Stressing Symbolic Scheduling Techniques within Aircraft Maintenance Optimization

Scheduling, or planning, is widely recognized as a very important step in several domains such as high level synthesis, real-time systems, and every-day applications. Given a problem described by a number of actions and their relationships, finding a schedule, or a plan, means to find a way to perform all the actions minimizing a specific cost function. The goal of this paper is to develop, analyze and compare different scheduling techniques on a new scheduling/planning problem. The new application domain is aircraft maintenance. It shares with previous ones the underlying problem definition, but it also unveils brand new challenging characteristics, and a different optimization target. We show how to model the problem in a suitable way, and how to solve it with different methodologies going from Satisfiability solvers and Binary Decision Diagrams, to Timed Automata and Coloured Petri Nets. New ideas are put forward in the different domains having efficiency and scalability as main targets. Experimental results stress the different techniques, showing their application range and limits, and defining advantages and disadvantages of the underlying models. Overall, general-purpose tools have been easily applied to our problem, but failed as far as efficiency was concerned. The satisfiability-based approach proved to be faster and more scalable, being able to solve instances 3− 4 times larger. To sum up, our contributions range from modeling the aircraft maintenance problem as a scheduling instance, to coding this problem with home-made and general-purpose tools, to dovetailing exact and heuristic techniques, and comparing these techniques in terms of efficiency and scalability.