CPDLC: APPLICATION AND FUTURE TECH LOG In part two of our series, we explore how CPDLC works for pilots By Captain Jeremy Feldman, Log Board member I t seems that use of CPDLC (controller pilot data link communications) by Air Navigation Service Providers (ANSPs) has increased over the past few months, and the requirement for pilots who are CPDLC equipped to log on is growing. We know that some members will be very knowledgeable on the use of CPDLC and some will not know much at all. So, what exactly can CPDLC do for us as pilots, and how does it work? There are currently six data link services available, although not all are implemented: n Data Link Initiation Capability (DLIC) (see figure 1, below right). This allows an aircraft to establish initial communications with an Air Traffic Service Unit (ATSU), before any further use of data link like a virtual handshake between the aircraft and ground station. Image courtesy of The Eurocontrol Maastricht UAC First 40 Years of Operations nATC Clearances Service (ACL) ACL provides the service that pilots primarily use to communicate with ATC (figure 2, opposite page). It is the operational driver that can allow ATC to issue clearances, notifications and instructions. Likewise, pilots can send reports and acknowledgements, and make requests. Typical messages include, but are not Image showing a frequency limited to: change request, made over data link, on the A380 oClimb/descent clearances oNavigation, turns and headings clearances oSSR instructions oReplies both from aircraft and ATSU to each other oATC Communications Management Service (ACM) The frequent pilot users of CPDLC will probably have been instructed to change frequency and contact the next ATSU. This is a partially automated service and is provided by ACM. nATC Microphone Check Service (AMC) This allows controllers to issue a request to all logged on aircraft simultaneously, to ask pilots to check for a stuck microphone that may be blocking a voice channel. nDeparture Clearance (DCL) Used on the ground before start up, DCL allows pilots to request departure clearance from the ATSU. It uses automation to help the controllers issue the respective clearance. nDownstream Clearance Service (DSC) Not that common at present, it allows flight crews to contact another ATS unit, not yet providing a control service, and request a clearance. An example would be gaining prior clearance to enter North Atlantic airspace and other oceanic clearances. The DLS-IR (Data Link Services Implementing Rule) has only mandated the first four services to be operational, although Departure Clearance and Downstream Clearance Service will probably be authorised in the future. Aircraft A Cl ea ran ce W ilc o 3 ATSU 1 4 ATSU 2 2 ic km tuc ks ec Ch e ns po es nr go st Lo ue eq nr go Lo Figure 1: Data link initiation capability. Image courtesy of ATC Data Link Operational Guidance insupport of DLS Regulation No 29/2009 2 3 Figure 2: ATC clearances Image courtesy of ATC Data Link Operational Guidance in support of DLS Regulation No 29/2009 Image courtesy of ATC Data Link Operational Guidance in support of DLS Regulation No 29/2009 Ch ec ks tuc km ic 2 1 Aircraft B 1 Sector Advantages and disadvantages The benefits seem numerous; a lower number of voice communications can make for a quieter working environment. As clearances are digital, there is perhaps also less scope for misunderstandings involving clearances and frequency changes. So this, apparently, is all great. Bring on the automation, reduce pilot and controller workload, increase respective situational awareness and all is good, right? Well, maybe but as with all forms of automation, the limitations and associated risks of the systems must be thoroughly understood. How many of us have heard that familiar ping and the CPDLC disconnected message, followed by the system reconnecting and then disconnecting? This is not just an annoyance for pilots, but also for the corresponding ATSUs. Analysis by the European Aviation Safety Agency (EASA) determined that this problem was, in part, caused by single radio frequency interference. EASA has since mandated a multifrequency standard to alleviate this problem, and its one of the reasons the fullimplementation date of the Data Link Services Implementing Rule was pushed back to February 2018. Nonetheless, the service does seem to randomly disconnect, and the incessant alert ping over time may reduce the pilots sensitivity to it. The problem, of course, comes when the ping is driven by an actual aircraft fault requiring attention and the pilots assume it is just that annoying CPDLC again. One of the more noticeable changes pilots may observe is that frequencies become quieter as more aircraft are logged on, so fewer voice communications can make for a quieter working environment. While this may make for a more peaceful ride, the lack of information about other aircraft on frequency, their altitude and flight path may remove a key element of situational awareness. Although this threat is perhaps more prevalent in the terminal environment, it is still worth bearing in mind. It is also worthwhile to note that the ATC operator issuing the CPDLC clearance may not always be the same voice controller. In the scenario where a voice clearance is issued at the same time as conflicting CPDLC clearance is issued, this may lead to confusion at best, and a potential loss of separation or of communications at worst. We work in a rapidly changing, dynamic environment; consequently, automated systems are trained to issue CPDLC clearances only when it is not time critical and the controlled aircraft is above FL285. When a CPDLC clearance is issued, the pilots have a certain amount of time in which to acknowledge and accept the clearance. Within Maastricht airspace, this is currently 40 seconds for ATN-B1 and 120 seconds for FANS-2. So CPDLC clearances are issued with this time delay in mind and the threat of time-critical clearance conflicts should be lower. As the controller workload will also be lower, the risk of a conflicting CPDLC and voice clearance should not beso prevalent. Nonetheless, if in doubt, revert to voice. Another scenario exists where the automated system issues a frequency change to the pilots before the automated system is ready to release the aircraft to the next sector. In this case, the aircraft pilot may have changed frequency before the controller has verbally corrected the mistake to the pilot. Although this is unlikely, there is still a risk of this occurring. Some clearances can contain more than one instruction. For example: Turn left heading 350, climb FL 310. While this digital message removes any ambiguity and reduces the risk of a level bust, the options available to the pilot are to accept the clearance in whole, or reject it in whole. If the pilots are happy to accept the heading but not the climb, the clearance message will have to be rejected and this can lead to higher overall workload. That being said, the overall time saved for automated systems is evidently greater than the occasional aircraft that rejects the whole clearance. When using voice to voice, the opening to any communication is usually to address the receiving call sign, followed by the message. This is the mental trigger that gets our attention for the rest of the message. Clearly, there is no mental trigger for datalink and, as such, there is the risk thatthe automated system could send the clearance tothe wrong aircraft, by way of pushing thewrong button. Because the pilots do not hear their call sign, we can only assume the clearance is designated for us when itappears. The automated systems can always follow this up with a Disregard CPDLC Message but, still, there is a slight risk of this happening. Plans for future development At present, ATN-B1 and Link 2000+/ FANS-2 is the protocol mandated by the European Commission and is being adoptedby the FAA. Meanwhile, the skies are only getting busier and demand for greater efficiency is ever increasing. Plansare afoot for the next generation ATN-B2 and even ATN-B3 which aimsto roll out beyond 2028. What will this do for pilots, and how will it affect our working day? A few applications are being developed. Perhaps the most significant are Data Link Taxi (D-TAXI), Oceanic Clearance Delivery (OCL) and 4-Dimensional Trajectory Data Link (4DTRAD), which will enable aircraft to be cleared with greater point-to-point direct tracks, reducing flight time and improving efficiency (see image, above). Ever increasing demand for air space being used in a more efficient manner can only drive forward the use of automation and, so, the necessity for data link communications. The rise of the machines and automation is inevitable, and we should be open to adopting it. We are all too well aware of the effects of reliance on automation and technology. Aslong as pilots are aware of the technological and human limitations of each system and remember who is in command then data link and CPDLC should help to make pilots lives, and those of our ATC colleagues, simpler. Thats something of which we can all be a FAN. Image courtesy of The SESAR Concept and i4D David Bowen Head of ATM Operations and Systems, SESAR Joint Undertaking