The search for slots

Helsinki airport

Finding more capacity in our airports has become an urgent necessity internationally. Professor Konstantinos G. Zografos at Lancaster University Management School and Principal Investigator of the OR-MASTER project, explains how this new £2.8 million project is developing a next generation approach to improve slot scheduling efficiency, freeing up capacity without relying only on physical expansion.

Helsinki airportDemand is expected to exceed current capacity in Europe by as many as 2.3 million flights (or 11%) according to the ‘most-likely’ growth forecast scenarios for 2030 at 138 Eurocontrol airports . What might be good news in terms of business growth for other commercial sectors is a serious problem for every provider associated with airports, every customer – and society as a whole. Almost one third of flights were delayed in the ECAC (European Civil Aviation Conference) area in 2013, with the average delay per delayed flight exceeding 26 minutes . Schedule disruptions and delays are exacerbated when busy airport pairs are involved. Almost half of flights delayed in 2013 for approximately 30 minutes were in the most affected airport pairs (for example, London Heathrow), according to 2014 Eurocontrol figures. Across the EU there were estimated to be 10.8 million minutes of delays in 2012 as a result of air traffic management issues. The cost was around €11.2 billion to airspace users and passengers, and 7.8 million tonnes of wasted CO2 .

The complexity of building new airport capacity – entangled with political, physical and institutional constraints in individual countries – means new, sustainable approaches are needed to make the most efficient use of existing resources. The drive to optimise the allocation of resources has been happening, of course, for many years. The airport community alongside policy makers and researchers have been working on capacity based around the framework of voluntary scheduling guidelines developed and evolved over the years under the auspices of IATA.

To understand why the current approach isn’t delivering the levels of efficiency needed in an age of congestion, it’s important to understand how this works. A slot identifies a time interval, specific date and time, during which a carrier is permitted to use the airport infrastructure for landing or take-off at a slot-controlled airport. A fundamental concept in the slot allocation process is the ‘declared capacity’ and particularly its rationing and allocation on the basis of a complicated set of administrative rules, criteria, and priorities within an airport. Declared capacity represents an administrative measure of capacity specifying and controlling the number of slots available for allocation per unit of time. As a result, slot scheduling and the setting of optimum declared capacity levels are closely interdependent and both lie at the heart of optimising the allocation and use of scarce airport resources.

The existing slot allocation process does not properly match requested slots with those actually allocated to airlines. On top of that, slot misuse then sharpens the capacity shortage due to poor use of the scarce capacity. Even at airports where slot demand exceeds capacity, over 10% of the allocated slots go unused . ACI Europe estimates that slots unused due to their late return account for losses of around €20 million per season at large, congested European airports .

Most recently researchers have attempted to deal with the inefficiencies and limitations of this existing allocation practice by focusing on two alternatives (which have the potential to work together). One is introducing market-driven mechanisms such as congestion-based pricing schemes and auctioning of slots, leading to a situation where there are ‘winners’ and losers’ in the race for capacity. The other is improving the allocation efficiency of the IATA-based allocation mechanism via slot scheduling. Slot scheduling is a challenging area. There’s real potential to generate quick and drastic capacity utilisation improvements, but it involves huge and complex mathematical problems.

The slot scheduling procedures currently in use suffer from limitations. They are very simplistic in terms of accounting for the full range of factors and stakeholders involved, the constraints of operations and regulations; they don’t consider the inherent dependency of slots allocated to a network of airports; there’s no sense of the dynamic nature and uncertainty associated with airport capacity. As a result, the problems are multiplied, as difficulties and inefficiencies in different individual airports all feed into the overall network. Ultimately, slot scheduling procedures rely on an oversimplified interpretation and don’t reflect all complexities the real-world problems involve.

A new major research programme, funded by UK’s Engineering and Physical Science Research Council (EPSRC), will work to build new models, testing new potential solutions to the problem. The OR-MASTER Programme Grant (Mathematical Models and Algorithms for Allocating Scarce Airport Resources) will take into account the views of all stakeholders involved in and affected by the allocation of slots now and as changes in the context occur over time: individual airport operations, networks of airports, airline operations, air traffic management systems, airport authorities, civil aviation authorities, airlines and the travelling public. On a more technical level, this will involve models and algorithms capable of handling the dynamic nature of both demand and capacity, as well as the uncertainty of air transport operations. Most importantly, models will need to reflect the inherent interaction and ‘complementarity’ of slots at the airport network level, the simultaneous consideration of multiple objectives (such as schedule delay, operational/queuing delay, resource utilisation, fairness and equity, environmental externalities); alternative formulations of the objectives (non-linear cost functions for delay, for example) and the constraints (rolling capacity, turnaround, flight connectivity) of the allocation problem, coping with the trade-off between complexity and accuracy of the solution. The research has been funded by the EPSRC (Engineering and Physical Sciences Research Council) and is to be led by a team at Lancaster University Management School, working with Computing, Science and Mathematics researchers at the University of Stirling.

The scale of the computational and mathematical challenges involved shouldn’t be underestimated. The airport scheduling problem has been on the agenda for the scientific community for some time. A UK-based project is important because of its role as a world-leader in Operational Research, the use of advanced analytical techniques to improve decision-making, and its ability to link news ideas and theory to practice in computer sciences.

Over the six years of the project, OR-MASTER will involve close collaboration between Lancaster University, the University of Stirling, and a host of organisations internationally that will support the project providing real-world data, insights and expertise: National Air Traffic Services (NATS) in the UK; Eurocontrol (managing air traffic across Europe); Park Air Systems; KLM Air France; Zurich Airport and Athens International Airport; the research organisations linked to the national air navigation services for Italy and Spain (SICTA and CRIDA); SESAR (Single European Sky research body); the Airport Services Association; Goldair Handling; ACI Europe (Airports Council International); the HALA! SESAR network of leading researchers in Europe working in the area of Air Traffic Management automation; NEXTOR II (National Centre of Excellence for Aviation Operations Research); the MIT International Centre for Air Transport Research (MIT-ICAT) in the USA; and DLR, the German Airspace Research Centre. The project represents a strong partnership between academia, the air transport industry and policy makers.

At Lancaster, OR-MASTER will be undertaken by the Centre for Transport and Logistics (CENTRAL) Research (a newly established Research Centre in the Department of Management Science), and will link into existing Operational Research-related development initiatives and expertise at Lancaster University: the STOR-i doctoral training centre (one of the new generation of Centres for Doctoral Training supported by funding from the EPSRC) and the Lancaster Data Science Institute, which is generating new interdisciplinary approaches to address data-driven research challenges around the world. A key role is being played by Professor Kevin Glazebrook who has had a significant influence on the development of Operational Research in the UK. In 2006, he was the founder and initial director of NATCOR.

The air transport system is working within tight confines that aren’t likely to be loosened to any great extent in the near future. The industry needs to work smarter, take advantage of mathematical models and computational capabilities, to reduce delays and increase schedule reliability – both of which will lead to lower operating costs and improved revenues.

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