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The negative response `UNABLE' from the controller to the crew's request for a higher level, in accordance with the SPOM, would have reduced the possibility of error in the pre-formatted CPDLC message selected by the controller for transmission to the crew of OEB.

The use of HF radio and CPDLC combined with a high controller workload and similar radiotelephony callsigns of the two aircraft involved in the communications exchanges possibly contributed to the confusion to which the controller referred and may have resulted in the transmission of the incorrect CPDLC message.

When an individual controller combines a number of positions, diverse scenarios and increasing workloads can quickly distract controllers. Controllers and supervisors need to be vigilant so that ATC positions can be separated to facilitate effective workload management.

The monitoring of TCAS and high situational awareness by all the crews involved proved to be an effective defence for the aviation system.

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