ATPL Theory

Acars In Flight

Typical step-by-step ACARS flow during a flight, showing what each on-board component is doing. (Exact sequence varies by airline/aircraft, but this is the common pattern.)

1) Power-up at the gate

  • CMU/ATSU boots and starts managing datalink.
  • It looks for the best available network (usually VHF on the ground; SATCOM/HF may be available too).
  • CDU/MCDU or DCDU becomes the crew interface for datalink pages/menus.
  • Annunciators (message lights/captions) are armed so the crew gets alerted to incoming messages.

2) Preflight initialization

  • Crew (or airline systems) may request/receive operational messages like:

    • Weather (METAR/TAF, winds)
    • NOTAMs
    • Gate/dispatch messages

  • Those uplinks arrive to the CMU, then display on CDU/DCDU.

  • If required, messages can be printed automatically or manually.

3) Clearance phase (at the gate / before push)

  • Crew requests or receives ATC clearance via datalink (where CPDLC is used).

  • Flow:

    1. Ground sends clearance → VHF/SATCOM/HF
    2. CMU receives and routes it
    3. Message appears on CDU/DCDU
    4. ATC message annunciator alerts the crew
    5. Crew reviews, may ACCEPT/REJECT (depending on procedure)
    6. If accepted, some aircraft can load parts into the FMS (or crew enters/loads data)

4) Pushback and taxi out

  • ACARS commonly sends automated airline messages (AOC) such as:

    • OUT time (gate departure timestamp)
    • Aircraft status snapshots

  • The maintenance computer may begin routine downlinks if faults exist.

  • Crew may receive last-minute updates:

    • Route amendments
    • Weather changes
    • Operational instructions

5) Takeoff and climb

  • When airborne, ACARS often sends the OFF time (wheels off).

  • CMU may switch to the best link as coverage changes:

    • Often VHF initially, then SATCOM if VHF fades (over ocean/remote)
    • HF can be used in some long-range operations

  • If ATC sends a datalink message, the annunciator alerts the crew and the message appears on the CDU/DCDU.

6) Enroute cruise (most active phase)

Typical datalink traffic in cruise:

A) Position reporting

  • The FMS provides position data.

  • A position report is generated and routed by the CMU to the ground.

  • This can be:

    • scheduled (periodic),
    • event-based,
    • or requested.

B) ATC / CPDLC messages (where used)

  • ATC uplinks instructions/requests (e.g., “climb to…”, “proceed direct…”).
  • Crew reads on CDU/DCDU, responds (WILCO/UNABLE/REQUEST).
  • Some clearances can be loaded into the FMS (aircraft dependent + airline SOP).

C) Airline operational control (AOC)

  • Dispatch may uplink:

    • New wind/temperature data
    • Reroutes, alternates
    • Turbulence/ride reports

  • Crew may downlink:

    • ride reports
    • fuel/status updates

  • Printing is used when the crew wants hard copy (or if auto-print is enabled).

D) Maintenance and diagnostics

  • If a fault occurs, the maintenance computer can automatically downlink fault codes.
  • Ground maintenance may uplink a request to run a diagnostic routine.
  • Results are then downlinked again through the CMU.

7) Descent preparation

  • Crew may request:

    • Destination weather
    • Arrival/approach updates
    • Gate info

  • ATC may send changes (speed/level/route) via datalink.

  • If the aircraft supports it, relevant items may be loaded/entered into the FMS.

8) Approach and landing

  • ACARS may send the ON time (wheels on).

  • Network choice may shift back toward VHF as you return to coverage.

  • Any late messages still follow the same path:

    • Link → CMU → CDU/DCDU (+ annunciator) → optional print / crew response / FMS update.

9) Taxi in and shutdown

  • ACARS sends the IN time (arrive at gate).
  • Final maintenance/status messages may be downlinked.
  • Some systems transmit post-flight reports automatically (performance, faults, operational summaries).
  • Crew prints anything required for records.

Quick “who does what” during all phases

  • CMU/ATSU: picks the link + routes messages
  • FMS: source/destination for flight data (position, route, some loads)
  • CDU/MCDU or DCDU: crew reads/responds/sends
  • Annunciator: “you have a message”
  • Printer: hard copy of uplinks
  • Maintenance computer: faults/diagnostics to/from ground