Analysis of Decision Postponement Strategies for Aircraft Assignment under Uncertainty

The ability to effectively match supply and demand can lead to significant revenue benefits in the airline industry. Airline supply management deals with assigning the right resources (i.e., aircraft and crew) to the right routes in the flight network. Due to certain crew regulations, operating characteristics, and constraints of the airline companies, these supply management decisions need to be made well in advance of departures, at a time when demand is highly uncertain. However, demand forecasts improve markedly over time, as more information on demand patterns is gathered. Thus, exploiting the flexibilities in the system that allows the partial postponement of supply decisions to a later time, when more accurate demand information is obtained, can significantly improve the airline's revenue.

In this thesis, we propose and analyze the Demand Driven Swapping (DDS) approach that aims at improving the airline's revenue by reducing the supply-demand mismatches through dynamically swapping aircraft as departures approach. This research has been done in collaboration with our industrial partner, the United Airlines Research and Development Division.

Due to the proximity to departures, the DDS problem is restricted by two main constraints:

1. the initial crew schedule needs to be kept intact (due to certain union contracts); and
2. airport services and operations need to be preserved to the greatest extent possible.

As a result, only a limited number of simple swaps can be performed between aircraft types of the same family (i.e. crew-compatible aircraft types). However, the swaps can be potentially performed on a daily basis given the initial fleet assignments. Clearly, the swapping criteria, frequency, and timing will highly impact the revenue benefits of the DDS approach. When the swapping decisions are made several weeks prior to departures (i.e., 4-6 weeks before departures), they will not cause much disturbance to the operations, but will be performed under highly uncertain demand information. On the other hand, swapping decisions that are delayed to a time later (i.e., 1-3 weeks before departures) will decrease the possibility of bad swaps, but will result in larger costs due to the higher disruptions to airport services and operations.

Thus our research objective is to provide guidelines and principles on how the flexible capacity should be managed in the system. For this purpose, we study the effectiveness of different swapping strategies, characterized in terms of their frequency and timing, for hedging against the demand uncertainty.

We first study stylized analytical models to gain insights into the critical parameters that affect these benefits. Simulation models are then conducted to test the validity of our analytical findings as well as to analyze more complex strategies and assess the dynamic performance of these strategies.

The analytical results indicate that strategies that make the swapping decision early in time (in order to minimize disturbances to the operations) perform very well on routes, where the demand uncertainty is low and the expected demands on the legs are well-balanced. Otherwise, a swapping strategy, which revises the swapping decision over time, should be implemented. Our simulation results, based on real data obtained from United Airlines, confirm the analytical findings.