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

I want to walk you through the Future Air Navigation Systems — FANS — chapter, and we're starting with the practical, day-to-day side of it: the messages and the logon procedure. This is the part you'll actually see in the cockpit, so let's get the terminology straight first. We have two message types here. The message from the aircraft to the ATSU — the Air Traffic Services Unit — is always called the downlink, or DL message. The message from the ATSU to the aircraft is always called the uplink, or UL message. And here's the subtle point: that naming holds even if the aircraft is on the ground. It's not about altitude, it's about direction — downlink is aircraft-to-ground, uplink is ground-to-aircraft, always. Now let's look at the actual message formats you'd see. First, a Maintenance Report. The example shows a snag — that's the "SNAG" line — with a reference like 0999/31, the route KJFK to EGLL, and the aircraft registration G-BOAC. Then there's a fault code, FAL 35310102, and a free-text entry. Look at the oxygen example: "310L OXYGEN BTL AT STN6 HAS BEEN USED FOR A PAX IN NEED." That's a maintenance snag written in free text — a 310-litre oxygen bottle at station 6 was used for a passenger in need. So the maintenance report is how the crew logs a defect or an event for the engineers. Next, the Rerouting Message. The example is SK 9999, Dublin to Copenhagen, STD 1000 UTC — that's the scheduled time of departure. The aircraft has been re-routed due to flow regulations. A new routing has been filed, EIDW to EKCH via SAS852, and the ATS plan has been filed accordingly. There's no restriction on this routing at present. Then you see the instruction: the crew must confirm receipt of the rerouting message to Copenhagen — CPH — via datalink, using a flow confirm message or another report. There's a phone number and a SITA address as backup. And note the new flight time: 0149 — one hour forty-nine minutes. That's the rerouting message, sent by the central flow management unit. Now the heart of this chapter — the Logon procedure. To gain access to the data link service, the pilot must perform a manual logon. Here's the sequence. You input the four-digit ICAO address for the ATSU into the FMS — the Flight Management System. The FMS then sends the logon message to the ATSU. That 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 — the AFN. When do you do this manual logon? Three situations. First, on first contact with the ATSU on the ground. Second, when entering an area with CPDLC from a non-CPDLC area — CPDLC is Controller-Pilot Data Link Communications, the text-messaging system between controller and pilot. And third, if there's 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 the manual logon is only the initial contact; after that, handoffs are automatic. Now let's step back and look at the big picture — what FANS actually is. The aim of FANS is to provide an integrated air traffic control system in areas where radar is not available. How? 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 instead of radar painting the aircraft, the aircraft tells ATC where it is via satellite, and they talk over the same satellite link. This gives ATC continuous information on aircraft positions and direct pilot-to-controller communications. And here's the payoff: when in operation, this allows separation distances to be significantly reduced. But there's a safety net built in — if position accuracy is degraded for any reason, the aircraft still have TCAS — the Traffic Collision Avoidance System — to warn of any potential conflict. So FANS shrinks the separation, and TCAS is the backstop if the position data goes bad. Let me show you the message flow so you can see how the logon handshake fits together. So to tie it together: the pilot manually logs on with the four-digit ICAO address, the AFN handshake completes, and from then on the system handles transfers automatically. The messages are always downlink or uplink by direction, not by whether you're airborne. And the whole point of FANS is to give ATC continuous position data and direct communications where radar doesn't reach, so they can safely reduce separation — with TCAS standing by if position accuracy ever degrades.