GEN 3.4.3.2.1   Mobile service

The air traffic services units maintain a continuous watch on their stated frequencies during the published hours of service unless otherwise notified, however low level flights in areas with mountainous terrain may encounter difficulties in establishing and maintaining radio communication with relevant air traffic services units.

VHF-equipment (transmitter and receiver) used in the aeronautical band (118.000 - 137.000 MHZ) shall comply with the specifications prescribed in ICAO Annex 10, Vol. III, Part II, Chapter 2.

GEN 3.4.3.6.1   General

CPDLC is part of the service for ATN via VHF Data Link Mode 2 (VDL M2) equipped aircraft.

The CPDLC application provides a means of communication between the air traffic controller and the pilot, using a predefined data link message set. This application includes a set of clearance/information/request message elements which correspond to the phraseologies used in the radiotelephony environment as described in ICAO Doc 4444, ref. Chapter 12 "Phraseologies" and Chapter 14 "Controller-Pilot Data Link Communications (CPDLC)".

Basis for CPDLC within Zagreb FIR is ATN/FANS B+. Data communication via FANS 1A will be not supported. If an aircraft logs on with both ATN B2 and ATN B1 capability, Croatia Control will offer ATN B1 CPDLC service. If an aircraft logs on with ATN B2 capability only, Croatia Control will not offer any CPDLC service – neither ATN B2 nor ATN B1.

GEN 3.4.3.6.2   Area of Application

CPDLC services are available for all certified aircraft operating within Zagreb FIR in the airspace of Zagreb CTA (Zagreb ACC), Zagreb TMA, Pula TMA, Zadar TMA, Split TMA and Dubrovnik TMA (commonly named ATCC Zagreb units) (refer to AIP Croatia ENR 2.1 and ENR 6.1-1).

Within FIR Zagreb above FL285, IFR flights shall be equipped with CPDLC. Below FL285 the use of CPDLC is not mandatory but appreciated and is conducted at the discretion of the ATS-Unit and at the initiative of the pilots concerned.

Within FIR Zagreb it is mandatory for all flight crews of data link equipped aircraft to log on to Zagreb data link address (LDZO).

Full coverage of the CPDLC services might not be available for aircraft operating in certain parts of the respective AoR below FL205.

GEN 3.4.3.6.3   Flight plan

In order to use the CPDLC services, following shall be filed in the respective items of the flight plan:

• Item 10a - J1 for the CPDLC ATN VDL Mode 2 capable aircraft;
• Item 18 - the indicator CODE/followed by the aircraft 24-bit address expressed in the form of alphanumerical code of six hexadecimal characters.

Aircraft which are exempt of the DLS IR (EC IR2023/1770) mandate and can't use ATN B1 CPDLC shall file letter “Z” in item 10 and the indicator „DAT/CPDLCX“ in item 18.

GEN 3.4.3.6.4   CPDLC use

In ATCC Zagreb units AoR voice communication and/or radiotelephony instructions have priority over CPDLC instructions at all times. However, a clearance requested via CPDLC should subsequently be issued via CPDLC and a clearance requested via radiotelephony should also be issued via radiotelephony.

Voice read-back is not required for any CPDLC instruction.

Clearances shall not be executed until the WILCO message has been sent.

If uncertainty arises regarding a data link message, voice communication shall always be used. CPDLC exchanges with ATCC Zagreb units shall only be conducted when the aircraft is actually under control and responsibility of referenced unit.

Pilots can expect to receive data uplink messages especially during periods of high traffic volume. Pilots are requested to always confirm data uplink messages as soon as possible with WILCO.

When using CPDLC, the maximum dialogue time is 120 seconds. CPDLC shall only be used for non-time-critical requests, i.e. requests that do not require the immediate reaction of the pilot. Nevertheless, as in radiotelephony, the CPDLC messages shall be answered with the least possible delay. If the downlink request is cut off because the time limit was exceeded, the pilot shall repeat the request via radiotelephony.

GEN 3.4.3.6.5   CPDLC services

The following CPDLC services are provided in Zagreb FIR:

• Data Link Initiation Capability (DLIC)
• ATC Communications Management (ACM)
• ATC Clearances and Instructions (ACL)
• ATC Microphone Check (AMC)
  

DLIC log-on

All data link equipped aircraft are required to log on to LDZO.

CPDLC shall be established in due time to ensure that the aircraft is communicating with the appropriate ATC unit. Log-on shall be initiated by the pilot. Pilots shall log-on using their ICAO call sign as filed in the flight plan. Pilots shall not use a two-letter IATA flight ID, neither insert a leading zero (0) into the call sign, as these actions will result in a failed log-on.

At the convenience of the controller a Start-CPDLC may be performed to enable the CPDLC message sending for any particular flight in the designated CPDLC airspace.

Data link capability is displayed on the radar screens at ATCC Zagreb and the controllers may request appropriately equipped aircraft to LOG ON. CPDLC capability shall not be mentioned on VHF.

ATC Communications Management (ACM)

When an aircraft is transferred by data link to an adjacent sector/ATC unit, the pilot shall acknowledge the instruction by WILCO, and shall then contact the next sector/ATC unit by voice communication on the instructed VHF channel. If the pilot is unable to comply with this data link instruction, he shall revert to voice communication to inform ATC.

If the controller needs to transfer the aircraft without replying to any downlink message(s) outstanding, the system has the capability to automatically send the response message “REQUEST AGAIN WITH NEXT UNIT” in order to close the pending dialogue, followed by the CONTACT message.

ATC Clearances and Instructions (ACL)

Pilots may receive an ATC instruction via data uplink messages. Pilots may request changes to flight levels (climb or descent) or clearance direct to a point on their route via data downlink messages.

ATC Microphone Check (AMC)

A “check stuck microphone” instruction may be sent by ATC in circumstances where an aircraft is inadvertently blocking a voice communication channel.

If the "check stuck microphone" instruction relates to the RTF channel currently being used, the pilot shall check that their radio equipment is not causing the blockage. If the "check stuck microphone" instruction does not relate to the RTF channel being used, no further action by the pilot is required.

GEN 3.4.3.6.6   Message restrictions and error management

If the ground system receives a message that is not supported, or constitutes an error to the technical rules for CPDLC communication, flight crew will receive an automatic reply indicating the nature of the error and, if applicable, required actions.

Pilots shall not use free-format free-text messages when communicating with Zagreb ATCC units via CPDLC. Use of such free-text messages will result in an error response.

GEN 3.4.3.6.7   CPDLC failure

Some failures may result in termination of the existing data link connections with aircraft that are under control of a sector. In this case, it will not be possible for ATC to re-initiate dialogues via CPDLC unless the pilot re-initiates the data link logon process in order to re-establish data link connection.

In case of a CPDLC failure, CPDLC clearances that have not yet been confirmed shall be repeated over voice communication and/or confirmed. If either the pilot or ATC consider that CPDLC should not be used in the prevailing circumstances, CPDLC shall be suspended or terminated and the other party shall be informed by voice communication.

In case of a scheduled shutdown or an unexpected failure of the CPDLC system, ATC will instruct all aircraft equipped with data link to return to voice communication. In case of an on-board failure of CPDLC, the pilot shall return to voice communication and inform ATC.

GEN 3.4.3.6.8   CPDLC disconnection (log-off)

CPDLC disconnection is automatic upon transfer of communication to adjacent ATC unit, no pilot action is required. Between Zagreb ATCC units and adjacent CPDLC equipped ATC units the ACM service is used.

GEN 3.4.3.6.9   CPDLC Messages

The controller or pilot shall construct CPDLC messages using the defined message set. The following set of uplink/downlink messages may be expected when using CPDLC with Zagreb ATCC units:

1. ATC uplink clearances and instructions

Vertical clearances:

• MAINTAIN /level/
• CLIMB TO /level/
• DESCEND TO /level/

Route modifications:

• TURN /direction/ HEADING /degrees/
• FLY HEADING /degrees/
• PROCEED DIRECT TO /position/
• CLEARED TO /position/ VIA /route clearance/

Contact/monitor/surveillance requests:

• CONTACT /unit name/ /frequency/
• MONITOR /unit name/ /frequency/
• SQUAWK /code/
• SQUAWK IDENT

Air traffic advisories:

• CHECK STUCK MICROPHONE /frequency/

Other uplinked messages supported by ATM system:

• UNABLE
• STANDBY
• ROGER
• REQUEST AGAIN WITH NEXT UNIT
  

2. Pilot downlink request

The following downlink requests may be sent by pilots using CPDLC with Zagreb ATCC units:

Vertical requests:

• REQUEST /level/
• REQUEST CLIMB TO /level/
• REQUEST DESCENT TO /level/

Route modifications requests:

• REQUEST DIRECT TO /position/

Other downlinked messages supported by ATM system:

• PAN PAN PAN
• MAYDAY MAYDAY MAYDAY
• SQUAWKING 7500
• DUE TO WEATHER
• DUE TO AIRCRAFT PERFORMANCE
• WILCO
• UNABLE
• STANDBY
• ROGER