only the first two funded. The third step, Build II, had called for 114 services and continued use of VDL-2. Together with Build III, it would implement CPDLC at tower, terminal and oceanic air traffic control facilities. As envisioned, Build III, which would offer a complete message set, would have used the Next-Generation Air/Ground Communications (NEXCOM) equipment, including VDL-3 technology.

The FAA is moving cautiously, however, focusing on the first two phases before going on to the next. Hawthorne says: "Build II is not defined. We are in the initial stages of investment analysis work. The vision for Build II is to look at a spiral development effort that will eventually provide gate-to-gate capabilities for CPDLC. We will eventually work in the terminal environment with Build II."

Build II will be phased in, but services, domains and timeframe have yet to be determined, says Hawthorne. "It's hard to say when we will complete the analysis since we don't know what we're getting into," he adds, maintaining: "Build III doesn't exist."

Compatibility issues

FAA officials say they are in close contact with Eurocontrol to ensure that both systems are compatible. The FAA is also watching closely the Preliminary Eurocontrol Test of Air/ Ground Data Link (PETAL) programme - Europe's ground-breaking validation of air-ground datalinks in an operational ATC environment. The three-phase project is designed to obtain first-hand, factual data on operational benefits, requirements, human factors, procedures and problems associated with CPDLC.

The DoT IG office says: "The PETAL trials represent an important step in the development of CPDLC, and we believe that the FAA must actively monitor these tests to reduce future costs and prevent duplication of effort. At a minimum, these tests will generate data that will prove useful in deliberations about CPDLC."

Source: Flight International