Route Adherence Monitoring (RAM) is a controller advisory tool designed to assist in the early identification of a variation between the actual and the expected trajectory.
RAM is beneficial to controllers as it provides advance warning in case of lateral deviation which could lead to loss of separation (the aircraft turns towards another one), airspace infringement (the aircraft turns towards an SUA or a volume of controlled airspace) or increased risk of CFIT (the aircraft turns towards high terrain). Thus, the reason for the deviation may be clarified and appropriate corrective action negotiated and implemented.
Unlike cleared level deviation and Mode S selected level deviation, RAM is available for all correlated aircraft and does not depend on the onboard equipment (transponder). Naturally, its effectiveness depends on the planned trajectory and its prompt update with any clearances issued.
On the situation display, the RAM is normally represented in the track label of the aircraft concerned. This is either done using a dedicated symbol or by a string above the label (e.g "RAM", "DEV", etc.) and a conspicuous colour (e.g. yellow, orange, or red) is used to draw the controller's attention.
The pictures above represent a variety of methods to indicate RAM. These are not taken from a real ATS system and are for representative purposes only.
- A "RAM" string above the track label
- Highlighted background of the route field in the aircraft label (containing the POINT waypoint)
- Arrows around the route field in the aircraft label and using yellow colour for that field
There are various reasons for the activation of a RAM alert. While some of them indicate a clearance non-compliance, this is not always the case. Therefore, in case of such warning, the controller must first identify the cause and then take appropriate action. A non-exhaustive list of such situations includes:
- The controller issues a route clearance (vectoring, direct routing, etc.) but for some reason (e.g. workload) does not input it into the system. When the aircraft complies with the new clearance, the system will see that as a deviation.
- The flight plan information is not up-to-date. The system will initially construct the trajectory based on FPL information. If a change (CHG) message has been sent by the operator but it has not been received by the ATS unit, then the trajectory may be created based on obsolete data. Naturally, following the flight planned route will result in a deviation alert.
- The controller updates the trajectory with the intended clearance but for some reason does not issue it to the pilot. This may be due to e.g. distraction by another (more urgent) situation, or due to the fact that the trajectory is modified by the planner controller while the executive controller is busy doing something else (poor teamwork).
- The system fails to auto-update the trajectory. In this case the path flown by the aircraft will differ from the calculated one so an alert will be displayed. The controller will be able to compensate by manually updating the trajectory.
- The pilot deviates from the current flight plan in order to avoid adverse weather
- The pilot mistakenly accepts a clearance intended for another aircraft due to callsign confusion
The main consequence of RAM is that the trajectory recalculation for the flight is no longer valid. This results in the unavailability/unreliability of a number of ATS system features, such as:
- MTCD. As this tool is trajectory-based, an invalid trajectory would result in false alarms and missed conflicts. This leads to increased controller workload and reduced capacity.
- Estimate calculation and OLDI messages. A wrong trajectory makes it impossible for the system to accurately calculate and update the estimates for the exit points.
- Colour-coding. ATS systems, especially in multi-sector (en-route) units, use different track label colours to focus controller attention on important flights and dim those that are of little or no interest. In case of deviation, however, it is likely that inappropriate colour will be used, which could lead to e.g. a flight unexpectedly entering the sector and being in conflict with another aircraft or an [[SUA]].