Each ATS unit (or sector if the unit is split) operates on a separate frequency. Therefore, when an aircraft approaches the boundary, the pilot needs to change the operating frequency to that of the next unit or sector. This process is called transfer of communication and is effected either by
- a voice instruction ([callsign] CONTACT [unit name] [frequency]), or
- a CPDLC message, or
- a special procedure, described e.g. in the AIP. For example, when an aircraft is about to start up at a controlled aerodrome, the pilot is supposed to contact the appropriate ATC unit (tower or ground). Similarly, when the aircraft is about to enter airspace where ATS is provided (most often controlled airspace but not necessarily), the frequency of the appropriate ATS unit should be aquired in advance from e.g. the relevant AIP.
The transfer of communication is done in accordance with the procedures established in Letters of Agreement or in local procedures (e.g. operations manuals). Within controlled airspace it usually happens a few minutes before the Transfer of Control is effected. This creates a potenially dangerous situation where one controller is responsible for the safety of the flight (but is unable to issue instruction) and another is in contact with the crew (but the aircraft is not yet within the area of responsibility). It is therefore important that the accepting controller does not introduce flight profile changes before the aircraft passes the transfer of control point unless these are coordinated with the transferring controller or a release has been obtained. This does not mean that no communication is to be carried out. There are a number of things that can be safely communicated, e.g.:
- instructions to be carried out after the transfer of control point has been passed. Note that when issuing these, the after part must be emphasized and correctly acknowledged.
- exchange of information, e.g. ATIS designator, meteorological information, runway in use, etc.
- confirmation of entry or diplomatic clearance. Note that in case there is an issue with any of these the controllers need to coorinate what happens next (e.g. a holding pattern, return to the departure aerodrome, deviation to an alternate, etc.).
Another risk with communication transfer is that for a (usually short) period of time no controller is in contact with the flight crew. It is therefore crucial that aircraft being transferred are clear of any potential conflict. While this is applicable to air traffic control in general, the transfer of communication phase is an inherently riskier situation and therefore extra vigilance should be exercised. Also, the risk of Loss of Communication should be taken into account.
The accepting unit (or sector) is normally not supposed to inform the transferring unit (or sector) that communication with the aircraft has been established. However, if communication has not been established as expected, the accepting unit (or sector) should notify the transferring unit (or sector).
While not mandatory, it is generally safer not to combine communication transfer with other instructions, e.g. level changes. The controller may not be able to correct an errorneous instruction (or a wrong readback) if the pilot changes the frequency quickly enough. If this happens, the situation may quickly deteriorate while neither controller is in contact with the aircraft and both are therefore unable to issue corrective instructions. The situation can be further compounded if there is an issue with the frequency change (e.g. wrong frequency being set) or if the next frequency is congested.
Transfer of communication of several aircraft one right after the other may cause frequency blocking (due to simultaneous transmissions) in the downstream sector (or unit). This risk can be mitigated (but not elliminated since most sectors and units have more than one neighbour) by:
- System support. For example, the OLDI message MAS (manual assume) can indicate that the aircraft being transferred has made initial contact so the next transfer can commence.
- Time management. If a bunch of aircraft are approaching the boundary, the transfer of control of the fist one may take place a little earlier than usual so that proper interval (e.g. 10-20 seconds) between consecutive transfers is maintained.
In case the flight time within a sector (or unit) is very short (e.g. 1-3 minutes) communication may be transferred directly between the two adjacent sectors (or units). This procedure is sometimes called "skipping". The following aspects need to be considered if it is to be used:
- The sector or unit being skipped remains responsible for the safety of the flight even though it is not in contact with the crew. Therefore it should be kept fully informed about this traffic. Any profile change must be coordinated with this sector or unit.
- The sector or unit being skipped must ensure separation between this traffic and the other aircraft within their area of responsibility.
Special attention should be paid to the handover process during controller training stressing the importance of proper timing of frequency change, e.g.:
- If two climbing, vertically separated aircraft are to be transferred, the higher is to be transferred first. This allows the accepting controller to reclear a higher level thus providing continuous climb.
- If two descending, vertically separated aircraft are to be transferred, the lower is to be transferred first. This allows the accepting controller to reclear a lower level thus providing continuous descent.
- If an aircraft is climbing to a level below the requested one due to another aircraft above, the maintaining flight is to be transferred first and the climbing (which is cleared to a safe level below) is transferred shortly before the boundary. This serves as a reminder to the accepting controller not to reclear the climbing aircraft to a higher level unless horizontal separation is applied.
- If there is a potential conflict near the boundary, a common solution should be coordinated. This includes agreeing on the frequency the aircraft will be until clear of each other. This may mean that the transferring controller may (after coordination) remain in contact with an aircraft after it has left their area of responsibility.