Optimisation Approaches for Robust Airline Crew Scheduling

To use aircrew members efficiently and economically airlines are required to solve many optimisation problems. One of these is the crew pairing problem or Tour-of-Duty planning problem. In the crew pairing problem, each flight in a pre-determined flight schedule has a fixed origin, destination, scheduled time of departure and arrival. The scheduled flights have to be combined into pairings or Tours-of-Duty (ToDs) that a single crew member can operate. The pairings have to respect contract and legal regulations, and need to be constructed in such a way that each flight is in exactly one ToD. Optimisation tools to find minimal cost sets of ToDs have been implemented in the airline industry for many years. The crew pairing problem is solved well before the flight schedule becomes operational. In this planning stage, all flights are assumed to have departure times that are both fixed and known. This assumption is often proven wrong in actual operation, and the cost minimised crew schedule often cannot be operated as planned. Pairings in a cost minimised crew schedule often include short ground times between consecutive flights. This is because pairings are usually less expensive if crews spend less time on the ground between arrival and departure of two consecutive flights, hence the total working or operating hours are maximised. Such crew schedules happen to become sub-optimal in actual operation, as they are easily disrupted and chain impacts of delays are usually found as a result. If an airline provides connection times between consecutive flights which just satisfy the minimum time legally required, a late arriving flight will cause the subsequent flights to depart late. Not only will the downstream flight which operates on the delayed aircraft depart late, but also the late arriving crew members who are changing aircraft will board late for their outgoing flights. After a few aircraft changes, many flights may be delayed by the initially minor delay. Disruptions may result in flight cancellation and/or may require the use of reserve crews to get back on schedule and originally scheduled crew might not be able to continue their duty because of rule violations. As a result, substantial unplanned costs, such as overtime, fuel costs and compensations for parking and passengers with delayed or canceled flights, can be incurred. Therefore, airlines do not only require minimum cost solutions, but are also very interested in robust solutions. The robust ToD planning problem is the problem of obtaining aircrew schedules in planning that are not necessarily optimal in terms of the planned crew cost but that yield low actual cost in operation. Approaches to robust aircrew scheduling have been developed only recently using different measures of robustness. At the beginning of this thesis (Chapters 1 to 4), we provide the reader some background material on the airline scheduling process and crew scheduling. In Chapter 5, we give an overview of disruption management and review the literature on robust airline scheduling. In Chapter 6, we study and analyse some historical data provided by a New Zealand domestic airline and build a statistical flight delay model for the comparison of robust crew schedules in Chapter 7.