In both civil and military aviation, maintenance plays a large role in ensuring continued safe operation and accounts for a significant portion of operating costs. Typically, a conservative planned maintenance (PM) program is initially developed to ensure the aircraft reliability but this often leads to over-maintenance. With more in-service experience, operators seek to customize the maintenance interval accordingly in order to reduce workload and cost without compromising safety. With prevailing use of health usage monitoring systems (HUMS), the maintenance can even transit from PM to condition-based maintenance (CBM) where further safety and costs benefit may be reaped. Whilst some guidance for such changes exists, it remains challenging for maintainers in practice as suggested methods often require significant component failure or test data; which are unavailable or too expensive to obtain. As such, this research reviews the challenges faced by maintainers when extending PM intervals or implementing CBM and seeks ways to support decision making for the changes. For PM, the challenge to extend the maintenance interval with little or no past failure is addressed. Existing reliability methods were reviewed and two improved methods to estimate the reliability lower confidence bounds were developed. The first approach adopts the use of Monte Carlo simulation applied to the Weibull plot equation while the second uses a probabilistic damage accumulation model together with bootstrap techniques. Both methods are used to assess the reliability of extending the replacement interval of a gearbox bearing and are shown to perform better than existing methods as they provide tighter reliability confidence bounds. For CBM, a survey on sensor technologies and diagnostic algorithms showed that vibration-based sensor is most widely used to detect fault. The study then demonstrates a CBM implementation using vibration-based HUMS data from in-service helicopters. Analysis of the FFT spectra shows that the fault patterns corresponding to progressing stages of bearing wear can be clearly observed. The fault patterns are extracted as features for unsupervised classification using Gaussian Mixture Models and used to infer the different bearing health states. Signal detection theory was then applied onto the classified feature to determine the detection thresholds for fault diagnosis. A simplistic prognostic model using trend extrapolation to determine the replacement lead-time is then performed and use for maintenance planning. In an effort to ease the implementation of CBM, ways to improve prognostics application is explored. The Switching Kalman Filter (SKF) was adapted for both diagnostic and prognostic under an autonomous framework that requires little user input. The SKF uses multiple dynamical models with each one describing a different stage of bearing wear. The most probable wear process is then inferred from the extracted feature data using Bayesian estimation. As different stages of bearing wear can be tracked using the dynamical behavior of the measurements, pre-established threshold for fault detection is no longer required for diagnostics. The SKF approach provides maintainers with more information for decision-making as a probabilistic measure of the wear processes are available. It also offers the opportunity to predict RUL more accurately by distinguishing between the wear stages and performing prediction only when rapid and unstable wear is detected. The SKF approach is demonstrated using in-service feature data from the AH64D TRGB and the results have shown the proposed methods to be a promising tool for maintenance decision-making. As an extension of research on methodologies to improve PM and CBM decision support, a thioether mist lubrication is explored for its feasibility as a backup lubrication system for helicopters. The aim is to reduce the mishap severity category which in turn eases the extension of PM interval or its replacement with a CBM task. An experimental setup was developed to test the thermal properties of a spur gearbox with thioether mist lubrication under various load and speed conditions and it was shown that only a very small volumetric flow of lubricant is required to preserve the gears from damage in oil starved environment. As such, a thioether based mist backup system can potentially reduce the risk of oil starvation failures significantly.

Over the last few years, the reduction of operational costs and control of pollutant emissions have become central issues for the commercial aviation industry, and as a result, airlines have been increasingly focusing their attention on operational fuel saving techniques. However, even though the practical implementation and economic potential of these techniques have been exemplified in a number of papers, little research has been dedicated to a systemic investigation of the effects of operational fuel saving on the human component of the system, i.e., the flight crew of an aircraft. This research examines this area, and investigates the human factors aspects in context with the application of operational fuel saving on the Airbus A 320 series aircraft. The study presents a detailed analysis of the flight crew's performance and motivational factors related to the topic of interest, which were investigated by means of an online survey and a controlled simulator experiment. Results of the analysis revealed that the application of operational fuel saving imposes a number of latent performance impairments on the flight crew. Motivational factors were shown to be disrupted by the flight crew's inability to achieve satisfaction from the application of operational fuel saving. The implications of these findings are wide-ranging, as they show, in essence, that the system's safety and efficiency relies solely on the flight crew's cognitive flexibility and workload compensation capability, while structured analyses and conceptual frameworks in regard to the human factors aspects of operational fuel saving are absent.

The aviation industry is in the process of developing the All-Electric-Aircraft through a succession of increasingly More-Electric-Aircraft. Through this evolution, previously pneumatically or hydraulically powered functions are being implemented by electrical systems. This requires an increase in electrical power generation and distribution. To meet this increasing demand at competitive power densities, a change of generator technology may soon be required. Permanent Magnet (PM) machines, due to their power density, efficiency and simple robust construction, are a strong candidate technology for future aircraft electrical power generation. A variable frequency AC system based around a non-salient PM machine and a parallel shunt regulator is assessed and analysed to examine the effect of generator electromagnetic parameters on the rating of the regulator. With the correct pairing of PM machine and converter, it is anticipated that a system could be conceived which provides the necessary performance to meet the coming needs of the evolving aerospace industry at a competitive mass density. Key to achieving an optimised converter is the realisation of a machine with a specific PM flux linkage and inductance. Surface mounted PM machines offer a robust and simple machine construction. This topology is often employed where electromagnetic saliency is not required. However, their design is currently constrained by the effect of rotor containment structures on the machine's magnetic circuit. Alternative containment materials with a degree of magnetic permeability are calculated to reduce the effective air-gap usually presented by the containment material. Cold-rolled AISI 304L stainless steel exhibits such magnetic propelties. A prototype rotor is constructed incorporating a containment sleeve constructed of this material in a laminated construction. The machine is then tested to verify the analysis and to demonstrate shunt regulation of a PM generator. A further investigation is then conducted to assess the suitability of salient machine designs for operation as shunt regulated generators before conclusions are drawn and suggestions for future research are proposed.

In order to analyse the turbine blade life, the damage due to the combined thermal and mechanical loads should be adequately accounted for. This is more challenging when detailed component geometry is limited. Therefore, a compromise between the level of geometric detail and the complexity of the lifing method to be implemented would be necessary. This thesis therefore focuses on how the life assessment of aero engine turbine blades can be done, considering the balance between available design inputs and adequate level of fidelity. Accordingly, the thesis contributes to developing a generic turbine blade lifing method that is based on the engine thermodynamic cycle; as well as integrating critical design/technological factors and operational parameters that influence the aero engine blade life. To this end, thermo-mechanical fatigue was identified as the critical damage phenomenon driving the life of the turbine blade. The developed approach integrates software tools and numerical models created using the minimum design information typically available at the early design stages. Using finite element analysis of an idealised blade geometry, the approach captures relevant impacts of thermal gradients and thermal stresses that contribute to the Thermo-mechanical Fatigue damage on the gas turbine blade. The blade life is evaluated using the Neu/Sehitoglu Thermo-mechanical Fatigue model that considers damage accumulation due to fatigue, oxidation, and creep. The leading edge is examined as a critical part of the blade to estimate the damage severity for different design factors and operational parameters. The outputs of the research can be used to better understand how the environment and the operating conditions of the aircraft affect the blade life consumption and therefore what is the impact on the maintenance cost and the availability of the propulsion system. This research also finds that the environmental (oxidation) effect drives the blade life and the blade coolant side was the critical location. Furthermore, a parametric and sensitivity study of the Neu/Sehitoglu model parameters suggests that in addition to four previously reported parameters, the sensitivity of the phasing to oxidation damage would be critical to overall blade life.

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The present work focuses on preliminary weight estimation methods that enable the feasibility studies of novel aero engines. The key contributions can be found in the analysis of the existing preliminary weight estimation methods, the development of a new preliminary weight estimation method and the study on the feasibility of a Geared Turbofan (GTF) engine. In more detail, the existing preliminary weight estimation methods are examined in the first part of the thesis, aiming to define their suitability for current turbofan engines, but also for future engine arrangements. For this purpose, they are examined not only quantitatively, to verify their accuracy, but also qualitatively to figure out if they are able to reflect the key thermodynamic and design parameter variations on weight. Apart from NASA WATE no method achieves either the required accuracy, or simulates the weight trends. Realising the need for a more accurate, robust, flexible and extensible method, a new "component based" method that performs basic component design to estimate engine weight, is devised. Its accuracy is verified by comparing the whole engine weight prediction and estimated component design against the publicly available data of two major turbofan engines and the weight predictions of existing weight estimation methods. ATLAS, the tool based on the above method was used to estimate weight over a range of Bypass Ratio (BPR) and Turbine Entry Temperature (TET) values for a Direct Drive Turbofan (DDTF) and a GTF two spool arrangement, reaching the following conclusions: The adjustments of Low Pressure Turbine (LPT) number of stages or geometry are not sufficient, if high stage isentropic efficiency values are targeted at high BPR values. For the examined engine model, with the given weight estimation methodology, the weight reduction, when a gearbox is introduced at a DDTF, depends on the reduction of LPT stages, with the other components having negligible impact. However, it should be noted that a constant fan diameter was assumed for both configurations. A fan loss model and more detailed weight estimation of frames, shafts and control and accessories is required to verify this conclusion. The comparison of a DDTF and a GTF engine is representative only if the cycles corresponding to the installed performance optima are considered. Engines with the same thermodynamic cycle could only be compared when the optima cannot be reached, due to geometry restrictions.

The continuous growth of air transport has raised concerns about global aircraft fuel consumption, emissions and noise. Industry’s efforts have identified that to reduce future emissions and the impact of aircraft operations on the environment will require contribution from: a) New technologies with better efficiency b) Improved asset management and c) Greener manufacturing and recycling processes. This research falls under asset management and involves aircraft trajectory optimisation. Most aircraft trajectory optimisation studies concentrate on optimising fuel burn, emissions and noise. Fuel burn is the dominant contributor to operating costs. During the course of this work, no work was found to better understand from an operator’s perspective how the optimal solutions for minimising fuel burn and protecting the environment will impact on engine useful life and the engine operating costs. Also no work was found to understand how engine component degradation will impact on the optimised solutions for fuel burn and engine life. The contribution to knowledge from this research is a) the assessment of the impact of airport severity factors on engine life consumption and aircraft performance and b) the assessment and quantification of the change in engine life usage when optimising for flight mission fuel burn and the change in flight mission fuel burn when optimising for engine life usage; in both cases the effects of engine component degradation are considered and assessed. The trade-offs between mission fuel burn and engine life optimised trajectories are presented here for a clean (new) engine for three routes (London–Madrid, London–Ankara and London–Abu Dhabi). The engine life calculated was the HPT blade life and HPT disc life due to creep, fatigue and oxidation failure modes independent of each other. Mission fuel burn and engine life trajectory optimisation assessments were conducted to incorporate the effects of degradation after 3000, 4500 and 5250cycles of operation. Further assessments were made linking aircraft performance to airport severity factors for the clean engine, after 3000cycles and after 5250cycles. A techno-economic environmental risk assessment approach was used. The results indicate that airports at higher altitudes e.g. Cairo, suffer more severity due to higher operating temperatures, but benefit from less climb fuel burn and lower operating costs. The severity and fuel burn for take-off at airports with higher ambient temperatures was found to be more due to the higher operating temperatures required. The operating cost at these airports was thus higher. The fuel burn optimised trajectories were found to be achieved at higher operating temperatures with reduced blade life (due to creep, fatigue and oxidation). In particular, for London–Madrid, the blade creep and blade oxidation lives were found to reduce by -3.4% and -2.1% respectively. The blade oxidation life optimised trajectories showed increase in fuel burn of +3.6% and +4.9% for London–Madrid and London–Ankara respectively. The blade creep life optimised trajectories for London–Abu Dhabi were found to benefit from less fuel burn during climb. The disc creep life optimised trajectories showed benefit in fuel burn for London–Ankara and London–Abu Dhabi. The conclusions from the study are:  High OAT and high altitude airports such as Abu Dhabi require higher operating temperatures which have severe consequences on the engine component life, fuel burn and emissions.  Fuel burn optimised trajectories have a negative effect on the blade life due to creep, fatigue and oxidation due to higher maximum operating temperatures. However, the reduction in fuel burn outweighs the drop in life, thus benefitting to the operating costs.  Optimising for blade creep life benefits the fuel burn for London–Abu Dhabi due to less fuel burn at climb  The blade oxidation life optimised trajectories are detrimental to the fuel burn due to slower cruise speeds and more time spent at cruise and descent  The disc creep life optimised trajectories benefit the fuel burn for London – Ankara and London–Abu Dhabi due to flying at higher cruise altitudes and burning less fuel. The recommendations from this research include making improvements to the framework such as a) Integrating the lifing methodologies because in reality the failure modes are not entirely independent of each other but do interact b) Develop and incorporate a diagnostics and prognostics tool to predict levels of degradation c) Using actual waypoints and incorporate horizontal trajectory profiles d) Future studies can include noise as an objective, which though mentioned has not been within the scope of this work. e) A key driver to lower operating costs is a considerable reduction in fuel burn. Maintenance costs will inevitably rise with engine life consumption. Further study of the trade-offs between fuel burn and engine life is therefore recommended.

Routine airframe Non-Destructive Testing (NDT) procedures are costly and prone to human error. Guided wave structural health monitoring (GWSHM) shows great promise to in future assist these carefully regulated aerospace NDT practices. Using automatic GWSHM to both detect and localise damage can better focus the human NDT effort and ultimately lead to safer operation of airframes. The thesis presents structural health monitoring techniques for airframes using measurements of guided waves. Work is presented on both metal plates and carbon fibre reinforced plastic panels. An active GWSHM method is considered in its capability to detect and localise damage by measurements of scattered Lamb waves from artificially placed damage. The contribution to knowledge on active GWSHM has been towards effective and practical strategies for placing a low number of transducers into arrays suitable for global coverage. Much early active GWSHM studies often adopted a uniformly sparse distribution of transducer elements, perhaps in an attempt to gain the best possible global coverage. In this thesis, active GWSHM performance has been evaluated for arrays of different geometry and has shown that a uniformly sparse distribution of transducer elements may not be the most effective strategy when using a minimal number of sensors. Simulated and artificial damage, placed with different orientations over a large area, has been used to test candidate array layouts. It finds the layout optimal for damage detection is not necessarily the layout optimal for damage localisation. The zeroth order anti-symmetric Lamb wave mode has been used at low frequency-thickness. The mode, referred to as the flexural mode when propagating with low frequency-thickness, is favoured for its short wave length and long range. At low frequency-thickness this mode is quickly outrun by its symmetric counterpart, causing coherent noise in the signals recorded. Baseline subtraction is used to suppress the coherent noise before imaging. Benign structural features, that would usually hinder damage-localisation from an image, are actually found to assist damage localisation for some array layouts when using the reference baseline signal subtraction technique. A passive GWSHM method is considered in its capability to localise impacts. Impact events on carbon fibre panels are localised using a low frequency passive array. The technique is suggested for evaluating damage from tyre-burst or propeller debris impacts to airframe surfaces. It is particularly relevant to new airframe designs that have significant usage of composite materials on their outer surface. Historically the aerospace sector has readily adopted time of arrival estimation methods similar to those found on a standard oscilloscope. As an example, acoustic emission monitoring, in recent decades has routinely used threshold-crossing as a means of time of arrival measurement. An alternative is presented requiring the whole time series to be post-processed. It extracts an alternative arrival time from propagating waves resulting from the impact, which can be used in time-difference of arrival algorithms. This method is shown to be more reliable and accurate for impact localisation than historical techniques.

In this work rapid computational methods to support the designers in taking more informed decisions within the design process of a large aircraft manufacturer are developed. Since the design takes place in a soda-technical environment, a description of the existing multidisciplinary wing shape design process during the concept and pm-design phase is first provided, adopting a systems approach. The systems approach includes a thorough analysis of the actors involved, requirements, and several system methodologies that have been adopted to characterise the problem situation. Then, rapid, robust, and user friendly methods that fit into the existing design process are developed to aid designers to perform some specific tasks more efficiently. These include a method to modify the twist of a wing to match a target lift distribution, a method to take into account the effects of the structure flexibility, and a method to include the effects of propellers slipstream on the wing lift coefficient distribution. The commonality between these techniques is that they exploit a pm-computed aerodynamic database, generated using high fidelity simulations, coupled with a lifting line module to correct for 3-D effects. The optimum twist distribution is achieved through a correlation between the change in twist and downwash span distributions. The flexibility and propeller swirl effects on the wing lift distribution are taken into account by coupling the aerodynamic databases with a reduced structural model and a blade element momentum theory module respectively. Since the databases are available as a standard product of the design process, no extra effort is required by the designer for their generation. The methods proposed are applied to a number of test cases showing industrial complexity, and results compared against more expensive high fidelity simulations. Results show a satisfactory level of agreement for a vastly reduced cost. The methods proposed in this work have been implemented in an industrial context and made available to wing designers.

This thesis investigates the notion of fuel-reduction through formation flight for commercial aircraft, addressing the problems of global routing and assignment. A two stage centralised approach is presented, firstly, assuming a reduction in observed cost by flying in formation the routes, including rendezvous and break points, are calculated to minimise a total cost. The interconnected assignment problem then takes a set of flights, their possible formations and corresponding costs and optimally allocates them into a cost-minimising formation fleet. An analytic geometric approach is used to develop a scalable methodology for the formation routing problem enabling the quick calculation of costs. The rapid evaluation allows the large scale fleet assignment problem to be solved via a Mixed Integer Linear Program in reasonable time. A Transatlantic case study shows possible formation fuel savings against solo flight of around 8.7% and 13.1 % for formations up to size two and three respectively. Further case studies of three distinct sets of flights show that encouraging levels of saving can still be achieved by flights with varied distances , geographical locations and formation drag-reduction levels. For the more complex task of routing through wind, results show that the analytic approach can act as a reasonable estimate to the assignment problem, allowing higherfidelity and computationally more intensive routing methods to be introduced via a post-process, significantly reducing solve time. Methods for mitigating the impact of uncertainty in aircraft take-off times are explored, where a state-space approach, solved using value iteration, can provide optimal speed-policies for aircraft to follow for any possible realisation of delay. Additionally portfolio optimisation provides a method for formations to be assigned to simultaneously maximise reward and minimise the associated risk. Finally the calculation of efficient frontiers allows matching of reward to desired levels of risk-aversion.