Future Air Navigation Systems (FANS) — Page 330, Lesson 384

I want to walk you through the Future Air Navigation Systems, or FANS, chapter. We're going to start with the practical side of how the system actually works in the cockpit, then look at what FANS is trying to achieve overall. Let's begin with the maintenance and rerouting messages you see on the page. These are real examples of the data link messages that flow between the aircraft and the ground. Look at the maintenance report first. It starts with a header like "3E03 SNAG 0999/31 KJFK/EGLL .G-BOAC." That's the message type, a defect report number, the origin and destination airports — KJFK is New York JFK, EGLL is London Heathrow — and the aircraft registration, G-BOAC. Then you see the free text section. The message ID is 123456, the info field is the actual text, and there's a confirm flag set to Y, meaning confirmation is required. The routing field shows the route, and there's a CTOT field, which stands for Calculated Take-Off Time, and it's blank here. Now look at the rerouting message. This is a real-world example of flow management. The airline SK 9999 is flying Dublin to Copenhagen, with a scheduled time of departure, STD, of 1000 UTC. The message says it's re-routed due to flow regulations. The new routing is filed in the ATS plan, and there's no restriction on it at present. The message tells the crew they must confirm receipt of the rerouting message to Copenhagen via datalink, using a flow confirm report or another report, or by phone. It gives the new flight time as 0149, that's one hour forty-nine minutes. This is the kind of operational message that flows over the data link system. Now let's get to the core of the chapter. The first thing you need to understand is the logon procedure. To gain access to the data link service, the pilot must perform a manual logon. This requires you to input the four-digit ICAO address for the Air Traffic Services Unit, the ATSU, into the Flight Management System, the FMS. The FMS then sends the logon message to that ATSU. The logon message includes the aircraft address and information on the services the aircraft equipment supports. The ATSU acknowledges the logon message, then sends a connection request message back to the aircraft. The aircraft responds with a connection confirm message, and the process is complete. This whole handshake is known as the ATS facilities notification, abbreviated AFN. Now, when do you perform this initial logon? It's done on first contact with the ATSU on the ground, when entering an area with CPDLC from a non-CPDLC area, and if there has been any interruption to the service — that is, if the link is broken. Once the service is established, the equipment will perform an automatic transfer to subsequent CPDLC-capable ATSUs. So you only do the manual logon once; after that, the system hands you off automatically. One critical piece of terminology here: the message from the aircraft to the ATSU is always referred to as the downlink message, abbreviated DL. The message from the ATSU to the aircraft is always the uplink message, UL. And this is true even if the aircraft is on the ground. So don't think of downlink and uplink in terms of altitude — it's purely about direction of transmission relative to the aircraft. Now let's step back and look at the big picture. What is FANS trying to achieve? The aim of FANS is to provide an integrated air traffic control system in areas where radar is not available. It does this by using GNSS, the Global Navigation Satellite System, to define aircraft position, combined with data link and voice communications through geostationary and lower orbit satellites, providing global coverage to ATC centres. So the key idea is this: in oceanic or remote areas where radar simply doesn't reach, FANS gives ATC continuous information on aircraft positions and enables direct pilot-to-controller communications. When this is in operation, it allows separation distances to be significantly reduced, because controllers have accurate, continuous position data. And here's the safety net: if position accuracy is degraded for any reason, the aircraft still have TCAS, the Traffic Collision Avoidance System, to warn of any potential conflict. Let me tie this together for you. The logon procedure is your gateway into the system — it's how you establish that data link with the ground. The downlink and uplink terminology is how you talk about the messages once the link is up. And the whole purpose of FANS is to give you that integrated ATC capability where radar can't reach, using satellite navigation and communications to keep separation safe, with TCAS as the backup if position accuracy degrades.