Downstream Clearance Via Datalink

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Downstream Clearance Via Datalink

36TH ANNUAL CONFERENCE, Taipei, Taiwan, 17-21 March 1997

WP No. 115

Downstream Clearance Via Datalink


This paper reports on developments regarding Downstream Clearances (DSC) via datalink that have taken place in the ICAO Automatic Dependent Surveillance Panel (ADSP) and proposes that IFATCA adopts a policy statement to be included in the Technical Manual.

During the past year while the ICAO ADSP was completing the Draft ICAO Manual of Air Traffic Services (ATS) Data Link Applications, there was significant internal IFATCA debate among members of SC1 and various IFATCA representatives regarding Downstream Clearances. Resulting from this debate, the IFATCA Member of the ADSP formulated the bulk of the operational requirements for DSC that were eventually included in the draft Manual. This draft Manual is now being reviewed by the ICAO Air Navigation Commission (ANC) for publication.

Some background is necessary to understand the details of the debate regarding Downstream Clearances delivered via data link.

The ADS Panel has been working for several years developing the draft Manual that is proposed to become the ICAO Guidance Material for implementation of Air Traffic Services over a data link system. One of the fundamental operational principles that has been incorporated from the beginning of this work is that any ATS data link system must only allow one ATSU (ATS Unit) to be capable of communicating with a given aircraft at any one time. This requirement was felt to be necessary to ensure that there would be no confusion by the aircrew that a clearance or instruction delivered by the data link system actually came from the proper controller (or controller team).

Currently there are States that require aircraft to receive Downstream Clearances via radio before entering their FIR (Flight Information Region). This communication is done while the aircrew remains in contact with the Executive Controller in the current ATSU by using a second radio to contact a controller in an ATSU of the FIR that the aircraft will later enter.

Note: There have been incidents reported in which the aircrew confused the altitude portion of the Downstream Clearance as an instruction from the current Executive Controller and began a climb/descent to an altitude to which they had not been cleared.

As the draft Manual matured and capabilities of proposed data link systems became better understood, the States that use Downstream Clearances expressed their requirement that a data link system also be able to accommodate this capability. To do this would require the capability for the aircraft to open a second data link connection with the Downstream ATSU (D-ATSU) while maintaining the connection with the Current ATSU (C-ATSU).

There are numerous operational constraints to the use of Downstream Clearances that were included in the draft Manual to ensure that this data link capability is as safe as possible. These include:

  • There can only be one Downstream Clearance connection at any one time;
  • The Downstream Clearance connection can only be initiated by the aircrew, not from the ground.

Any data link message sent via a DSC link must be readily identifiable as such on the display of both the sender and the recipient of the message.

There will be procedures that prevent the pilot from executing a clearance received via a DSC link until the aircraft enters the airspace of the control authority from which the Downstream Clearance was received. If the information received by an airborne user via a DSC link requires action while still in the airspace of the current control authority, the clearance for such action must be obtained from that current control authority.

The ground system must have the ability to reject any request for DSC. This functionality is necessary because there are numerous States that do not use Downstream Clearances via voice today nor do they plan to use DSC via data link. These States must have the technical capability to prevent aircraft from connecting with their ATSU’s if they deem that DSC links are inappropriate for their airspace.

The data link system described in the draft Manual uses a pre-defined set of messages for Controller Pilot Data Link Communications (CPDLC). There are several hundred messages in this Message Set that cover the spectrum of ATC communications. Since the Message Set was designed to be used between the aircraft and the current Executive Controller, there are messages that are clearly inappropriate to be sent over a Downstream Clearance connection. These include any message that would instruct the aircraft to alter its current trajectory or speed. These messages are listed in the draft Manual and are prohibited from use in isolation; however, they can be concatenated with other CPDLC message elements that make it clear that they are to executed at a later time or location.

An example of this is that Message Element # 20, CLIMB TO [level], cannot be sent in isolation over a DSC link. It can, however, be concatenated with Message Elements #77, AT [position] PROCEED DIRECT TO [position], and #165, THEN. Procedural constraints would require that the position listed in the message be within the airspace of the controller issuing the Downstream Clearance.

Another area in which Downstream Clearances has required some adjustment of heretofore fundamental data link operational requirements is that of concurrent direct controller pilot voice communications. A Generic Operational Requirement in the draft Manual states,

“In any data link based ATS, provision must always be made for direct pilot-controller voice communications.” The purpose of this requirement is clearly that the voice and data link communications mentioned are with the same controller. However, among the many Annex and Doc 4444 changes proposed by the ADS Panel is this one for Annex 11, Chapter 3, para., “Aircraft shall maintain the necessary two-way communication with the current air traffic control unit whilst obtaining a downstream clearance.” In the same section is contained par. “Recommendation. – Where practicable, and where data link communications are used to facilitate downstream clearance delivery, two-way voice communications between the pilot and the air traffic control unit providing the downstream clearance should be available.”


This Committee should review the material included in the Discussion section of this Working Paper and decide whether the proposed operational constraints on a data link Downstream Clearance capability are appropriate and sufficient for it to be used safely.

Last Update: September 28, 2020  

March 4, 2020   629   Jean-Francois Lepage    1997    

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