FANS, CPDLC and ADS

The bigger picture: CNS/ATM

FANS is aviation’s piece of a wider ICAO plan: the communication, navigation, surveillance / air traffic management (CNS/ATM) concept, the standing programme to modernise how aircraft talk, find their way, are watched, and are managed, largely by moving each of those jobs from analogue voice and ground radar onto data links and satellites. The syllabus asks only that the concept exists and frames what follows: FANS is the flight-deck face of it.

Two versions

FANS comes in two versions, named twice each for historical reasons (one name per manufacturer line):

FANS A/FANS 1 — the original oceanic generation; it uses the ACARS network of the previous lesson.

FANS B/FANS 2 — the later generation for busy continental airspace; it uses the ACARS network and the aeronautical telecommunication network (ATN), a newer, faster data network built for air traffic services.

Same ideas, better plumbing where the traffic is densest.

The three applications

FANS A/FANS 1 carries three applications, and the figure shows their shapes:

ATS facility notification (AFN) — the log-on: the one-off exchange that introduces the aircraft to an air traffic control unit (ATCU) and declares what it can do. Everything else waits until AFN has succeeded.

Automatic dependent surveillance (ADS) — the watching: the aircraft downlinks its own position and intentions automatically, under contracts the ground sets. Dependent, because the surveillance depends on the aircraft reporting itself.

CPDLC — the talking: controller–pilot data-link communications, the two-way exchange of clearances, requests and instructions as messages.

Compared with the old world

The syllabus asks for two comparisons:

ADS against secondary surveillance radar (SSR) — SSR, the classic surveillance, works by a ground radar interrogating the aircraft’s transponder and reading the reply, so it exists only within range of a radar head, which oceans and remote areas do not have. ADS needs no radar at all: the aircraft reports its own navigation-system position wherever a data link reaches, which is why oceanic surveillance became possible. The dependence reverses too: SSR measures the position itself at the ground station, the aircraft needing only an operating transponder that replies, while ADS depends entirely on the aircraft’s own navigation equipment reporting its position.
CPDLC against VHF voice — the previous lesson’s comparison, applied: CPDLC is unambiguous, logged and immune to congestion and accent, and it works far beyond VHF’s line of sight over whatever link is available; voice remains faster for a quick exchange and is the required fall-back when the dialogue must happen now.

Who uses what

The applications are separable on the ground side: an ATCU can use the ADS application only, or the CPDLC application only, or both of them. The one thing that is not optional is the introduction: AFN is not part of the choice, because no other application runs until the log-on has happened. A unit watching without talking, talking without watching, or doing both are all real configurations met in service.

Logging on: the AFN process

The AFN process is flown ahead of need: typically some minutes before entering the airspace, the crew initiates the log-on to the ATCU named in the procedures, usually through the CDU/MCDU pages. The notification message carries typical data:

flight identification — as filed in the flight plan, so the ground can match it.
aircraft registration and unique aircraft address.
the aircraft’s data-link capabilities — which applications and versions it supports, so the ATCU knows what it may use.

A successful AFN hands the connection over; a failed one is the cue to revert to voice procedures before the boundary, not after it.

CPDLC in practice

The typical CPDLC messages are the controller’s ordinary business in text:

level clearances (climb, descend, maintain).
route and direct-to instructions.
speed instructions.
frequency-change instructions.
crossing constraints.
requests and reports — the aircraft’s own, going the other way.
free text — for what the standard set does not cover.

The work practices are what the examiners care about: both pilots read the message, on the DCDU, before anyone answers; the clearance is checked against the plan and the aircraft’s performance; the answer is one of the standard responses:

WILCO — will comply.
UNABLE — cannot comply.
STANDBY — wait.

And a clearance that changes the trajectory is loaded and cross-checked before it is flown. A message half-read by one pilot and answered by reflex is the failure mode the discipline exists to prevent.

ADS contracts

ADS reporting is governed by contracts that the ATCU sets, beyond the control of the pilot: the ground decides what it wants to hear and the aircraft complies automatically. Three contract types share that shape:

Periodic — data sent at set time intervals: the routine heartbeat of oceanic surveillance.
On demand — data sent when the ATCU requests it: a one-off snapshot between heartbeats.
On event — data sent when something happens: a heading change, a climb initiated, a level left; the aircraft volunteers the news the moment the event occurs.

The fourth contract breaks the pattern, deliberately. The emergency mode exists to signal distress and step up the reporting when the crew needs the ground watching closely, and its difference from the other contracts is exactly where the control sits: the ATCU-controlled contracts are invisible to and beyond the pilot; the emergency mode is the one the crew can trigger from the aircraft, by its own selection (and on some aircraft it is armed automatically by other emergency actions). The ground sets the routine watching; the crew can always escalate it.