ABSTRACT
Aircraft mechanical problems, severe weather, crew sickness, airport curfews, and security are among the problems that force an airline to delay or even cancel their regular published flights. On an average day in the United States, approximately 15–20% of all flights experience significant delays (more than 15 minutes) and approximately 1–3% of all flights are cancelled (Yu et al. 2003). The scheduling methodologies described in chapters 6 and 7 provide an airline with a very efficient plan, high utilization of resources, and very tight aircraft and crew assignments. In many cases a small perturbation in this plan, such as unavailability of an aircraft or crew, results in major disruption to the scheduled flights. The airlines adopt a combination of tactics such as flight delays, flight cancellations, aircraft substitutions, ferry flights (flying an empty aircraft to a point of need), and aircraft diversions to return to their published scheduled flights as soon as possible. Since these activities are not pre-planned and occur only when there is a disruption in the schedule, they are called irregular operations. The recovery time can span from the time the disruption occurs up to the time the airline gets back to its original schedule.
