(15 pages) 

 CAEP.8.WP.015.2.en.doc

 COMMITTEE ON AVIATION ENVIRONMENTAL PROTECTION (CAEP) 

EIGHTH MEETING 

Montréal, 1 to 12 February 2010 

Agenda Item 2: Review of technical proposals relating to aircraft engine emissions 

ENVIRONMENTAL AND ECONOMIC ASSESSMENT OF THE NOX STRINGENCY 

SCENARIOS 

(Presented by Rapporteurs of MODTF and FESG) 

SUMMARY 

This paper presents an overview of the analysis conducted by MODTF and 

FESG of the cost impacts, emissions reductions and environmental trade-offs 

of the NOx stringency scenarios under consideration for CAEP/81

. 

Action by CAEP is in paragraph 7. 

1. INTRODUCTION 

1.1 This paper presents an overview of the assessment of the environmental benefits and economic 

costs of stringency scenarios to reduce emissions of nitrogen oxides (NOx) relative to the ICAO CAEP/6 

standard. It briefly discusses the models, databases, methods and assumptions used by MODTF and 

FESG to conduct the environmental benefit and economic cost assessments. The emissions reduction 

results from the MODTF assessment and the FESG assessment of the cost impact are presented along 

with a discussion on environmental trade-offs. Finally the cost-effectiveness results are determined and 

presented as costs per tonne of NOx reduction during the ICAO Landing Take Off (LTO) cycle, including 

the ranking of the stringency scenarios. Results are reported separately for the small engine group (thrust 

rating of 26.7<Foo< 89kN)2

, large engine group and combined groups. The main conclusions of the 

analysis are reported in Appendix A. All tables and graphics are provided in Appendix B. 

 

1

 Detailed descriptions of the NOx stringency assessment process, assumptions, and results are provided in [CAEP/8-IP/13]

“NOx Stringency” and [CAEP/8-IP/14] “Economic Assessment of the NOx Stringency Scenarios”. 2

 Foo – Thrust rating. Thrust is expressed in kilonewtons (kN). 

International Civil Aviation Organization

WORKING PAPER 

CAEP/8-WP/15 

2/12/09
CAEP/8-WP/15 - 2 -

2. BACKGROUND 

2.1 As part of the NOx stringency assessment framework, WG3 provided 10 scenarios for modelling, 

as shown in Table 1. For the large engine category, the CAEP Steering Group in 2007 agreed to analysis 

of 5%, 10% and 15% stringency with a slope of 2.0 above 30 OPR3

, and 10%, 15% and 20% stringency at 

a slope of 2.2. A corresponding selection of small engine stringencies was agreed for each scenario. The 

10 stringency scenarios were analyzed for each of 3 future years, 2016, 2026 and 2036. The assessment 

framework also called for two stringency introduction dates, 31 December 2012 and 31 December 2016. 

2.2 A Modification Status (MS) methodology was developed by WG3 for assessing engine 

technology responses to the CAEP/8 NOx stringency scenarios. It includes Minor Changes (MS1), Scaled 

Proven Technology (MS2) and New Technology (MS3). 

2.3 The MS methodology covers the situation where an engine family fails to comply with a NOx 

stringency scenario and proposes the different categories of response that may be used to bring into 

compliance any in-production engine family failing a new proposed CAEP/8 NOx stringency standard. 

Only MS1 and MS2 technology responses were needed by the engines in the small engine group to allow 

them to comply with the NOx stringency scenarios, whereas, for the large engine group at the higher 

stringency scenarios, all three MS technology responses (MS1, MS2 and MS3) were required to make 

these engines comply with the stringency scenarios as can be observed in Figure 1. 

2.4 For each stringency scenario outlined in Table 1, MODTF conducted NOx modelling separately 

in the terminal area below 3,000 ft and above 3,000 ft. In addition to modelling NOx, the environmental 

trade-offs associated with the proposed stringencies were also modelled. These included fuel burn tradeoffs below and above 3,000 ft (taking into account a possible fuel burn penalty, as per recommendations 

from WG3), as well as the associated noise trade-offs (taking into account a possible noise penalty, as per 

recommendations from WG1). The NOx, fuel burn and noise results presented herein represent the 

consensus view of MODTF and are consistent across multiple models4

. The analysis was conducted for 

all affected engines as well as separately for the large and small engine groups. 

2.5 Costs assessed by FESG included non-recurring costs incurred by manufacturers and recurring 

costs incurred by manufacturers and airlines. FESG incorporated a cost-effectiveness approach where cost 

impacts incurred by manufacturers and operators were compared with the change in the NOx emissions. 

This allows scenarios to be ranked based on total industry cost per tonne of NOx reduced. 

2.6 A listing of assumptions used for the CAEP/8 NOx stringency cost-effectiveness analysis is 

included in CAEP/8-IP/14. A thirty year time horizon through 2036 was used to calculate and assess the 

Net Present Value (NPV) of the industry costs and aggregate NOx emissions reductions. The aggregate 

NOx emissions reductions were computed by FESG using the modelled results from 2006, 2016, 2026 

and 2036 assuming linear interpolation between the modelled years. 

3. EMISSIONS REDUCTIONS RESULTS 

3.1 Figures 2 through 9 provide MODTF consensus results for NOx modelling below and above 

3,000 ft. Results are presented in terms of metric tonnes and percentage NOx reduced for each stringency 

scenario listed in Table 1, relative to the baseline no stringency case. Solid bars show reduction results for 

 

3

 OPR – Overall Engine Pressure Ratio. 

4

 Individual model results are provided in [CAEP/8-IP/13]. 
- 3 - CAEP/8-WP/15 

a NOx stringency implementation date of 31 December 2016, and hashed bars show additional NOx 

reduction for an earlier implementation date of 31 December 2012. 

3.2 It should be noted that the quoted absolute values of NOx savings are sensitive to the modelling 

assumptions. The primary function of the data is to allow comparison between the various scenarios and 

implementation dates. A detailed listing of the MODTF assumptions is provided in CAEP/8-IP/13. 

3.3 Figures 2 through 5 provide the NOx reduced for all engines, large engines, and small engines for 

the below 3,000 ft case (presented separately in terms of metric tonnes, and all engines percentage NOx 

reduced). These results are calculated based on the MODTF Local Air Quality (LAQ) consensus model 

computed using ICAO default times-in-mode. The total NOx reduced below 3,000 ft ranges from about 

6,300 metric tonnes to over 114,000 metric tonnes for all engines, or from 1.4% to 9.8% below the 

baseline no stringency case. As can be seen, the total savings for the large engines are about two orders of 

magnitude higher than for the small engines. Another way of looking at this is that the large engines 

account for about 99 percent of the total NOx savings across all scenarios. 

3.4 Figures 6 and 7 provide the NOx reduced for all engines for the above 3,000 ft case. These are 

MODTF consensus results from AEDT5

. The total NOx reduced above 3,000 ft ranges from about 54,000 

to over 773,000 metric tonnes for all engines, or from 1.5% to 10.1% below the baseline no stringency 

case – about the same percentage as described in Paragraph 3.3 for the below 3,000 ft case. 

4. COST RESULTS 

4.1 Cost impacts were estimated for each stringency scenario listed in Table 1 for the two 

implementation dates for small and large engine categories separately, with a range of values for a 

number of key assumptions, including non-recurring costs, fuel burn penalty, fuel price, loss of resale 

value (LRV) and a variety of discount rates. As summarized in Table 2, total cost impacts for small and 

large engines combined are broadly similar for stringency scenarios 1 through 5, but for scenarios 6 

through 10 costs increase steeply, driven by non-recurring costs for engines driven by MS3 technology 

response. While efforts were made to comprehensively quantify all cost impacts, some costs were not 

included. For example, increased industry operational costs in scenarios involving higher fuel burn were 

partially itemized to include fuel costs and costs associated with loss in payload for payload limited 

flights. However, carbon costs for additional CO2 emissions such as those resulting from the inclusion of 

airlines in the EU Emissions Trading Scheme were not itemized and consequently were not included in 

the cost roll-up, but its effects could be assumed to be approximated by the sensitivity cases for the fuel 

prices discussed in CAEP/8-IP/14. 

5. ENVIRONMENTAL TRADE-OFFS 

5.1 An important part of the NOx stringency assessment is the consideration of environmental tradeoffs between the various NOx stringency scenarios, fuel burn, and noise. WG3 recommended a fuel burn 

penalty range of between 0% and 0.5% for engine families requiring a major (MS3) modification. Figures 

8 and 9 provide the maximum potential fuel burn penalty for the full-flight case, presented in terms of 

total fuel and percent, respectively. These are MODTF consensus results from AEDT. In accordance with 

WG3 recommendations, the MS3 fuel burn penalty only applies to large engines and only for scenarios 6 

through 10. As can be seen in Figures 8 and 9, the maximum potential fuel burn penalty ranges from 

about 28,000 metric tonnes to 1.1 Mt (1.1 x 106

 metric tonnes), or from 0.01% to 0.19% relative to the 

 

5

 AEDT – Aviation Environmental Design Tool. 
CAEP/8-WP/15 - 4 -

baseline no stringency case. This translated into additional CO2 emissions of between 88,000 metric 

tonnes and 3.5 Mt. In accordance with WG3 recommendations the minimum fuel burn penalty is zero. 

5.2 WG1 recommended a noise penalty range of between 0 decibel (dB) and 0.5 dB per certification 

point for 10% of engines requiring a major (MS3) modification, i.e. 10% of all engines. As with fuel 

burn, the MS3 noise penalty only applies to large engines and only for scenarios 6 through 10. The effect 

of the MS3 noise penalty on the 55, 60 and 65 DNL6

 contour area expressed as a percentage change was 

less than 0.12%. Based on these findings, MODTF concluded that the analysis indicates there is no noise 

trade-off associated with any of the CAEP/8 NOx stringency scenarios. This conclusion has been verified 

at the global, regional and airport levels. 

6. COST-EFFECTIVENESS RESULTS 

6.1 The cost-effectiveness results are dominated by large engines, which account for approximately 

99% of the benefits. Scenarios 1 through 5 are the most cost effective, all providing relatively low cost 

per tonne of NOx reduced. For scenarios 6 and 7, cost per tonne of NOx reduced increased by a factor of 

3 to 4, using a 3% discount rate. Scenarios 8, 9 and 10 result in a further doubling of cost per tonne of 

NOx reduced. Cost-effectiveness rankings for large and small engines are shown in Tables 3 and 4 

respectively. Although the analysis concentrates on NOx reduction up to 3,000 ft, stringencies also have 

an effect on climb/cruise NOx emissions. If these were taken into account, the total reduction achieved 

would increase by an approximate factor of 7 to 8, and the costs per tonne would diminish accordingly7

. 

6.2 The early implementation date of 2012 gives overall lower values for the costs per tonne of NOx 

reduced. This is enabled by an extra four years of NOx reduction compared to 2016 implementation, 

coupled with roughly the same costs for both implementation dates. This implies that an early 

implementation year would be more cost effective. However, in the approach used, it is assumed that the 

non-recurring costs for the technology responses needed start four years in advance of implementation 

(from 2009). This may mean that in practice a somewhat later date than 2012 is more obvious, 

particularly for those scenarios involving MS3 modifications. The CAEP/6 decision made in 2004 with an 

implementation date of 2008 would be consistent with a CAEP/8 implementation date of 2014. 

6.3 The example in Figure 10 includes large engine results for both 2012 and 2016 implementation 

dates. Gold columns represent cost uncertainty bands for the 10 stringency scenarios based on a 2016 

implementation date; whereas, the red columns represent the uncertainty bands for a 2012 implementation 

date. The “fan lines” indicate lines of constant cost per tonne of NOx reduction. From analysis of Figures 

10 and 11, two conclusions emerge: (1) The small engines contribute approximately 1% of the aggregate 

NOx reduction benefit. Additionally, while the total costs to make small engines compliant are low, their 

cost-effectiveness is worse, by a factor ranging from 30% to as high as 200%. (2) The discount rate does 

not affect the ranking of NOx stringency scenarios, but higher discount rates give lower present value to 

the NOx reduction in the future years.8

 Similarly none of the other sensitivity tests performed influence 

the ranking of scenarios. 

7. ACTION BY THE CAEP 

7.1 The CAEP is invited to: 

 

6

 DNL – Day-Night Noise Level. 

7

 For more details, refer to [CAEP/8-IP/14]. 8

 For details on the effect of discount rates on the results refer to [CAEP/8-IP/14].
- 5 - CAEP/8-WP/15 

a) take note of the material presented in this paper; 

b) accept the results of the NOx stringency assessment performed jointly by MODTF 

and FESG; and 

c) consider the results of the assessment performed jointly by FESG and MODTF in the 

NOx stringency discussions. 

— — — — — — — — 
 

CAEP/8-WP/15 

Appendix A 

English only

APPENDIX A 

SUMMARY AND MAIN CONCLUSIONS 

CAEP has tasked FESG and MODTF to study the effects of a number of NOx stringency scenarios9,10. In 

this regard MODTF, with substantial input from the other CAEP working groups, studied the 

consequences of defined stringency scenarios on the LTO11-NOx emissions, climb and cruise NOx 

emissions, and the associated noise, fuel use and CO2 emissions. FESG assessed the cost impacts of the 

NOx stringency scenarios, and brought the results together with the analysis of emissions reductions 

undertaken by MODTF modellers to present cost-effectiveness results. This paper presents an overview 

of the analysis of cost impacts, emissions reductions and environmental trade-offs of the NOx stringency 

scenarios together with the resulting cost-effectiveness findings, with cost per tonne of NOx reduced and 

rankings presented for each stringency scenario. Detailed descriptions of the NOx stringency assessment 

process, assumptions, and results are provided in CAEP/8-IP/13 and IP/1412. 

The main conclusions are: 

a) the large engines dominate the NOx reductions; 

b) combined cost-effectiveness results fall into three general groupings as follows: 

1) cost per tonne NOx reduced is lowest for stringency scenarios 1-5, 

2) increases by a factor of 3-4 for scenarios 6-7, and 

3) further doubles for scenarios 8-10. 

 — — — — — — — —

 

9

 Ref. [CAEP-SG/20071-SD/5]. “Summary of Discussions and Decisions of the Fifth Meeting of the Steering Group Meeting. 

Friday, 30 November 2007”. First Steering Group meeting of the CAEP/8 cycle held in Zurich (Switzerland) on November 

26-30, 2007. 

Appendix E. Modelling and Databases – MODTF, p. 19. 

Task MOD.06. Conduct policy option analyses as requested by CAEP. This effort requires coordinating work including a 

specific framework and set of assumptions required to support CAEP/8 analyses (WG1/2/3/FESG). 

Appendix F. Forecast and Economic Analysis – FESG, p.22. 

Task F.07. Perform a cost-effectiveness analysis of potential NOX stringency options. 

10 More details on the work programmes of MODTF and FESG for CAEP/8 are provided in [CAEP/8-WP/05] “MODTF Work 

Programme Overview” and [CAEP/8-WP/04] “Report of the FESG”. 11 LTO – Landing and take-off. 

12 Ref. [CAEP/8-IP/13] “NOx Stringency” and [CAEP/8-IP/14] “Economic Assessment of the NOx Stringency Scenarios”. 
 

CAEP/8-WP/15 

Appendix B 

English only

APPENDIX B 

NOX STRINGENCY ASSESSMENT RESULTS 

TABLES AND GRAPHICS 

Table 1. NOx Stringency Scenarios 

NOx Stringency 

Scenario 

Small Engines Large Engines 

[26.7 kN / 89 kN Foo]13,14 OPR15>30 Slope16

1 -5% / -5% -5% 2 

2 -10% / -10% -10% 2.2 

3 -10% / -10% -10% 2 

4 -5% / -15% -15% 2.2 

5 -15% / -15% -15% 2.2 

6 -5% / -15% -15% 2 

7 -15% / -15% -15% 2 

8 -10% / -20% -20% 2.2 

9 -15% / -20% -20% 2.2 

10 -20% / -20% -20% 2.2 

Table 2 - Cost Results 

NOx Stringency 

Scenarios 

Low Cost Estimate ($M) 

3% discount, 2016, LRV17

High Cost Estimate ($M) 

3% discount, 2016, LRV 

1 - 5 $ 1,922 $ 2,500 

6 - 7 $ 6,412 $ 9,470 

8 - 10 $ 10,878 $ 21,507 

Table 3 – Cost-Effectiveness Ranking, Large Engines 

Ranking Stringency Reference NOx Reduction % / Slope of Dp/Foo 

1 NS01, NS02, NS03, NS04, NS05 -5% / 2.0, -10% / 2.2, -10% / 2.0, -15% / 2.2 

2 NS06, NS07 -15% / 2.0 

3 NS08, NS09, NS10 -20% / 2.2 

 

13 Foo – Thrust rating. For engine emissions purposes, the maximum power/thrust available for takeoff under normal operating 

conditions at ISA (International Standard Atmosphere) sea level static conditions without the use of water injection as 

approved by the certificating authority. Thrust is expressed in kilonewtons (kN). 14 Incremental stringency options defined for small engines with thrust ratings (Foo) comprised between 26.7 kN and 89 kN. 15 OPR – Overall Engine Pressure Ratio. The engine pressure ratio is defined as the ratio of the mean total pressure at the last

compressor discharge plane of the compressor to the mean total pressure at the compressor entry plane, when the engine is 

developing its takeoff thrust rating (in ISA sea-level static conditions). 16 Slope of the line of the NOx stringency options at engine pressure ratio (PR) greater than 30. 17 LRV – Loss in Resale Value. 
CAEP/8-WP/15 

Appendix B B-2 

Table 4 – Cost-Effectiveness Ranking, Small Engines 

Ranking Stringency Reference NOx Reduction % 

1 NS01, NS04, NS06 -5% / -5%, -5% / -15% 

2 NS08 -10% / -20% 

3 NS02, NS03 -10% / -10% 

4 NS10 -20% / -20% 

5 NS09 -15% / -20% 

6 NS05, NS07 -15% / -15% 

Figure 1 - Number of Engine Families Requiring an MS Technology Response 

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

NOx Stringency Option

Number of Engine Families

MS3

MS2

MS1

Large Engines

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

NOx Stringency Option

Number of Engine Families

MS2

MS1

Small Engines

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

NOx Stringency Option

Number of Engine Families

MS3

MS2

MS1

Large Engines

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

NOx Stringency Option

Number of Engine Families

MS2

MS1

Small Engines

Figure 2 - Total Below 3,000 ft. NOx Reduced Relative to Baseline 

All Engines 
B-3 

CAEP/8-WP/15 

Appendix B

Figure 3 - Percentage of Below 3,000 ft. NOx Reduced Relative to Baseline 

All Engines 

Figure 4 - Total Below 3,000 ft. NOx Reduced Relative to Baseline 

Large Engines 
CAEP/8-WP/15 

Appendix B B-4 

Figure 5 - Total Below 3,000 ft. NOx Reduced Relative to Baseline 

Small Engines 

Figure 6 - Total Above 3,000 ft. NOx Reduced Relative to Baseline 

All Engines 
B-5 

CAEP/8-WP/15 

Appendix B

Figure 7 - Percentage of Above 3,000 ft. NOx Reduced Relative to Baseline 

All Engines 

Figure 8 - Maximum Potential Full Flight Fuel Burn Penalty Relative to Baseline 

All Engines 
CAEP/8-WP/15 

Appendix B B-6 

Figure 9 - Percentage of Maximum Potential Full Flight Fuel Burn Penalty Relative to Baseline 

All Engines 

Figure 10 - NOx Cost-Effectiveness – Large Engines 

$-

$5,000

$10,000

$15,000

$20,000

$25,000

0.0 0.2 0.4 0.6 0.8 1.0

$20,000/tonne

$15,000/tonne

$10,000/tonne

$5,000/tonne

cost effectiveness

$25,000/tonne

$30,000/tonne

$35,000/tonne

$40,000/tonne

*Assumes loss in resale value LTO NOx Reduction

(Million Tonnes, 2009 – 2036)

Industry Cost*

($ Million, 2009 – 2036)

$45,000/tonne

$50,000/tonne

$55,000/tonne

$60,000/tonne

Large Engines

• 3% discount rate

• Low/high cost range

Large Engines

• 3% discount rate

• Low/high cost range

NS

6-7

NS

8-10

NS

1-5

2016 2012
B-7 

CAEP/8-WP/15 

Appendix B

Figure 11 - NOx Cost-Effectiveness – Small Engines 

$-

$100

$200

$300

$400

$500

0 2000 4000 6000 8000 10000

$40,000/tonne

$30,000/tonne

$20,000/tonne

$10,000/tonne

cost effectiveness

$50,000/tonne

$60,000/tonne

$70,000/tonne

$80,000/tonne

*Assumes loss in resale value LTO NOx Reduction

(Tonnes, 2009 – 2036)

Industry Cost*

($ Million, 2009 – 2036)

Small Engines

• 3% discount rate

• 2016 stringency effectivity

• Low/high cost range

Small Engines

• 3% discount rate

• 2016 stringency effectivity

• Low/high cost range

NS

2,3

NS

9

NS

1,4,6

NS

8

NS

10

NS

5,7

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