Probe (What-if) Controller Tool

Probe (What-if) Controller Tool


Probe (what-if) is a tool that allows the controller to test whether a planned clearance would cause a conflict without making any real change to the system.

Principle of Operation

Probe generally shares the same human-machine interface with the clearance tools (trajectory modification, level allocation, etc.). The controller is presented with a choice - either enter the modification or trigger a what-if check. This may be done e.g. by using different mouse buttons (left=execute, right=probe) or by adding a special button to all input features (if the button is pressed, then use Probe, if not - proceed with the change) or some other solution.

If Probe is chosen, the software creates a new, temporary trajectory and sends it to the MTCD subsystem. The MTCD processes the proposed change and provides information about the potential conflicts (or lack thereof) that would happen should the clearance be issued. The controller may then choose to proceed with the clearance or to go for an alternate plan.

Picture 1 - The controller modifies the trajectory and sends the result to the Probe function. Picture 2 - The Probe function has found a potential conflict, the controller is about to cancel the modification.

Probe-like features have been available in the ATS systems for some time. For example, inputting a cleared level (without issuing a clearance) would send the controller input to the MTCD subsystem. If a conflict is found, the controller may revert to the previous value. Therefore, this feature may be considered as a what-if predecessor. The difference between those and the "real" Probe function is that the latter presents the results of the two situations (actual and hypothetical) at the same time to allow comparison. Also, undoing the change when using Probe could take considerably less time in case of route modification.


Probe has the potential to increase safety and reduce controller workload in some situations as it:

  • Allows the controller to test a (complex) clearance before making any real changes or cause unwanted coordinations.
  • Provides the controller with a representation of the actual and a would-be situation simultaneously.
  • Provides one-click option for confirmation or rejection of the new trajectory.
  • Provides defence against blind spots.

Issues and Limitations

  • Probe shares any MTCD-related issues. If the latter often shows false alarms the controllers may refrain from using it as they would not want to put extra effort and get unreliable results.
  • Probe is a fairly complex calculation and therefore may require a few seconds to complete. This may be too long for the controllers to wait as they are usually quickly switching between tasks.
  • Probe is subject to uncertainty due to the clearance not being issued yet (unlike e.g. MTCD, TCT, etc. which use the current flight parameters). Therefore, it is not suitable for situations where a mile or two make the difference between a safe situation and loss of separation.
  • Depending on local implementation, Probe may only be applicable to vertical clearance and not be available for horizontal manoeuvres.
  • Double controller input is required (one for Probe activation and another for the confirming the modification).
  • HMI output may not always be easy to interpret, e.g. if there are several conflicts in the original trajectory and several other on the one being tested, the screen may become too cluttered.

Related Articles


SKYbrary Partners:

Safety knowledge contributed by: