Philip de Bruin | Searcharxiv

arXiv2026

Simulation Strategies for an Efficient Local Search to solve Stochastic Scheduling Problems

In scheduling problems, deterministic task durations are often assumed. This usually does not capture reality and may lead to schedules that are not robust to (small) changes to these task lengths. The use of stochastic task durations therefore seems preferable. Including these in local search, which is the way to find good solutions for difficult scheduling problems, is not straightforward, though. The objective value becomes stochastic then too, and computing the expected value is often not possible. One way out of this it to approximate this value by using simulation. This is quite easy to implement in a local search algorithm, but it may require many simulations each iteration to get a reliable estimate. Hence such an approach comes with a performance penalty. In this paper, we study techniques to limit the number of simulations. Besides comparing known techniques, we propose our own method for this, which is based on $t$-tests. We evaluate these techniques on the Stochastic Parallel Machine Scheduling Problem and the Stochastic Electric Vehicle Scheduling Problem. In these case studies, we show the effectiveness of using such methods to reduce runtime while retaining solution quality. Our method using $t$-tests turns out to be most effective in both problems.

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arXiv2025

Integrated Airline Fleet and Crew Recovery through Local Search

Airline operations are prone to delays and disruptions, since the schedules are generally tight and depend on a lot of resources. When disruptions occur, the flight schedule needs to be adjusted such that the operation can continue. Since this happens during the day of operations, this needs to be done as close to real time as possible, posing a challenge with respect to computation time. Moreover, to limit the impact of disruptions, we want a solution with minimal cost and passenger impact. Since airline operations include many interlinked decisions, an integrated approach leads to better overall solutions. We specifically look at resolving these disruptions in both the aircraft and crew schedules. Resolving these disruptions is complex, especially when it is done in an integrated way, i.e. including multiple different resources. To solve this problem in an integrated manner, we developed a fast simulated annealing approach. To the best of our knowledge, we are the first to develop a local search approach to resolve airline disruptions in an integrated way. This approach is compared with traditional approaches, and an experimental study is done to evaluate different neighbour generation methods, and to investigate different recovery scenarios and strategies. The comparison is done using real world data from KLM Royal Dutch Airlines. Here, we show that our approach resolves disruptions quickly and in a cost-efficient manner, and that it outperforms traditional approaches. Compared to naive delay propagation, our method saves 40% in non-performance costs. Moreover, while most airlines use tools that consider resources separately, our approach shows that integrated disruption management is possible within 30 seconds.

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