Controller–Pilot Data Link Communications (CPDLC)

Definition

CPDLC is a form of digital communication between pilots and air traffic controllers using text-based data link messages rather than voice radio. It is primarily used in oceanic, remote, and increasingly in high-density enroute airspace, where VHF communication is limited or voice traffic is congested. CPDLC allows clear, unambiguous exchanges of clearances, instructions, and requests, improving efficiency and safety in modern airspace.

Purpose

  • Provide reliable communication beyond VHF coverage
  • Reduce radio frequency congestion
  • Eliminate language and accent misunderstandings
  • Enable delayed but recorded communication, with full audit trail
  • Support future air navigation systems (FANS) and SESAR/NextGen initiatives

Where CPDLC Is Used

  • North Atlantic (NAT) Tracks
  • Europe (Upper Airspace, e.g., Maastricht UAC, Karlsruhe)
  • Australia, New Zealand, Canada, Singapore FIR
  • High-seas and oceanic FIRs like Shanwick, Gander, Reykjavik

CPDLC Communication Types

Category Examples
ATC clearances “Climb to FL350”, “Maintain heading 270”
Frequency changes “Contact Vienna Radar on 132.050”
Route changes “Proceed direct to waypoint MOLUS”
Position reports “Position KOKOS at 1234Z, FL360, estimating DOLIP at 1306Z”
Requests from pilot “Request higher FL”, “Request direct destination”

How It Works

  • Aircraft logs onto CPDLC service using ACARS or satcom
  • Controller initiates contact (e.g., “Contact via CPDLC”)
  • Messages are sent and received via cockpit interface (FMS or dedicated CDU)
  • Each message is displayed, acknowledged, and stored
  • All communication is time-stamped and traceable

CPDLC vs. Voice Radio

Feature CPDLC VHF Radio
Medium Data/text Voice
Range Global (satcom/HF) Line-of-sight (~200 NM)
Congestion Low High in busy FIRs
Delay Minor (seconds) Real-time
Language issues Minimal Possible misunderstandings
Recordkeeping Automatic Manual logs or voice recording

Phraseology and Examples

  • Message received: “Climb to FL360” → Pilot response: “WILCO” (will comply)
  • Pilot sends: “Request direct to RUDUS” → ATC response: “Proceed direct RUDUS”

All messages use standard ICAO message sets, ensuring uniformity.

Limitations

  • ❌ Not available in all FIRs or for all aircraft
  • ❌ Not designed for time-critical emergencies (use voice instead)
  • ❌ Requires certified equipment and crew proficiency
  • ✅ Complements but does not replace voice radio completely

Benefits

  • ✅ Reduces pilot and controller workload
  • ✅ Increases clarity and accuracy of communications
  • ✅ Enhances safety and efficiency in high-volume and remote airspace
  • ✅ Allows better flight plan optimization and real-time rerouting

Tips for Pilots

  • Log on with correct ICAO codes and identification
  • Always monitor voice frequencies as backup
  • Practice with LearnATC advanced ATC simulation to get familiar with message handling
  • Be prepared to switch to voice if message response delays occur