Call sign similarity service

We help to reduce the incidence of call sign confusion events and improve the safety of the network.

The use of similar call signs by aircraft operating in the same area on the same radio frequency is referred to as 'call sign similarity'. The danger of a pilot taking and acting on a clearance intended for another aircraft due to this phenomenon is a common occurrence and can lead to flight safety incidents.

The purpose of the call sign similarity service is to help aircraft operators (AOs) to de-conflict similar call signs embedded in their schedules. This helps to reduce the incidence of call sign confusion events and improves the safety of the Network.

Service levels

The Call Sign Management Cell (CSMC), as part of the Network Manager operations centre (NMOC), delivers three levels of service.

Service level 0

The main objective is to raise awareness about CSS reduction processes. In particular this involves providing the following:

  • publication of the CSS rules to be applied in the process of detection and resolution of conflicts;
  • support aircraft operators (AOs) in implementing call sign similarity reduction processes;
  • provide feedback and monitor the results of the implementation and of the use of the de-confliction strategies during the season.

Service level 1

Service Level 1 provides support to de-conflict or eliminate similar call signs within a single aircraft operator's schedule, prior to the start of the summer and winter season. The CSMC facilitates and monitors this process with the help of the Call Sign Similarity Tool (CSST). As part of this service level, the CSMC also provides users with a limited feedback on priority conflicts with other CSST users.

Service level 2

Service level 2 will provide support to de-conflict or eliminate similar call signs between different aircraft operators’ schedules, prior to the start of the summer and winter season. This service level will be dependent on the success of Service level 1 operations.

CSS users group

To drive the development of the CSS service and its supporting CSS tool, a CSS users group has been established. The target membership of this user group is air navigation service providers (ANSPs), airspace users, airports, and military stakeholders as appropriate. Representatives from the International Air Transport Association (IATA) and the United Nations' International Civil Aviation Organisation's (ICAO's) Paris office are also welcome to join as observers to the CSS users group. Contact us to request access to the group.

Call Sign Similarity Rules

One key element in reducing the number of call sign similarities (CSS) is the publication of an agreed set of call sign similarity rules i.e. agreed definitions of what constitutes a ‘similarity’. These rules have been identified by analysis of safety reports concerning call sign confusion and by the call sign similarity user group (CSS UG).

In addition to the rules for detection of similarities, the CSS users group has determined some basic principles that describe the acceptable formats for the creation or correction of call signs.

A strict application of these rules within the CSST application would cause a blocking for airline schedules as it would not leave enough solution space to resolve similar flight numbers and call signs. Therefore, these rules have been refined with certain parameters within the CSST to reflect operational feedback. This fine-tuning seeks to make a balance between capturing those similarities, which are considered most likely to lead to real time incidents on frequency whilst leaving sufficient solution space to deconflict an airline schedule.

Call Sign Similarity Rules

Single aircraft operator similarity rules

Single aircraft operator (AO) similarity rules are a set of rules for detecting similarities between two (or more) call signs within a single aircraft operator's schedule. These rules are applicable to all flights in all airspaces.

Local similarity rules

In some cases, the air navigation service providers (ANSPs) might put specific constraints on the composition of call signs.

For example, the ANSP might not be able to accept mixed alphanumeric call signs or might wish to avoid confusion with other data included within a radar label. These rules are applicable to a specific airspace and is therefore referred to as a ‘local’ rule. Depending on the specific circumstances, these rules are differ in terms of their importance.

Single call sign similarity rules

A similarity event does not have to involve two call signs. A call sign can be similar to different entities commonly broadcast within an ATC clearance (e.g. runway designator, a flight level or VHF frequency). While these similarity events can certainly cause confusion, they are generally considered less severe as they will not normally cause one aircraft to take the clearance intended for another. Because there is usually only one aircraft involved, recovery is easier. They are also more predictable for the pilot and/or controller as the entities concerned are always present. Depending upon the detail, single call sign similarity rules are either applicable to all flights or to a specific airspace.


Service level 0 is public. Service level 1 is available upon subscription and signing of a service agreement and includes access to the CSS tool.


Call sign similarity tool

Monitoring our performance

The CSMC has established procedures with participating aircraft operators to monitor the operational effectiveness of the CSS service and tool. The anticipated improvements in safety performance will be monitored by the EUROCONTROL voluntary air traffic management incident report (EVAIR) scheme.

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