1. 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.
Sustainability Indicator Set and Corresponding Metrics for
Aircraft 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 setof 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 suggesta 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.

2. I. Introduction
Sustainability is constantly gaining importance in civil aviation especially as the market share of aviation as
a mode of transportation increases overtime. 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 otherfactors 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 underthree 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 anotheris instrumental in the development of sustainable aircraft for the future.
II. Motivation and Objective
The goal of this paper is to develop a systemwhereby 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 oftheir broader appeal in addressing all aspects of
sustainability.
III. Indicator Selection Methodology
In order to choose and assess a newindicator 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 otherresearch into similar topics.

3. 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 otherresearchers into the subject of
sustainability, these indicators appear to be most effective in measuring sustainability of aircraft in each of these

4. categories. Indicators which were decided to be the best for the social sustainability pillars are: PassengerSafety, 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 stakeholderinput 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 aviation
2) Determine which factors are most important when addressing sustainability
3) Understanding why certain factors are more important than others
4) Noting suggestions forpossible 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 anotherliterature 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.

5. Figure 3. Flow chart illustrating the process taken to create and apply the survey
Following the steps ofthe outline, a final survey was created and sent out to industry experts and
stakeholders.A snapshot ofthe final survey can be seen in Figure 4 below as an example of the document
stakeholders would have received.

6. 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 fromthe 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 considerin 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. Anotherimportant 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 essentialto 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 whetherthe metrics are
acceptable and meet the criteria. This can be done by testing the candidate metrics in modeling environments. In the

7. further discussion ofthe 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 conceptualtool called MICADO was employed..
Figure 6. Direct Operating Cost Contributors
MICADO or (Multidisciplinary Integrated Conceptual Aircraft design and Optimization Environment) is a
conceptualsoftware 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

8. MICADO was given a baseline configuration for a small, single aisle passengeraircraft 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 showthe
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

9. 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 conceptuallevel. By using MICADO which is similar to
conceptualdesign 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

10. 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 studentsat 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 teamare 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.