A decision support system needs to know the current state of the departure system in order to make a decision about the re-sequencing. Aircraft in the holding area should be included. Aircraft on the taxiways can optionally be included. Due to the separation rules, aircraft that have recently taken off need to be kept in the system until they can no longer affect the take-off times of the aircraft awaiting take-off. When large minimum departure intervals are in force on some departure routes this may mean keeping aircraft within the system for up to ten minutes or more after departure.
It is envisaged that all of the data below will be available to a real decision support system, for instance through an interface with existing systems such as the electronic strips the controllers use to record the chosen take-off sequence or the ground radar aircraft tracking systems. There may be a degree of uncertainty associated with some data, such as taxi times. The accuracy of taxi time predictions could be improved by implementing an intermediate prediction system between the ground positional data system and the decision support system. Such a system could consider taxiway congestion and the current positions and taxi speeds of aircraft in order to improve the accuracy. The design of such a system is beyond the scope of this thesis.
The available data should include:
• The weight class of the aircraft, so that wake vortex separations can be determined. • The speed group and allocated departure route of the aircraft, so that route-based separa-
tions can be determined.
• Any CTOT take-off time-slot associated with the aircraft. • The take-off time of the aircraft if it has already taken off.
• The arrival time at the holding area if the aircraft is within the holding area. So that the delay at the holding area can be predicted.
• The predicted (possibly inaccurate) arrival time and holding area entrance for any taxiing aircraft, if taxiing aircraft are being included in the system.
• The position of the aircraft in any previously chosen take-off sequence, enabling sequences that are closer to previous suggestions or closer to a controller’s current plans to be pre- ferred.
• The current position of any aircraft that is within the holding area. The manoeuvring available to these aircraft is, obviously, more restricted than that available to aircraft still on the taxiways as aircraft within the holding area may have already passed decision points where paths through the holding area diverge.
• Optionally, any fixed paths through the holding area. The simulation used to test the system (described in chapter 7) fixes the paths at the point aircraft reach the holding area and provides this information to the decision support system. A live system need not be so inflexible but could estimate the allocated paths from the movement of the aircraft or could allow it to be specified by controllers if this is a desirable feature.
The push-back time was not included in the list above but could be made available and used to favour take-off sequences which are more equitable in terms of time from stand to take-off rather than aiming for equity of holding area delay. This was not used by the decision support system presented in this thesis, although the simulation used to evaluate the performance of the system (described in chapter 7) does require the push-back time information. It is used to determine when to add aircraft into the system and for simulating a taxi time prediction system with specific levels of uncertainty.
2.15
System Outputs
In a live situation, with a real runway controller present, the decision support system would display to the controller only the suggested take-off sequence, and possibly an estimated take-off time. One approach could be to annotate the electronic strips that the controllers currently use with a suggested take-off sequence number. If a separate display is used it may be useful to also show the allocated CTOT and the characteristics of the aircraft so that the controller can immediately judge the value of the suggested sequence without having to cross-reference two different displays.
When running with the simulation described in chapter 7, the decision support system has to provide more information as it is effectively acting as a runway controller simulation. This means that it has to provide information about the instructions to be given to pilots, and it even predicts the movement of the aircraft within the holding area in response to the sequencing decisions.
2.16
Other Issues
The runway controller has to perform a difficult task under tight time constraints. In many cases, the controller will be weighing the effects of contradictory constraints in an attempt to find the best take-off sequence. Objectives such as improving throughput or delay must often be weighed against complying with the CTOTs and against controlling the amount of overtaking.
The system is (and can only be) intended to be advisory. The runway controller has a lot of information available and some of it may be hard to capture for input into an automated system. A decision support system will also need to accept some kind of feedback from the controller so that the controller can reject or correct schedules with which they disagree.
Finally, the departure process is a dynamic system where aircraft are removed from the system at some point after take-off and more aircraft are added as they are released from the stands. Any system will need to respond to changing situations, for instance aircraft being delayed longer than expected or developing problems.