U.S. POSITION ON THE DEVELOPMENT OF ICAO’S AIRCRAFT CO2 STANDARD 

(Presented by the United States) 

SUMMARY

The purpose of this paper is to:

1) provide the U.S. position on the development of ICAO’s aircraft CO2

Standard;

2) recommend a purpose of the CO2 Standard to provide the CO2TG

policy guidance to complete its work;

3) identify major points of disagreement requiring resolution in order to

move the work forward; and

4) suggest possible next steps to ensure the work is completed in an

expeditious manner.

Action by the CAEP-SG is in paragraph 4.

1. INTRODUCTION

1.1 Since the meeting of the Steering Group in 2010, the WG3 has focused the majority of 

their resources on CO2TG assigned remits, specifically to develop an aircraft CO2 Certification 

Requirement. A concentrated schedule of teleconferences, workshops, and meetings was followed to 

accomplish this work, which focused mainly on conducting technical analyses to select a certification 

framework as a basis for the development of the certification requirement. Such a certification framework 

historically consists of a metric (e.g. DP/F00 for NOx, EPNdB for noise), correlating parameter 

(e.g. overall pressure ratio for NOx, MTOW for noise), and evaluation option (i.e. representative landing 

and takeoff (LTO) cycle for NOx, combination of flyover, lateral, and approach measurement for noise). 

1.2 The CO2TG initially attempted to target each specific part of the certification framework 

separately. However, the CO2TG recently concluded that the analysis of metrics, correlating parameters 

(CPs), and evaluation options (EOs) form a highly complex and interdependent system (i.e. Metric 

System). As a result, the CO2TG agreed that further work must assess metrics, CPs, and EOs as a whole 
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rather than individually. The analysis of metric systems ought to help recognize interdependencies and 

potential consequences, isolate effects, and aid the understanding of differences of opinions within the 

working group. The CO2TG also constructed a set of Key Criteria (KC) to be used in the process as a 

guide to identify and select candidate metric systems for aircraft certification purposes. 

1.3 At the conclusion of the sixth and most recent meeting of WG3/CO2TG, on 

July 18 to 22, 2011 in Hartford, Connecticut, the group narrowed down to four favoured metric systems 

(MS)1. These metrics systems are listed in the Table I below, and include metrics based on both mission 

fuel and Specific Air Range (SAR). 

Table I: Favoured CO2 Metric System Submissions 2

Metric 

System # Metric Correlating Parameter(s) Evaluation option

MS #1 1/SAR MTOW 75% (MTOW-MinW)+MinW3

MS #2 1/SAR (MTOW+MZFW)/2 (MTOW+MZFW)/2

MS #3 1/SAR TBD TBD

MS #4

Mission Fuel Payload (MZFW-SAEW) and Distance

3 test points at ZFW = SAEW + 0.7 (MZFW-SAEW) with 

fuel mass of:

i) 1/3(MTOW-MZFW)

ii) 1/2(MTOW-MZFW)

iii) 2/3(MTOW-MZFW)

>1500ft flight phases

2. DISCUSSION

2.1 While progress has been made by the CO2TG to down-select to three well defined metric 

systems (and one to be determined), significant differences in opinion exist and have prevented the group 

from identifying a single metric system as the basis for a CO2 certification requirement. Discussions 

regarding the appropriateness of the metric systems have been focused on how the metric systems relate 

to the CO2TG KC, however, the differences in opinion persist as to the true ability of each MS to meet 

several of these criteria. In particular, the considerations include: 

2.1.1 The purpose of the CO2 Standard so as to inform the development of the aircraft CO2 

Certification Requirement; 

2.1.2 Whether the metric needs to explicitly represent fuel efficiency across all relevant phases 

of flight (i.e. Mission-based) or if it is sufficient to represent fuel efficiency at a cruise condition 

(1/SAR); 

2.1.3 How to determine if a metric system is transport capability neutral; 

2.1.4 Whether empty weight reductions need to be explicitly rewarded, through increased 

margin to a notional limit line, and also whether empty weight increases should be penalized through 

decreased margin to a notional limit line, similar to aerodynamic or propulsive efficiency changes; 

2.1.5 Whether the evaluation options should be based on day to day operational patterns 

versus measuring the performance at maximum capability; and 

1

 CAEP9_WG3_CO2- - 4 - 6_WP13 

2

 Where SAR is specific air range, MTOW is maximum takeoff weight, MZFW is maximum zero fuel weight, and MinW is 

minimum weight 

3

 Possible use of SAEW instead of MinW, where SAEW is standard aircraft empty weight 
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2.1.6 The complexity and extent of the burden required from certification authorities to certify 

new parameters and promulgate national regulations, develop processes to implement the regulations, and 

train certification engineers to oversee implementation and verification. 

2.2 Some of the divergence of opinions cannot be answered solely through technical 

analyses, but may require policy direction. The CO2TG requires guidance from CAEP on policy 

questions in order to successfully carry out its technical work. The U.S. believes that CAEP should 

provide the CO2TG direction on an agreed purpose of the CO2 Standard. The U.S. believes that the 

purpose of the CO2 Standard is to achieve CO2 emission reductions from the aviation sector beyond 

expected ―business as usual‖ – i.e., a standard that achieves CO2 emission reductions from the aviation 

sector beyond reductions that would otherwise be achieved in the absence of the standard. This would be 

analysed using the ICAO criteria of technical feasibility, environmental benefit, cost effectiveness, and 

impacts of interdependencies. Similarly, in anticipation of the next phase of CO2TG work involving the 

CO2 Standard’s applicability and stringency, the U.S. believes that CAEP should reaffirm the importance 

of analysing the potential applicability of a CO2 Standard to new in-production aircraft as well as new 

types. 

2.3 To address all of the criteria and these considerations the U.S. believes that the CO2

Standard metric system should be feasible to implement and demonstrated by data and rigorous 

quantitative analysis. 

2.4 To address considerations 2.1.1 and 2.1.4, the U.S. believes that the CO2 Standard metric 

system should reflect all relevant phases of flight and real-world operations of aircraft as much as 

possible. 

2.5 Consensus within the group has not been reached because some believe that technical 

evidence evaluating the degree to which the three well defined MS meet the KC has not been presented 

by the members that support the different systems. This has been expressed in particular with respect to 

the mission-based approach. Therefore, the U.S. believes it is important to direct the CO2TG to focus 

future data gathering and analysis on the most robust metric system that reflects all relevant phases of 

flight and real-world operations as much as possible, is feasible to implement, and is demonstrated by 

data and rigorous quantitative analysis. This work should be performed without prejudice toward a final 

metric system selection. Furthermore, there is not yet technical consensus on all of the evidence that has 

been provided within the task group; the aforementioned differences in opinion appear to be preventing a 

technical consensus in several aspects. 

2.6 A specific area of disagreement that distinctively surfaced is whether changes in transport 

capability, in absence of changes in technology, could be explicitly rewarded by the metric system via an 

increased margin to any assumed notional limit line. Any metric systems that are determined to not be 

transport capability neutral for any assumed notional limit line should require an analysis for a 

comprehensive assessment of potential consequences before down-selecting to the final metric system. 

2.7 It should be noted that some metric systems that are not neutral to transport capability for 

any assumed notional limit line could drive the design and development of aircraft in unintended 

directions, such as influencing the design based on transport capabilities, which may result in responses to 

a notional limit line without promoting technology improvements. This could have potential implications 

on the air transportation system and its stakeholders, including airline purchases, aircraft utilization, 

operations and routing, air transportation system congestion and delay, safety, and system-wide fuel burn, 

local air quality, and noise. The U.S. believes that the CO2TG should eliminate any non-transport 
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capability neutral metric systems, as inappropriate for consideration at this point given all the 

uncertainties of this approach. 

2.8 The U.S. believes that CAEP should maintain a schedule for the CO2 Standard that will 

produce international agreement on a standard in 2014. As part of this approach, WG3 needs to decide on 

a metric system at the November WG3 meeting, and CO2TG should work on an aggressive schedule to do 

the necessary analytical work in advance of that meeting. If further work by CO2TG members concludes 

that it is not feasible to down-select to a single metric system within a reasonable amount of time, the 

working group and CO2TG should clearly state to Steering Group its reasoning with identification of 

pros/cons, existing gaps in technical opinions, and basis for the technical gaps. WG3 should also work 

with MDG and FESG to identify schedule efficiencies to recover from delays, including moving forward 

with the sample problem. 

3. CONCLUSION

3.1 The CO2TG has conducted a significant amount of work toward achieving its remits; 

however there are still major points of technical disagreement that prevent the CO2TG from reaching a 

consensus on one single metric system. 

3.2 The CO2TG should identify a clear plan to work on the development of an aircraft CO2

certification requirement to achieve a down-select of one metric system by the end of their next scheduled 

meeting in November. This work plan should include key points that must be addressed in order to 

resolve disagreements and subsequently down-select to a single metric system to take forward to a sample 

problem analysis. 

4. ACTION BY THE CAEP-SG

4.1 The CAEP-SG is invited to: 

a) agree that the purpose of the CO2 Standard is to achieve CO2 emission reductions

from the aviation sector beyond expected ―business as usual‖ – i.e., a standard that

achieves CO2 emission reductions from the aviation sector beyond reductions that

would otherwise be achieved in the absence of the standard. This would be analysed

using ICAO criteria of technical feasibility, environmental benefit, cost effectiveness,

and impacts of interdependencies.

b) reaffirm the importance of analysing the potential applicability of a CO2 Standard to

new in-production aircraft as well as new types;

c) agree that the CO2 metric system should reflect all relevant phases of flight and

real-world operations of aircraft as much as possible;

d) agree that the CO2 metric system should be feasible to implement and demonstrated

by data and rigorous quantitative analysis;

e) agree to eliminate from consideration any non-transport capability neutral metric

systems at this point given all the uncertainties of this approach;
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f) request an action plan by 5 October 2011 from WG3 CO2TG to deliver one downselected metric system by the end of their next scheduled meeting in November;

g) agree to maintain a schedule for the CO2 Standard that will produce international

agreement on a Standard in 2014; and

h) agree that WG3 should work with MDG and FESG to identify schedule efficiencies

to recover from delays, including moving forward with the sample problem.

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