Route Deviation Notification
This feature is designed to allow a user to obtain a Notification when a flight deviates laterally in distance away from the planned route, beyond a defined buffer region. The buffer region is set to a threshold of 50NM by default but can be user defined between 1 and 9999NM from the route. The threshold is measured to the shortest distance to a waypoint or the segment of the route joining 2 waypoints.
The Departure (DEPT) and Destination (DEST) airports have a 100NM buffer circle drawn around them where deviations will not be notified. This is due to changes or uncertainties in routing within those phases of flight and to avoid nuisance alerts. The system monitors for excursions outside the lateral or circular DEPT/DEST buffers in a rolling 1 min sampling of data, meaning that an alert could be generated with a slight time delay (approx. 1 or 2 mins).
The diagram below provides a visual depiction of how this deviation Notification functions.

Flight Deviated From Route provides a Notification when the flight deviates beyond the rules explained earlier.
Flight Recovered Back To Route provides a Notification when the flight returns into the buffer region as explained earlier.
The function of the deviation Notification is better explained in the diagram below. The blue flight plan between the triangular waypoints shows the last part of a route to a DEST. The dashed orange lines show the buffer region and threshold that has been user defined to 45NM.
The green dots are the tracked positions of the aircraft. The purple dashed line of the circle is the buffer region of 100NM surrounding the DEST (also applies to a DEPT).
There are 2 black squares surrounding 2 tracking positions, the first leaves the buffer region and generates a deviation Notification (Flight Deviated From Route).
The second black square tracked position returns to within the buffer region and generates a recovered Notification (Flight Recovered Back To Route).

Special conditions and limitations for Deviation Notifications
- If a flight has deviated outside of buffer and remains outside before entering directly into the 100NM DEST circle, then a Flight Recovered Back To Route notification will not be generated.
- If a flight is within the 100NM DEPT or DEST circle and then routes outside of the circle, then a Flight Deviated From Route Notification will be generated. This is only true if the flight is also outside of the set Route Deviation Threshold and buffer along the planned route.
- The deviation Notifications have not yet been implemented for routing to Alternate airports. If an Alternate airport route remains within the 100NM DEST circle, then no deviation Notification will be generated. However, if the routing to an Alternate airport falls outside of the 100NM DEST circle then a deviation Notification will be generated.
- Changes to the Route Deviation Threshold only take effect on flights that start tracking after the change is made. When a flight begins tracking then the current Route Deviation Threshold that is set will be used to generate Route Deviation Notifications. Subsequent changes of the Threshold in the settings will not take effect on any flights already tracking
- Users should be aware that ATC commanded direct routings could cause deviation Notifications. In the example below the flight has had a direct clearance between DELOG and TABOV, therefore missing out the routing GODEM-USODA-GONEK. This is only about 30NM away from the planned route, but could generate Route Deviation Notifications if a low threshold value is used. It can be seen how ATC direct routings could easily generate deviation Notifications.
