First, the context — Page 330, Lesson 386

I want to walk you through the Future Air Navigation Systems — FANS — and specifically the FANS A system, because this is where the book gets into the real detail. First, the context. We've just been talking about how, over oceanic and remote airspace, the separation between aircraft can be significantly reduced, and even if position accuracy is degraded for any reason, the aircraft still have TCAS — that's the Traffic Alert and Collision Avoidance System — to warn of any potential conflict. So the reduced separation is made safe by having TCAS as a backstop. Now, to give the required global coverage, there are two systems established: FANS A and FANS B. We're going to focus on FANS A. FANS A provides a communications, navigation and surveillance system — that's the CNS system — and an automatic dependent surveillance system, which is ADS. Let me unpack that. Communications utilize current frequency allocations in HF and VHF, and GNSS — the Global Navigation Satellite System — provides the navigation input for the surveillance. So the surveillance picture is built on navigation data from GNSS. Currently, FANS A is used by some AOCs — that's Airline Operations Centres — to monitor the progress of aircraft at all stages of flight. So, for example, the aircraft system will automatically inform the AOC of gate departure, take-off, landing and gate arrival. In-flight progress and the operation of on-board systems can also be monitored, and where necessary, messages can be passed to alert or assist the crew when potential problems are detected. FANS A is used over oceanic and remote airspace, and it's transmitted over the ACARS network, operated by ARINC. ACARS is the Aircraft Communications Addressing and Reporting System, and ARINC is the organisation that operates that network. Now let's look at the components of the FANS A system. There are three: AFN, ADS, and CPDLC. AFN stands for ATS Facility Notification. ATS is Air Traffic Services. AFN is a contact message initiated by aircrew, or by an automatic trigger within the aircraft. Here's the key operational point: if acknowledgement has not been received within a pre-set time, or there is an erroneous reply, then an error message must be displayed to the aircrew. So the crew gets a clear indication that the notification didn't go through properly. Next is ADS — Automatic Dependent Surveillance. This is a controller set-up contract, called ADS-C, with an aircraft's FMS — the Flight Management System — without any pilot input, using automated and customized reports. The flight crew have no workload associated with this set-up. So the controller establishes the contract directly with the FMS, and the crew doesn't have to do anything. Now, the types of contract available are: Periodic, On Demand, On Event, and Emergency Mode. Let me explain each. Periodic means reports are sent at regular intervals. On Demand means a report is generated when the controller requests it. On Event means a report is triggered by a specific event, like a change in altitude or a deviation from the planned route. And Emergency Mode is the critical one — only the flight crew can declare and cancel ADS-C emergency reporting. Note this carefully: the flight crew can initiate the emergency mode, but the aircraft cannot initiate a contract. So the aircraft can't start an ADS-C contract on its own — that's always controller-initiated, except the emergency mode, which only the crew can declare and cancel. Finally, CPDLC — that's Controller-Pilot Data Link Communications. CPDLC permits data link messages to be generated for all stages of flight. The messages have a fixed format defined within the FMS and ATSU computers — ATSU is the Air Traffic Services Unit — and they're activated by the ATC controller or pilot, either as an instruction or as a response to a request. The messages are annotated according to whether a response is required or not. Let me give you the example from the book. If the pilot is instructed to report levelling at FL310 — that's flight level 310, 31,000 feet — then the message will remain open until the aircraft reaches FL310, when the FMS will send the response. The confirmation that the aircraft is at FL310 does not require a response from the controller, so the message will automatically close once received at the ground station. So you see the logic: an open message stays open until the condition is met, the FMS sends the response automatically, and if no further response is needed, the message closes itself at the ground station. That figure shows the elements of a CPDLC system — the ground earth station, the ATSU, the effective control, and the two links: the SATCOM link and the VHF COM link. So CPDLC can be carried over satellite communications or over VHF communications. That's the complete FANS A picture: AFN for the initial notification, ADS-C for automatic surveillance contracts, and CPDLC for the data link messaging between controller and pilot.