In operational airline crew scheduling, one of the major tasks is to allocate flight activities to individual crew members. This task is referred to as the crew rostering problem; an optimization problem that aims to assign flights to crew members in a cost-effective way, while also considering preferences of crewmembers. Flights are combined into multiple-day activities called pairings that consist of flight duty days and rest period days. For intercontinental long-haul destinations specifically, pairings may take more than a week. During flight duty days of these long-haul pairings, crew members are not situated at the airline base location. During rest period days, however, they have the opportunity to be at their homes or with their families. The pairings that crew members on long-haul crew divisions operate, thus, directly impact their life
apart from work as well as the locations at which they are situated during flight duty days. Therefore, crew preferences are highly valued by crew members, as these provide their only means to influence duties. This is a critical reason for crew preferences to be a recurrent topic in negotiations on collective labour
agreements between crew unions and airlines that influence crew resources. As crew costs are the biggest expense for airlines next to fuel (Belobaba et al., 2015), crew preferences are considered in an environment that is predominantly designed to manage crew resources as efficiently as possible. For long-haul cockpit crew in European airlines, crew preferences can be expressed via requests for the operation of specific pairings. However, the problem with these pairing requests is that the requests concern pairings that commence months later than the pairings considered by the crew rostering problem. This
problem is typically solved only one month before operation of the pairings. It is impractical to consider the pairing requests as part of the crew rostering optimization problem, since the problem size of the optimization problem becomes too large to be solved within practical computation time. In other words, a decision on whether or not to grant a certain pairing request has to be made with limited information available on the consequences of that decision since it concerns a part of the schedule that is not optimized for. When a pairing request is granted for a crew member, this invokes a pre-assigned pairing for this crew member. Such pre-assigned pairings constrain the construction of this crew member’s roster in the crew rostering problem, as only a limited number of options exist to efficiently construct this crew member’s roster. When many of these pre-assigned pairings exist in the schedule, this leads to inefficiencies in the schedule. Such inefficiencies cause loss days without productive activity in the schedule, which are an indicator of inefficient use of crew resources leading to higher crew costs.