The Controller Pilot Data Link Communications (CPDLC) program–a joint government/industry effort–has progressed in fits and starts, following conditions in the aviation industry. Like other Federal Aviation Administration (FAA) development programs, CPDLC felt the impact of the tragedies of Sept. 11, 2001, when airlines focused solely on economic survival. Now that economic conditions are improving, CPDLC has revived and once more is moving forward.
A key element of FAA’s Operational Evolution Plan (OEP–see sidebar, page 25), CPDLC would allow a gradual shift in controller/pilot communications from time- consuming and error-prone voice messages to faster, more accurate, digital data transmissions.
According to the original CPDLC schedule, the initial phase, called Build 1, was to start in June 2002. It was to include flight trials of CPDLC in the airspace controlled by the Miami air route traffic control center (ARTCC). Participating aircraft were to include four American Airlines Boeing 767-300ERs (already CPDLC-equipped for earlier Eurocontrol trials) and about 28 new American Boeing 737s equipped with Rockwell Collins avionics compatible with aeronautical telecommunications network (ATN) digital technology and VHF digital link Mode 2 (VDL-2). (ATN is a protocol specification to which the CPDLC application is being developed.)
Build 1 flight tests were to include four routine message types:
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Initial air traffic control (ATC)/aircraft contact,
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Altimeter setting,
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Communications transfer to the next ATC sector, and
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Text from an approved menu.
Basically, Build 1 was to be an 18-month "shakedown" period, during which any initial problems were to be resolved in this limited deployment service.
Under the pre-9/11 plan, Build 1 was to expand into Build 1A in December 2003, after which CPDLC equipment would be progressively installed at the remaining 19 ARTCCs across the United States. In addition, the earlier Build 1 message set would expand to include speed, heading and altitude assignments, pilot-initiated altitude requests and en-route clearances.
Build 1A was to run for roughly two years. During this time government and industry participants were to assess the performance of all system elements and to develop equipment and procedure standards and specifications for the final phase, called Build 2. By late 2005, Build 2 was to usher in full CPDLC operations throughout the United States, including expanded functions yet to be planned.
Everything was moving ahead in the project prior to Sept. 11. In August 2001, one of the four American Airlines B767s equipped for CPDLC made the first commercial airline flight, from Amsterdam to Paris, using CPDLC in the en-route airspace over Europe, as part of Eurocontrol’s Preliminary Eurocontrol Test of Air/ground data Link (PETAL) program. This flight was the culmination of an extended series of CPDLC tests by the Europeans, who largely pioneered and validated the concept, although there was close coordination of standards and procedures with FAA to ensure international commonality.
But after Sept. 11, the circumstances changed. Airline funds to purchase and install CPDLC equipment were quickly diverted to help stop financial hemorrhaging. A representative of American Airlines (which by October 2001 was losing about $12 million daily) stated at an industry conference that the company’s "top priority is survival." Consequently, FAA initiated a change in plans. It would hold a brief, four-month CPDLC test at Miami during 2002, in place of Build 1, using two FAA aircraft. The project then would be shut down until a delayed startup of Build 1A in 2005.
By mid-December, however, circumstances changed again. While traffic volumes were significantly lower than they had been before the attacks, recovery was quicker than expected. The massive financial losses of September and October also had slowed because of large-scale fleet and airline staff reductions. Although a full recovery was still a long way away, maximum operational efficiency had now replaced survival as the industry’s top priority. As a result, the airlines refocused attention on CPDLC.
At a CPDLC meeting, airline representatives asked FAA to conduct Build 1 as originally planned and to extend it to the rescheduled start date of Build 1A in December 2005, to eliminate gaps in CPDLC operations. The airlines also proposed that the additional messages planned for Build 1A be included in Build 1. Build 2 remains unfunded and in the conceptual stage.
The airline attendees have stated their commitment to CPDLC. American stated that it would equip a minimum of 16 aircraft by September 2002, and Delta Air Lines announced in early April that it will fit out four B767-400ERs for daily use of CPDLC by November 2002. The Delta installations will include Teledyne Controls communications management units and Thales-manufactured ARINC 750 VHF digital radios. FedEx, Continental and Southwest also have said they plan to equip aircraft with VDL-2 avionics and are interested in participation in Build 1. FedEx hopes to participate in the CPDLC program within a year; Continental and Southwest say they would like to take part, as well.
Under the new plan, Build 1 IDU (initial daily use) would now start in September 2002 (instead of June). In preparation for the IDU, ARINC has installed four VDL-2 ground stations with ATN capability in the Miami ARTCC support area and is installing five more in Florida and the Caribbean. (ARINC has a total of 50 new VDL-2 ground stations installed across the United States, each of which can be made ATN-capable with a software upgrade, when CPDLC becomes more widespread.)
A six-month shakedown test began March 1, 2002, in preparation for Build 1’s launch, and initial results are encouraging. In March a Miami ARTCC controller transmitted more than 100 operational data link messages to a CPDLC-equipped Convair 580 testbed aircraft from FAA’s Atlantic City Technical Center. Most messages were exchanged "flawlessly," with one-second, end-to-end response times, says FAA Free Flight program director John Thornton. The exchanges took place between the aircraft and an air traffic controller in a working sector of the ARTCC, rather than in a separate, dedicated test position at the center.
On April 23 matters progressed further, as Miami ARTCC controllers and pilots on an American Airlines B757 test flight, exchanged 70 to 100 digital operational messages using Collins’ CPDLC avionics. The exchanges provided operational service for initial contact, communications transfer, altimeter setting and informational free text. This was the first demonstration from a commercial airline of data link for ATC communications in U.S. airspace, ARINC says. (Conversely, in the Convair trial, CPDLC equipment was installed on a pallet in the back of the FAA aircraft.) Collins expects its communications management unit (CMU-900), VHF-920 data radio, and aircraft personality module (APM-900) to obtain supplemental type certification (STC) on American Airline B757 and 767 aircraft by the end of June 2002.
VDL-2 Vs. VDL-3
While the CPDLC project appears to be well launched, a major question remains as to which VHF digital data transmission technique FAA eventually will adopt for the National Airspace System (NAS). In Build 1 and Build 1A, VDL-2 data transmissions will be used, along with analog voice. FAA has planned to upgrade VHF voice with a VHF digital voice system based on VDL Mode 3 technology, which can transmit digital data messages, such as CPDLC. If this happens, the VDL-2 system would be phased out for CPDLC at some time well after 2010, observers say.
In Europe, however, Eurocontrol mandated that the conventional 25-KHz VHF analog voice channel spacing be subdivided into three 8.33-KHz channels, to alleviate frequency congestion. This also allows more flexibility in assigning dedicated channels for CPDLC and other data applications using VDL-2. As a result, all U.S. airline and corporate aircraft that fly to Europe carry 8.33-KHz-compatible VHF radios, and U.S. airlines have requested that FAA introduce 8.33-KHz spacing for VHF voice.
FAA, meanwhile, has opted to employ VDL Mode 3 in its NEXt generation COMmunications (NEXCOM) program. NEXCOM would allow four separate radio transmissions to occur simultaneously, without interfering with each other, over a single 25-KHz channel.
There remains a disagreement in the United States: U.S. airlines generally favor an 8.33-KHz/VDL-2 solution and FAA prefers a VDL Mode 3 solution. A high-level committee–called NARC, for NEXCOM Aviation Rulemaking Committee–recently recommended that VDL-2 be employed for CPDLC Build 1 and 1A and that a demonstration/validation of VDL Mode 3 be conducted to attain a better estimate of the digital link’s cost.
Some observers forecast a future European/U.S. battle over which mode of VDL should become a worldwide standard. However, production CPDLC avionics probably will be mulimode units, able to handle both techniques and automatically switch to whichever VDL mode is used in the incoming messages. On the other hand, while FAA accelerates NEXCOM development, it seems doubtful whether certified equipment will be available before 2006 or 2007, by which time the VDL-2/8.33-KHz combination may have a substantial market share.
How much would a VDL avionics installation in an aircraft cost? That’s still an unknown, as the equipment is not yet in full production. But estimates run from $80,000 to $150,000, depending on the installation’s complexity. A fairly quick payback is expected, as this avionics also will be used to upgrade the airline dispatch communications to a high-speed digital communications system with its own cost savings and increased capability.
Today, an airline’s departments–engineering, scheduling, flight planning, catering, etc.–use a data link to maintain virtually continuous contact with each of their aircraft. An engineering department, for example, can directly monitor an aircraft’s engines and other systems via the data link and then advise technicians at the aircraft’s destination about any additional required maintenance.
Airlines now use the tried-and-true aircraft communications addressing and reporting system (ACARS) for operational communications. While reliable, it is becoming congested. VDL-2 boasts more than 10 times the data rate of ACARS.
CPDLC would be more efficient than voice communications and would ease the congestion in the VHF aviation voice channels, many of which are operating at almost full capacity in Europe and North America. The switch from voice to data transmission of routine messages, such as altimeter settings or hand-
overs to the next ATC sector, could offload voice channels by between 40 and 70 percent. Since communications congestion contributes to ATC delays, the move to CPDLC would provide major operational benefits.
OEP: The Bigger Picture
June 2001, the Federal Aviation Administration (FAA) announced the Operational Evolution Plan (OEP), in which the Controller Pilot Data Link Communications (CPDLC) effort plays a major role. OEP is a broad-ranging initiative that, through the progressive introduction of advanced air traffic control (ATC) and aircraft systems, would establish a more efficient, higher-capacity, operating environment over the next 10 years. The result of extensive consultation with users, air traffic controllers, industry groups and manufacturers, the plan calls for FAA to invest $11.5 billion and industry, a comparable figure. OEP aims to increase system capacity by 30 percent by 2011, along with widespread implementation of Free Flight.