Sustainability Indicator Set and Corresponding Metrics forAircraft Design Assessment

Khambrel Simpson1 and Sayan Roy2

Georgia Institute of Technology, Atlanta, Georgia, 30313

Civil Aviation has become one of the most popular modes of transportation across the last several decades as air travel has become safer and carrying capacity has increased. This combination has lead to lower ticket prices, and made flying more accessible to the global population. Along with advancements in size and safety, new methods of modeling and analysis have been developed for propulsion, structures, and various other aircraft systems. These new technologies have allowed aircraft to become more lightweight and fuel-efficient. With the increased utilization of air transportation, new issues have arisen concerning the impact of the aviation industry upon the global economy, the environment, and society as a whole. However, there has never been a way to measure in a quantifiable manner the impact of an aircraft design within the realms of the environment, the economy, and society. As we transition into a time where the aviation industry has a profound impact, there becomes a need for aircraft designers to assess the sustainability of each aircraft design.

Each new aircraft design should take into consideration what impact there will be on these three criteria which define sustainability. As it currently stands there is no standard set of indicators which can assess the sustainability of an aircraft design at the conceptual design phase. In this paper, there will be discussion into a methodology which allows the selection of a set of indicators and corresponding metrics. The metrics can be tested in various software tools and real world applications to understand if they are effectively measuring the impact of the indicator. This paper will suggest a set of indicators and an example of a corresponding metric which can be used to measure sustainability. This standard would allow the designers to develop configurations with long-term sustainability impacts in mind.

1 Student, Georgia Institute of Technology. 332463 Georgia Tech Station. Atlanta, GA. Student Member.2 Student, Georgia Institute of Technology. 154 5th Street NW. Atlanta, GA. Student Member.

I. Introduction

Sustainability is constantly gaining importance in civil aviation especially as the market share of aviation as a mode of transportation increases over time. Greenhouse gas production, in particular, among many other impacts, by aircraft has been increasingly becoming a factor when determining the effect of sustainability of aircraft. However, there are still other factors which impact more than just the environment that are not currently considered. These include the societal impact such as airport noise contribution, and economic sustainability including such items as the cost of maintaining an aircraft over the entirety of its lifetime, or a design which incorporates recyclable components. The sustainability impacts of an aircraft design can be grouped under three pillars: Environment, Economic, and Societal.

Development of a sustainable aircraft today may involve the efficiency of the engine, a reduction in nitrous oxide emissions, or using materials which are less prone to disintegration or the expulsion of dangerous carcinogens. Evaluating an aircraft’s sustainable attributes as currently defined are often done at the end of the design cycle, during testing when figures such as emissions, material choices, and efficiency can be verified with hard data. However, decisions for improvements in sustainable characteristics may be made far in advance during the design cycle. Any engineer looking to take into account the impacts beyond environmental considerations, will need tools to assess the “sustainability” of aircraft. For an aircraft designer, the ability to quantifiably compare one aircraft design versus another is instrumental in the development of sustainable aircraft for the future.

II. Motivation and Objective

The goal of this paper is to develop a system whereby the “sustainability” of different aircraft designs can be assessed. This requires that sustainability be expanded to be more broad than the current definition which amounts to looking at specific areas such as emissions, and fuel efficiency. These are essentially relevant either to the profitability of the aircraft from the customer's point of view, or to conform to regulatory standards set by the FAA. Sustainability assessment criteria is to be divided among the three pillars mentioned earlier. Economic, Societal, and Environmental assessment were chosen because of their broader appeal in addressing all aspects of sustainability.

III. Indicator Selection Methodology

In order to choose and assess a new indicator set and corresponding metrics, there is a necessity for a new approach on selection. In this approach the first step is to delve into literature research and define the objectives of the indicator set. The purpose of the literature search is to fully understand the scope of the problem as well as identify which criteria are important in choosing indicators. It is also important to see what problems and approaches have been encountered by other research into similar topics.

Figure 1: Flow chart for Indicator and Metric Selection Process

During the literature search, we encountered some primary sources which delve into the subject of sustainability in the civil aviation transportation industry. Through the literature analysis, we started by defining the scope of our problem. The indicator set has to be relevant for the civil aviation industry, and be applicable to single aircraft designs. Fleet analysis is beyond the scope of this problem. When defining sustainability with relevance to impacts of civil aviation, it is easiest to break the impact into ‘sustainability pillars’ which are the societal, economic, and environmental impacts of the civil aviation industry. By dividing sustainability into these three pillars, it gives the ability to better select indicators which represent wholistically how sustainable an aircraft design is.

Figure 2: The three sustainability pillars and chosen indicator set

Through the extensive literature analysis and down selection, and indicator set was defined which represents each of the three ‘sustainability pillars’. Based on a work from other researchers into the subject of sustainability, these indicators appear to be most effective in measuring sustainability of aircraft in each of these

categories. Indicators which were decided to be the best for the social sustainability pillars are: Passenger Safety, Air Quality. Regional Equity, and Aircraft Noise. Under the economic pillar the indicators chosen are: Direct Operating Costs, Air Transport Volume, and Aviation Fuel Used. Under the environmental sustainability pillar, the indicators chosen are: Aircraft Disposal and Greenhouse Gas Emissions.

IV. Indicator Assessment Methodology

After identifying the indicator set, it is important to be able to validate the choice of indicators as a good one. The best approach to validating choice of indicators is to solicit stakeholder input into the indicator set and re-evaluate based on the input. The primary means to solicit stakeholder input is through the use of surveys. The survey allows input from the experts in the industry who actually stand to gain or lose from the creation of a new set of indicators which can measure aircraft sustainability. Therefore, it is important to avoid biases with such a target audience. With a survey, it is first important to identify the requirements for the survey that needs to be created. The primary requirements are:

1) To be able to determine stakeholder views on sustainable aviation2) Determine which factors are most important when addressing sustainability3) Understanding why certain factors are more important than others4) Noting suggestions for possible metrics for each indicator.

After identifying requirements, the next step is to identify which survey method best fits the requirements of the survey. During this step there was another literature analysis looking into possible survey methods which would best suit this application. Notice that Table 1 below provides a general description of several survey methods with potential issues of each as applied to the scope of this research.

Simple Random Sampling

Systematic Sampling

Delphi Method Cluster Sampling

General Description

Users obtain or create a list of respondents and randomly select from it to form a data set

A specific number of respondents are required for a statistically significant data sample

Uses a questionnaire to determine the relative importance of a variable given the total variable set

Takes random sections of a specifically sized sample, then samples each section individually

Potential Issues Infeasible for sampling large populations

Geared toward a more broad population

Delphi method requires a predefined set of opinions and actions

May not provide a good representation of the population

Table 1: Comparison of surveying techniques for use in stakeholder input solicitation

Based on the analysis of several survey techniques, it was decided that the Delphi Study was the approach which best meets the requirements set earlier. Based on the Delphi Study method, as seen in Figure 3, the process of creation for the survey was outlined.

Figure 3. Flow chart illustrating the process taken to create and apply the survey

Following the steps of the outline, a final survey was created and sent out to industry experts and stakeholders. A snapshot of the final survey can be seen in Figure 4 below as an example of the document stakeholders would have received.

Figure 4. Screenshot examples of part of the survey sent to stakeholders

This survey method provides a method of soliciting information from experts. However, designing a useful survey requires careful planning, and often many rounds of iterations. The benefits of this method, however, far outweigh the costs. This survey provides valuable insight into stakeholder mentality regarding specific sustainability indicators. The suggestions from the experts can help direct the quantitative part of metric analysis.

V. Metric Selection and Assessment Methodology

In relation to an indicator, a metric is a standard unit of measurement for that indicator which allows comparison. Before selecting metrics for indicators it is important to understand which criteria make up a good metric. Some general criteria to consider in this application are shown in the figure below.

Figure 5. Key criteria to consider in choosing a good metric

From the figure above, it is important to highlight some of the key criteria. One of the key criteria is to account for fundamental airplane design elements and capabilities. This is of importance because with any metric it must be able to account for different aircraft functionalities. For example, a jumbo jet will produce a greater amount of emissions than a corporate jet, however the metric should not favor the corporate jet simply for being a smaller aircraft. Another important criteria to take note of is to be fair across the set of stakeholders. Metrics lead to policy creation, therefore, before any policy can be created, it is essential to ensure fairness across each of the stakeholders.

Once the important criteria is decided upon, a good starting point is to once again begin literature analysis in order to investigate whether there are already existing metric systems which could be applied to the indicator. In the case that no metric exists, a new metric will have to be created through careful analysis and regard to the important criteria. Further in this paper, an example of such a situation is given and elaborated upon in order to explain the methodology.

Upon the generation of good candidate metrics there comes the need to assess whether the metrics are acceptable and meet the criteria. This can be done by testing the candidate metrics in modeling environments. In the

further discussion of the example, there will be a description of how candidate metrics are tested in a modeling environment.

VI. Evaluating Metrics Example

Direct Operating Costs (DOC) are a way to quantify the long term economic impact of an aircraft in terms of the cost associated with flight readiness. DOC includes fuel costs, maintenance, airframe ground handling, and insurance. Because of its widespread applicability it is chosen as the numerator, and input characteristics such as Range, Payload, MTOW are chosen as denominators for our candidate metrics. To analyze real aircraft missions in terms of candidate metrics a conceptual tool called MICADO was employed..

Figure 6. Direct Operating Cost Contributors

MICADO or (Multidisciplinary Integrated Conceptual Aircraft design and Optimization Environment) is a conceptual software created at the ILR (Institute for Aeronautics and Space Systems) at RWTH Aachen. It is used to analyze the effectivness of certain aircraft metrics in characterizing the economic sustainability of a specific mission. Given top-level aircraft requirements such as initial sizing, wing placement, and propulsion cycles, MICADO can create a more detailed model of aircraft capable of completing the required mission. MICADO then outputs aircraft performance analysis with parameters such as mass estimation, polar estimates, and general mission analysis. If this estimation meets the requirements initially set, then MICADO can output data such as Direct Operating Costs, noise, and exhaust emissions.

Figure 7. Matrix of Candidate Metrics

MICADO was given a baseline configuration for a small, single aisle passenger aircraft as its top level requirement. The fixed inputs provided include: a set thrust to weight ratio, and wing loading. Data from MICADO was able to show that metric systems with weight in their denominator were best able to show a clear separation when the Direct Operating Cost was improved. Candidate Metrics were also assessed qualitatively, based on their ability to be implemented by authorities, or how well they could be explained to the general public. Then the metrics were given a score from one to three, and rated on their qualitative properties. Figure 9 and 10 show the scores given to each candidate metric according to a number of qualitative criteria.

Figure 8. DOC/MTOW shows clear separation when DOC of an Aircraft Design Improvement.

Figure 9. Candidate Metric Qualitative Analysis

Figure 10. Candidate Metric Qualitative Analysis

VII. Possible Future Work

In this topic of research, there exists an opportunity to grow and develop upon many of the ideas presented in this paper. One area of further research is to look into a more rigorous metric testing method. This paper showed an example of one possible metric analysis, but if other methods yield promising results in helping to choose good metric, it would be good to explore such methods as alternatives.

VIII. Conclusion

By creating a method whereby existing indicators from literature can be taken and validated and then the metrics can be used to assess aircraft sustainability at the conceptual level. By using MICADO which is similar to conceptual design tools that a large aerospace company may have at their disposal, it gave the ability to evaluate the performance of an aircraft and apply a number of candidate metrics to it. As sustainability is a complex problem driven by technical constraints and the approval of stakeholders like the FAA, Airlines, and Aerospace Manufacturers it was important we validated our metrics through multiple avenues. For this reason we applied both qualitative and quantitative tests to candidate metrics. In our example of Direct Operating Costs, the qualitative scoring of Metrics according to our criteria led to certain ones performing far better than others. The top performing metrics are given below.

Figure 11. Top Performing Metrics According to Qualitative Analysis

Acknowledgements

This research is an undertaking by the Georgia Tech Aerospace Systems Design Laboratory and the RWTH-Aachen Institute of Aeronautics and Space Systems. This research is an international collaboration between four undergraduate students at the Georgia Institute of Technology in Atlanta, Georgia, and four graduate students from the RWTH-Aachen University in Aachen, Germany. The four undergraduate students on this team are Khambrel Simpson, Sayan Roy, Yuan Yao, and Lansing Wei. The four graduate students on this team are Yona Paproth, Sebastian Dufhaus, Volker Steinbrunn, and Phillip Sproten. Special thanks go to the research advisors from both sides, Bryan Boling and Katherine Franz, as well as Dr. Dimitri Mavris and Dr. Eike Stumpf.

The authors would also like to acknowledge that the primary sources used for this research and paper were Paul Grimley’s paper on Sustainability Indicators for Civil Aviation, and the Transportation Research Board’s paper about Sustainable Transportation Indicators. These two papers founded the basis of some of the literature analysis.

References

Grimley, Paul. “Indicators of Sustainable Development in Civil Aviation”. July 2006. Henry, Gary T. “Practical Sampling”. Newbury Park: Sage Publications, 1990. Print.Orlich, Donald C. “Designing Sensible Surveys”. Pleasantville, NY: Redgrave, 1978. Print.Kurzke, J. “Gas Turb 11 - Design and Off-Design Performance of Gas Turbines”. 2007.“Sustainable Transportation Indicators”. Transportation Research Board. November 10, 2008.