Communication Systems

Modern Datalink Communication Systems for ATPL Students

Modern airline communication has shifted from voice-heavy radio procedures to integrated datalink systems governed by ICAO standards and airline operating procedures. Under the FANS (Future Air Navigation System) concept, three core applications stand out: CPDLC (Controller–Pilot Data Link Communications), ACARS (Aircraft Communications Addressing and Reporting System), and ADS (Automatic Dependent Surveillance, typically ADS-C in FANS). CPDLC transmits formatted messages between the pilot and ATC, reducing frequency congestion and enhancing clarity; ACARS provides the air/ground messaging backbone for both AOC (Airline Operational Control) and ATC communications; and ADS automatically downlinks surveillance data to controllers. In this ecosystem, uplink means messages from the ground to the aircraft, while downlink flows from the aircraft to the ground. Digital services like D-ATIS deliver ATIS via datalink to the cockpit, streamlining information management within aircraft systems and flight deck procedures.

Before any ATC datalink exchange, the LOGON (notification) phase transmits aircraft identification and associated datalink capability so the ATS unit can establish a session. Once connected, CPDLC supports clearances and routine control instructions—commonly in oceanic and remote airspace, and increasingly for departure procedures in some regions—complementing or, in designated airspace, replacing routine voice per applicable aviation regulations and local AIPs. Time-critical or complex instructions may still require voice per operator SOPs and PANS-ATM guidance. Emergency communication is also supported: a MAYDAY can be sent via CPDLC’s emergency message set. On the ground side, ATC datalink routing is handled by service providers such as SITA and ARINC, whose networks interconnect to ensure end-to-end continuity. Physically, these services ride on media such as VDL Mode 2 VHF or SATCOM, with ACARS protocols commonly transporting FANS 1/A messages.

ADS enhances surveillance by automatically sending position, intent, and performance data without crew intervention to define content or report type; the FMS manages contracts (periodic, event, and demand) agreed with the ground system. Correct interpretation of CPDLC phraseology is essential for compliance and safe execution of procedures. For example, some uplinks direct when a climb must start, others when a level must be achieved, and some advise you to expect a forthcoming instruction rather than act immediately. Mastery of these distinctions is a core ATPL knowledge goal, ensuring pilots align aircraft trajectory management with controller intent while using aircraft systems effectively.

Interpreting common CPDLC uplinks

• CLIMB TO REACH [altitude] BY [time]: Plan and commence a climb so the specified level is reached at or before the stated time.
• AT [time] CLIMB TO AND MAINTAIN [altitude]: Initiate the climb at the stated time, then hold the cleared level.
• EXPECT CRUISE CLIMB AT [time]: Advisory only—anticipate a climb instruction at the specified time; do not climb until further clearance.
• AT [position] CLIMB TO [altitude] AND MAINTAIN: A standard CPDLC clearance tied to a waypoint or position report.

What this question bank covers

• FANS overview: CPDLC, ACARS, and ADS roles and architecture.
• ATC datalink routing via SITA/ARINC and continuity of ground networks.
• LOGON (notification) process and declaring datalink capability.
• Uplink vs downlink definitions and message flows.
• CPDLC-clearable items: Departure and Oceanic clearances; voice vs datalink policy.
• Emergency messaging: sending a MAYDAY via CPDLC.
• ADS automatic reporting principles and contract types.
• D-ATIS and operational uses of ACARS for AOC and ATC.
• Precise interpretation of time- and position-based climb messages.
• Relevant ATPL-level procedures aligned with ICAO PANS-ATM and operator SOPs.