The daily tail assignment problem under operational uncertainty using look-ahead maintenance constraints

The tail assignment problem is a critical part of the airline planning process that assigns specific aircraft to sequences of flights, called lines-of-flight, to satisfy operational constraints. The aim of this paper is to develop an operationally flexible method, based upon the one-day routes business model, to compute tail assignments that satisfy short-range—within the next three days—aircraft maintenance requirements. While maintenance plans commonly span multiple days, the methods used to compute tail assignments for the given plans can be overly complex and provide little recourse in the event of schedule perturbations. The presented approach addresses operational uncertainty by using solutions from the one-day routes aircraft maintenance routing approach as input. The daily tail assignment problem is solved with an objective to satisfy maintenance requirements explicitly for the current day and implicitly for the subsequent two days. A computational study will be performed to assess the performance of exact and heuristic solution algorithms that modify the input lines-of-flight to reduce maintenance misalignments. The daily tail assignment problem and the developed algorithms are demonstrated to compute solutions that effectively satisfy maintenance requirements when evaluated using input data collected from three different airlines.     

Lösung von Randwertproblemen der Schwingungsgleichung für ein Modell eines Triebwerkeinlaufs

We consider a simple model of a jet engine inlet consisting of two coaxial cylinders. The inner cylinder extends in both directions to infinity and represents the hub of a jet engine. The outer cylinder is assumed to be semiinfinite representing the engine's jacket. This arrangement is approached by a compressible and invisced gas moving at a velocity U less than the speed of sound. Two problems are formulated. In problem A a normal speed distribution is given in the compressor-inlet plane, in problem B we are given a pressure distribution. We look for the induced pressure and velocity fields. Using the acoustical approximation of the fundamental equations the Wiener-Hopf-method is applied then leading, after a multiplicative and additive decomposition procedure, to an infinite system of linear equations. These involve the expansion coefficients of the solution with respect to the eigenfunctions of the concentric cylindrical duct. Applying arguements from perturbation theory it is possible to extract the essential information from this system of equations. Apart from the possible exception of a denumerable set of values of the distance L of the blade-row from the inlet the problem has a unique solution.     

军用飞机维修质量管理的最优线性分派评估

在军机维修工作中,科学有效的管理,对及时完成维修任务,保障训练作战计划至关重要.在建立适合我军军机维修质量评估指标体系的基础上,定义了理想方案和贴近度,给出了排序频数的计算方法,进而将军机维修质量评估问题转化为最优线性分派问题来处理,从而为军机维修质量管理提供了一种科学、可靠的决策方法.     

COMPASS—the verification and validation of an operational analysis model for use in the prediction of Nimrod MRA4 operational effectiveness

Comprehensive Maritime Patrol Aircraft Systems Simulation (COMPASS) is a mathematical model of the Nimrod MRA4 weapon system that is being developed to enable the prediction of operational effectiveness. COMPASS will be used throughout the Nimrod MRA4's development life cycle to conduct tactics development and weapon system performance evaluation. Following Nimrod MRA4's entry into service, the COMPASS model may be used by the customer, the Ministry of Defence (MoD) Defence Procurement Agency (DPA), to assess operational effectiveness of potential system upgrades and to evaluate the effect of changes to the threat or operating environment. BAE SYSTEMS is developing tactical scenarios to test operational performance. The operational requirement is decomposed, at the highest practicable system level, into metrics that can be measured easily under test or trials conditions. Having quantified a system's capabilities in these lower level metrics, COMPASS can be modified to reflect them accurately. COMPASS uses these tactical scenarios to simulate the Nimrod MRA4 undertaking a number of Anti Submarine Warfare (ASW) and Anti Surface Unit Warfare (ASuW) activities. This paper describes the philosophy and process being applied to the development of a fully verified and validated COMPASS mathematical model. It also details the approach to using COMPASS to determine Nimrod MRA4 operational effectiveness through the construction of tactical scenarios.     

Operational aircraft maintenance routing problem with remaining time consideration

The aircraft maintenance routing problem is one of the most studied problems in the airline industry. Most of the studies focus on finding a unique rotation that will be repeated by each aircraft in the fleet with a certain lag. In practice, using a single rotation for the entire fleet is not applicable due to stochasticity and operational considerations in the airline industry. In this study, our aim is to develop a fast responsive methodology which provides maintenance feasible routes for each aircraft in the fleet over a weekly planning horizon with the objective of maximizing utilization of the total remaining flying time of fleet. For this purpose, we formulate an integer linear programming (ILP) model by modifying the connection network representation. The proposed model is solved by using branch-and-bound under different priority settings for variables to branch on. A heuristic method based on compressed annealing is applied to the same problem and a comparison of exact and heuristic methods are provided. The model and the heuristic method are extended to incorporate maintenance capacity constraints. Additionally, a rolling horizon based procedure is proposed to update the existing routes when some of the maintenance decisions are already fixed.     

Object-Oriented Modelling of Military Communications Networks

Military communications networks are large complex systems; their physical dynamics, composition, workload characteristics and operational goals make them very hard to model and analyse. It is particularly difficult to build a generalized model that can adequately fit each instance, generate a workload that exercises the network in a realistic way, and measure network effectiveness in terms of the operational goals. In this work, we describe our approach to building such a model for the US Marine Corps, and demonstrate the model's use by solving a particular equipment allocation problem. We combine object-oriented simulation with new concepts for workload modelling and measuring effectiveness that are based on the operational tasks the Marines perform.     

Gravity and the spiral model

The Spiral model, a generalization of Feynman's Chessboard model, is considered as a candidate for a realistic subquantum dynamical system. It can be shown that Dirac dynamics describe a subspace of the full dynamics of the model. Wave packet reduction and response to macroscopic measurement are both features which are being tested numerically. Assuming the model can mimic reduction, several new possibilities arise in the relation between quantum mechanics and gravity.     

A branch-and-price approach for operational aircraft maintenance routing

In recent years, considerable effort in the field of operations research has been paid to optimizing airline operations, including the logistics of an airline’s fleet of aircraft. We focus on the problem of aircraft routing, which involves generating and selecting a particular route for each aircraft of a sub-fleet that is already assigned to a set of feasible sequences of flight legs. Similar studies typically focus on long-term route planning. However, stochastic events such as severe weather changes, equipment failures, variable maintenance times, or even new regulations mandated by the Federal Aviation Administration (FAA) play havoc on these long-term plans. In addition, these long-term plans ignore detailed maintenance requirements by considering only one or two of the primary maintenance checks that must be performed on a regular, long-term basis. As a result, these plans are often ignored by personnel in airline operations who are forced on a daily basis to develop quick, ad hoc methods to address these maintenance requirements and other irregular events. To address this problem, we develop an operational aircraft maintenance routing problem formulation that includes maintenance resource availability constraints. We propose a branch-and-price algorithm for solving this problem, which, due to the resource constraints, entails a modification of the branch-on, follow-on branching rule typically used for solving similar problems. Through computational testing, we explore the efficiency of this solution approach under a combination of heuristic choices for column (route) generation and selection.     

Canadian forces global reach support hubs: facility location and aircraft routing models

The Canadian Forces (CF) is seeking to establish permanent and temporary operational support hubs at strategic locations around the globe to improve its logistics support effectiveness and responsiveness for deployed operations. This paper addresses two logistics problems associated with the hub-based support concept, namely, hub location optimization and aircraft routing problems. A discrete facility location model was developed to analyse the hub-based support effectiveness and to determine the optimal hub locations. An aircraft routing model was also developed to determine optimal aircraft routes for the movement of cargo and supplies from various support hubs to a theatre of operation. Both models were formulated using mixed integer nonlinear programming. Historical CF deployment and sustainment data were used to conduct the analysis and to illustrate the methodology. The study indicates that the hub-based support approach would offer potential cost avoidance on sustainment lift and could be an effective strategy for improvement of the CF's support capability. It also indicates that potential lift costs could be avoided through optimal routing of sustainment flights.     

Strategic planning in fractional aircraft ownership programs

In the fractional ownership model, the partial owner of an aircraft is entitled to certain flight hours per year, and the management company is responsible for all the operational considerations of the aircraft and for making an aircraft available to the owner at the requested time and place. In the recent years although the industry as a whole has experienced significant growth, most of the major fractional jet management companies have been unprofitable. To increase profitability a management company must minimize its operating costs and increase its crew and aircraft utilization. In this paper, we present a methodology for efficiently scheduling the available resources of a fractional jet management company that takes into consideration the details in real world situations. We then discuss several strategic planning issues, including aircraft maintenance, crew swapping, demand increase and differentiation, and analyze their effects on the resource utilization and profitability.     

Continuous management of airlift and tanker resources: A constraint-based approach

Efficient allocation of aircraft and aircrews to transportation missions is an important priority at the USAF Air Mobility Command (AMC), where airlift demand must increasingly be met with less capacity and at lower cost. In addition to presenting a formidable optimization problem, the AMC resource management problem is complicated by the fact that it is situated in a continuously executing environment. Mission requests are received (and must be acted upon) incrementally, and, once allocation decisions have been communicated to the executing agents, subsequent opportunities for optimizing resource usage must be balanced against the cost of solution change. In this paper, we describe the technical approach taken to this problem in the AMC barrel allocator, a scheduling tool developed to address this problem and provide support for day-to-day allocation and management of AMC resources. The system utilizes incremental and configurable constraint-based search procedures to provide a range of automated and semi-automated scheduling capabilities. Most basically, the system provides an efficient solution to the fleet scheduling problem. More importantly to continuous operations, it also provides techniques for selectively reoptimizing to accommodate higher priority missions while minimizing disruption to most previously scheduled missions, and for selectively “merging” previously planned missions to minimize nonproductive flying time. In situations where all mission requirements cannot be met, the system can generate and compare alternative constraint relaxation options. The barrel allocator technology is currently transitioning into operational use within AMC's Tanker/Airlift Control Center (TACC). A version of the barrel allocator supporting airlift allocation was first incorporated as an experimental module of the AMC's Consolidated Air Mobility Planning System (CAMPS) in September 2000. In May 2003, a new tanker allocation module is scheduled for initial operational release to users as part of CAMPS Release 5.4.     

Results from Bifurcation Analysis in Ship Dynamics

To assure safe ship operations on one hand side and to reduce operational restrictions to a minimum a proper knowledge of the ship's behavior in waves is necessary. One approach to achieve this is to generate an exact mathematical model of a ship and the fluid–structure–interaction in order to determine dangerous and safe operational conditions by analyzing the behavior with present numerical methods from nonlinear dynamics theory. Comparison between numerical simulations and experimental results as well as two sets of save and dangerous conditions for a given model of a real ship are shown. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Scheduling charter aircraft

We present a system that is used for scheduling charter aircraft. At the core of the system is an elastic set partitioning model that is embedded in a decision support system. The model assigns a set of flights that have to be performed to the available fleets of aircraft at minimal cost while satisfying all operational requirements. Flights that cannot be accommodated by the available fleets are sold off to other operators. The minimised costs include the cost of flying the aircraft, the cost of selling off flights, and penalties on violations of soft constraints. The system has been in daily operation for almost a year, and it provides high quality schedules and saves numerous hours to the schedulers.     

Dynamic preventive maintenance scheduling of the modules of fighter aircraft based on random effects regression model

Proper maintenance of fighter aircraft is an important issue to control theairpower. Typical maintenance policy applied is based on the constant schedulefor a given module. This kind of maintenance does not take into account varyingcharacteristics of the module over time. In this paper, we utilize the randomeffects Weibull regression model for non-constant MTBF (mean time betweenfailure) and MTTR (mean time to repair) in order to provide a dynamic preventivemaintenance schedule reflecting the module's varying characteristics in atimely manner. Our study is expected to contribute to ROKA (Republic of KoreaAirforce) in terms of improving the level of combat readiness of fighteraircraft.     

Visualization in mechanics: the dynamics of an unbalanced roller

It is well known that mechanical engineering students often find mechanics a difficult area to grasp. This article describes a system of equations describing the motion of a balanced and an unbalanced roller constrained by a pivot arm. A wide range of dynamics can be simulated with the model. The equations of motion are embedded in a graphical user interface for its numerical solution in MATLAB. This allows a student's focus to be on the influence of different parameters on the system dynamics. The simulation tool can be used as a dynamics demonstrator in a lecture or as an educational tool driven by the imagination of the student. By way of demonstration the simulation tool has been applied to a range of roller–pivot arm configurations. In addition, approximations to the equations of motion are explored and a second-order model is shown to be accurate for a limited range of parameters.     

Optimization of the design of recuperative heat exchangers in the exhaust nozzle of an aero engine

Today the needs for safer, cleaner and more affordable civil aero engines are found to be of great importance. Five years ago, the EU initiated an action for the design and the construction of efficient and environmentally friendly aero engines (EEFAE). One of the major European gas turbine industries, MTU, has presented a new technology for an advanced aero engine design, which uses an alternative thermodynamic cycle. The basis of this cycle is the adoption of a recuperation part with the use of a system of heat exchangers, installed in the exhaust nozzle of the aircraft engine. Thermal energy in the turbine exhaust is used in the recuperator to pre-heat the compressor outlet air before combustion. The benefits of this technique are focused on reduced pollutants and decreased fuel consumption. In this work, the procedure of the optimization of this installation, by means of the imposed pressure drop downstream the aircraft engine and the balanced mass inflow to the heat exchangers is presented. The optimization is based on experimental measurements in laboratory conditions and preliminary 2D CFD modeling for the flow inside the exhaust duct and through the heat exchangers. It is shown that with a careful approach, a better arrangement of the heat exchangers can be achieved in order to have a minimum pressure drop in the exhaust nozzle which can positively affect the engine’s performance.     

Seeking optimum project duration extensions

It is well-known that changes to a project's definition during the course of the project can cause significant disruption, and greatly extend a project's duration if no extra resources are committed to the project. However, it is also well-known that the opposite policy, of throwing as many resources as possible at the problem to try to keep to the original schedule, can be very expensive, and is often in fact counter-productive ($2000 hour). The underlying causes of the results for these two extreme policies are systemic, are often hard to quantify, and are often significantly under-estimated. This paper describes the system dynamics technique for modelling a project, using as a case-study a model drawn up for a delay and disruption claim; this model closely reflected the project both as budgetted and as actually occured. It then shows how this model could be used to find an optimum trade-off between the two extreme policies, giving an optimal project extension.     

Prioritization of medical equipment for maintenance decisions

Clinical engineering departments in hospitals are responsible for establishing and regulating a Medical Equipment Management Program to ensure that medical devices are safe and reliable. In order to mitigate functional failures, significant and critical devices should be identified and prioritized. In this paper, we present a multi-criteria decision-making model to prioritize medical devices according to their criticality. Devices with lower criticality scores can be assigned a lower priority in a maintenance management program. However, those with higher scores should be investigated in detail to find the reasons for their higher criticality, and appropriate actions, such as ‘preventive maintenance’, ‘user training’, ‘redesigning the device’, etc, should be taken. In this paper,we also describe how individual score values obtained for each criterion can be used to establish guidelines for appropriate maintenance strategies for different classes of devices. The information of 26 different medical devices is extracted from a hospital's maintenance management system to illustrate an application of the proposed model.     

Reversible dynamics in discrete dissipative systems

The random walk model of Brownian motion is an example of a stochastic system which exhibits intrinsically irreversible behaviour. In spite of this, a simple discrete version of the model has been shown to harbour dynamics which are reversible and are described by a discrete form of Schrödinger's equation. The reversible dynamics appear as second order effects in this diffusive model, and the usual relationship between macroscopic irreversibility and microscopic reversibility is itself reversed. This will be discussed in the context of the `Brussels' school' on irreversibility.