Experience has shown that Large Lateral Deviations and Large Longitudinal Errors have had significant influence on the outcome of safety assessments before and after implementation of PBN-based separation in a portion of airspace. Each ANSP will need to establish the means to detect and report the occurrence of large horizontal-plane deviations.

The surveillance information available to ATC units provides the basis for identifying large horizontal plane deviations. A programme for identifying large horizontal-plane deviations should be established and ATC units should report such events monthly.

One of the principal duty of an EMA is to ensure the existence of a programme to collect this information, assess the occurrences and initiate remedial action to correct systemic problems.