(44 pages) 

 CAEP.8.WP.018.2.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 TECHNICAL MANUAL (ETM), VOL II ON THE USE OF 

PROCEDURES IN THE EMISSIONS CERTIFICATION OF AIRCRAFT 

ENGINES

(Presented by WG3 Rapporteurs) 

SUMMARY 

The Working Paper describes the work of WG3 in development of the new 

Environmental Technical Manual (ETM), Vol II on the use of procedures in 

the emissions certification of aircraft engines. This is in fulfilment of remit 

E.07. 

Action by the CAEP is in paragraph 3. 

1. INTRODUCTION 

1.1 As part of the CAEP/7 work programme, WG3 had been requested to develop a second 

Volume of the Environmental Technical Manual (ETM) for emissions, consistent and compatible with 

the approach taken for noise (Vol I), including but not limited to guidance material in support of Annex 

16, Vol II. WG3 had consequently agreed on the structure, definitions of guidance material categories and 

principles to follow in developing the ETM. CAEP/7 endorsed the material developed thus far.

1.2 Regarding publication of this material, CAEP/7 accepted the Secretary’s proposal that 

since a revised version of Vol I of the ETM was anticipated, and in view of the limited nature of the new 

material intended for Vol II at present, the plan was to delay formal publication until both Volumes could 

be produced together. However, so that the material developed for Vol II could be used in the interim, it 

was proposed to publish it as a guideline on the ICAO website. It was considered that this procedure 

would still allow national authorities to incorporate the material in their own regulations by reference to 

the CAEP/7 report, if they so desired. 

International Civil Aviation Organization

WORKING PAPER 

CAEP/8-WP/18 

24/11/09
CAEP/8-WP/18 - 2 -

1.3 The CAEP/8 work programme included task E.07 which requested WG3 to further 

develop the Environmental Technical Manual (ETM) Vol II as guidance to support the ICAO Annex 16, 

Vol II certification requirements for aircraft engine emissions. 

1.4 WG3 remitted this to the Certification Task Group (CTG) that agreed to set up an ETM 

ad-hoc group to take forward this work principally though telecons in between WG3 meetings. The adhoc group has held a total of 17 telecons and this paper provides an overview of the main issues that have 

been considered since CAEP/7, with reference to the relevant part of Annex 16, Vol II (e.g. Chapter 2, 

Appendix 3, etc) 

1.5 Appendix A provides the full text of the updated ETM. 

2. OVERVIEW OF MAIN ISSUES DISCUSSED 

2.1 Date of manufacture (Chapters 1 and 2) 

2.1.1 The current Annex 16 Vol II applicability requirements are related to the “date of 

manufacture of the first individual production engine”, while for NOx production cut-off requirements the 

date of manufacture refers to “date of issue of the document attesting that the 

individual.....engine....conforms to the requirements of the type”. Explanatory information on this text 

and agreed interpretation has been incorporated into the ETM. 

2.2 Applicability of Certification Requirements (Chapter 2) 

2.2.1 This item was carried across from CAEP/7 (modernization item MOD6) and deals with 

the applicability of Standards for derivative engine models. The current wording of Annex 16 Vol II 

could be interpreted to mean that any change in an engine design would require a full emissions 

certification test programme. This does not reflect the real world as many changes which are considered 

major, according to airworthiness considerations, may have no or very small effects on the emissions 

performance of that product. 

2.2.2 Based on principles agreed to by WG3 that take into account past and current 

certification practice, guidance has been developed on how to treat these changes which have no or very 

little effect on emission levels. It also includes guidance on when new emissions levels would need to be 

determined, when a new certification test would be required, and whether the certification basis from the 

parent product would be retained or, due to substantial modifications, the latest standard applied. 

2.3 Process and criteria for NOx production cut-off exemptions 

(Chapter 2) 

2.3.1 In light of the ongoing discussions on a potential CAEP/6 production cut-off requirement, 

WG3 identified the need to develop guidance on the process and evaluation criteria for applications 

associated with NOx production cut-off exemptions. 

2.3.2 Significant discussions have taken place since SG2009 in order to harmonise the 

approach on a global basis and the agreed guidance has been incorporated in the ETM. 
- 3 - CAEP/8-WP/18 

2.4 Validation of emissions sampling rakes (Chapter 2) 

2.4.1 Annex 16 Vol II requires that “the applicant shall provide evidence to the certificating 

authority, by means of a detailed traverse, that the proposed probe design and position does provide a 

representative sample for each prescribed power setting”. This procedure is time consuming, costly and 

problematic with some engine designs. Further, the absence of a definition of the term “representative”, or 

an agreed test to demonstrate this, had resulted in a lack of clarity in the application of the requirements. 

2.4.2 ETM guidance has been agreed clarifying that a sample taken at the engine core exit 

plane, with a suitably designed rake, would provide more accurate and representative measurements than 

that from a rake measuring mixed flow (core and bypass). As such, future emissions certification projects 

should aim to measure at the core exit plane. Where mixed flow measurements are unavoidable, the ETM 

provides a default Smoke Number (SN) dilution correction based on the ICAO approved FOA. It was 

also agreed that, while 12 orifices is the minimum number for a sampling rake in Annex 16 Vol II, a more 

appropriate number would be 20 when validated by means of a detailed traverse. 

2.4.3 The required traverse measurements to demonstrate representative sampling are 

dependent on the rake design, number of sampling orifices and existing experience with similar engines 

or derivatives. These rake design requirements complement the system check performed in the carbon 

balance criteria. 

2.5 Status of supersonic engine emissions requirements 

(Chapter3) 

2.5.1 The CAEP Secretariat highlighted that they had received questions regarding the current 

status of the Vol II certification requirements for engines intended for propulsion at supersonic spends. 

2.5.2 WG3 has clarified their current position on this issue, which had previously been 

presented to CAEP/7, via the inclusion of text within the ETM. 

2.6 Smoke emission evaluation (Appendix 2) 

2.6.1 All engine manufacturers have confirmed that they use green tristimulus filters within 

their reflectometers, as per the requirements in SAE ARP 1179C on “Aircraft Gas Turbine Engine 

Exhaust Smoke Measurement”. This has been recognised as best practise in terms of ensuring an accurate 

correlation between filter reflectance and SAE Smoke Number measurements. The filter requirement 

relates back to the ANSI PH2.17, which has now been replaced by ISO 5-4, and was introduced to ensure 

that the photosensitive receptor provides for an effective spectral sensitivity which is equivalent to the 

average human eye. ETM guidance has been agreed specifically referring to the use of the filter as best 

practice. 

2.7 Instrumentation and measurement techniques for gaseous 

emissions (Appendix 3) 

2.7.1 Based on existing FAA AC34-1B guidance material, ETM guidance has been agreed for 

Annex 16 Vol II Appendix 3 on instrumentation and measurement techniques for gaseous emissions. The 

text includes a reference to SAE ARP 1256 on "Procedure for the Continuous Sampling and Measurement 

of Gaseous Emissions from Aircraft Turbine Engines". 
CAEP/8-WP/18 - 4 -

2.7.2 This work included the need to clarify Section 7 on the post processing of measured 

emissions data taken during a certification test. A worked example of the calculation procedure for 

correcting measured Emissions Indices (EIs) to reference conditions was produced by EASA, and 

validated by ICCAIA. It is based on combustor inlet conditions (T3, P3 and fuel flow) from a validated 

engine performance model and, again, makes reference to the relevant SAE ARP 1533 on “Procedure for 

the Analysis and Evaluation of Gaseous Emissions from Aircraft Engines”. In addition to being a method 

for calculating emissions data, this ARP also includes two fully worked examples of the matrix method 

for solving the combustion chemical equation. 

2.7.3 In coordination with SAE E-31, the ad-hoc group has also begun to develop guidance 

associated with the Attachments to Annex 16, Vol II, Appendix 3 which contain the specifications for all 

gaseous emissions analysers. This work has so far focused on Attachment A covering HC analysers. 

2.8 Outstanding issues for CAEP/9 

2.8.1 Several ongoing issues that have not been completed are expected to be carried over into 

the CAEP/9 work programme. These include evaluation of Optical Smoke Meters (OSM) for use in 

certification tests, and development of further guidance on Appendix 3 Attachments. 

3. ACTION BY THE CAEP 

3.1 The CAEP is invited to: 

a) Review and approve the full text of the ETM Vol II, as presented in Appendix A; and 

b) Recommend the publication of this document by ICAO. 

— — — — — — — —
 

CAEP/8-WP/18 

Appendix 

English only

APPENDIX 

ICAO EMISSIONS ENVIRONMENTAL TECHNICAL MANUAL 

GUIDELINES ON THE USE OF PROCEDURES IN THE 

EMISSIONS CERTIFICATION OF AIRCRAFT ENGINES 

TABLE OF CONTENTS 

Nomenclature 

SECTION 1. INTRODUCTION

1.2 Purpose 

1.3 Framework 

1.4 Emissions Compliance Demonstration Plans 

1.5 Emissions Certification Reports 

1.

SECTION 2. GUIDANCE MATERIAL 

Part I. DEFINITIONS AND SYMBOLS

PART II. VENTED FUEL [Reserved]

Part III. EMISSIONS CERTIFICATION

CHAPTER 1: Administration [Reserved] 

CHAPTER 2: Turbo-jet and turbofan engines intended for propulsion only at subsonic speeds 

2.

2.1 General 

2.1.1 Applicability 

2.1.2 Emissions involved [Reserved] 

2.1.3 Units of measurement 

2.1.4 Reference Conditions

2.1.4.1 Atmospheric conditions 

2.1.4.2 Thrust settings [Reserved] 

2.1.4.3 Reference emissions landing and take-off (LTO) cycle 

2.1.4.4 Fuel specification 

2.1.5 Test conditions [Reserved] 

2.2 Smoke 
CAEP/8-WP/18 

Appendix 18 A-2 

2.2.1 Applicability [Reserved] 

2.2.2 Regulatory smoke number [Reserved] 

2.3 Gaseous Emissions 

2.3.1 Applicability [Reserved] 

2.3.2 Regulatory levels 

2.4 Information Required 

2.4.1 General information [Reserved] 

2.4.2 Test information [Reserved] 

2.4.3 Derived information 

CHAPTER 3: Turbo-jet and turbofan engines intended for propulsion at supersonic speeds [Reserved] 

APPENDICES 

APPENDIX 1. Measurement of reference pressure ratio [Reserved] 

APPENDIX 2. Smoke emission evaluation 

1. Introduction and Definitions [Reserved 

2. Measurement of Smoke Emissions 

2.1 Sampling probe for smoke emissions 

2.2 Sampling line for smoke emissions 

2.3 Smoke analysis system 

2.4 Fuel specifications 

2.5 Smoke measurement procedures 

2.5.1 Engine operation 

2.5.2 Leakage and cleanliness checks 

2.5.3 Smoke measurement 

3. Calculation of smoke number from measured data 

4. Reporting of Data to the Certificating Authority [Reserved] 

APPENDIX 3. Instrumentation and measurement techniques for gaseous emissions 

1. Introduction 

2. Definitions [Reserved] 

3. Data Required 

3.2 Gaseous Emissions 

3.3 Other Information [Reserved] 
 A-3 CAEP/8-WP/18 Appendix 

4. General Arrangement of the System 

5. Description of Component Parts 

5.1 Sampling system 

5.1.1 Sampling probe 

5.1.2 Sampling lines [Reserved] 

5.2 HC Analyser [Reserved] 

5.3 CO and CO2 analysers [Reserved] 

5.4 NOx analysers [Reserved] 

5.5 Smoke analysis system [Reserved] 

6. General Test Procedures 

6.1 Engine operation [Reserved] 

6.2 Major instrument calibration [Reserved] 

6.3 Operation [Reserved] 

6.4 Carbon balance check [Reserved] 

7. Calculations 

7.1 Gaseous emissions [Reserved] 

7.1.1 General [Reserved] 

7.1.2 Basic Parameters [Reserved] 

7.1.3 Correction of emissions indices to reference conditions [Reserved] 

7.2 Control parameter functions [Reserved] 

7.3 Exceptions to the proposed procedures [Reserved] 

Attachment A to Appendix 3: Specification for HC Analyser 

1. General 

2. Synergistic Effects 

3. Optimisation of Detector Response and Alignment 

3. APPENDIX 4. Specification for fuel to be used in aircraft turbine engine emission 

testing 

4.

5. APPENDIX 5. Instrumentation and measurement techniques for gaseous emissions from 

afterburning gas turbine engines [Reserved] 

6.

7. APPENDIX 6. Compliance procedures for gaseous emissions and smoke [Reserved] 

8.
CAEP/8-WP/18 

Appendix 18 A-4 

NOMENCLATURE 

Symbols and Units 

SYMBOLS AND ABBREVIATIONS EMPLOYED IN VOLUME II OF THIS MANUAL ARE CONSISTENT WITH 

THOSE CONTAINED IN ANNEX 16 – ENVIRONMENTAL PROTECTION, VOLUME II – AIRCRAFT ENGINE 

EMISSIONS (THIRD EDITION, JULY 2008). 
 A-5 CAEP/8-WP/18 Appendix 

SECTION 1 - INTRODUCTION

3.2 Purpose 

3.2.1 The aim of Volume II of this manual is to promote uniformity in the implementation of ICAO 

Annex 16 – Environmental Protection, Volume II – Aircraft Engine Emissions, by providing guidance to 

certificating authorities and applicants regarding the intended meaning of the current Annex 16, Volume 

II Emissions Standards and those specific procedures that are deemed acceptable in demonstrating 

compliance to these Standards. 

3.2.2 This manual also provides guidance in the wider application of equivalent procedures that have 

been accepted as a technical means for demonstrating compliance with the emissions certification 

requirements of Annex 16, Volume II. Such equivalent procedures are referred to in Annex 16, Volume 

II, but are not dealt with in the same detail as in the Appendices which describe the emissions evaluation 

methods for compliance with the relevant Chapters of Annex 16, Volume II.

3.2.3 Annex 16, Volume II procedures must be used unless an equivalent procedure is approved by the 

certificating authority. Procedures presented in this manual should not be considered as limited only to 

those described herein, as this manual will be expanded as new procedures are developed. Also, their 

presentation does not infer limitation of their application or commitment by certificating authorities to 

their further use. 

3.2.4 References to Annex 16, Volume II relate to the Amendment 6 thereof. 

3.3 Framework 

3.3.1 The basic framework of Volume II of this manual is a replication of the ICAO Annex 16, Volume 

II structure in order to ensure easy reference between the requirements and guidance. References in the 

Table of Contents are only made to a part of the requirements when there is associated guidance material, 

otherwise the relevant paragraph has been “Reserved” for future use. There is minimal repetition of the 

requirement text in order to simplify the ETM content, lower maintenance costs, and reduce the danger of 

inconsistencies between Volume II and the ETM following future revisions. The requirement text is 

enclosed between 2 horizontal lines. 

3.3.2 This first Section provides general information while the second Section contains guidance 

material to ICAO Annex 16, Volume II. The format of the guidance material includes three types of 

information described as Explanatory Information, Equivalent Procedures and Technical Procedures. The 

Definitions of these three types of information are as follows: 

Explanatory Information 

• Explains ICAO Annex 16 Emissions Standards language 

• States current policies of regulatory authorities regarding compliance with ICAO Annex 16 

Emissions Standards 

• Provides awareness of critical issues for approval of applicants’ compliance methodology 

proposals 
CAEP/8-WP/18 

Appendix 18 A-6 

Equivalent Procedures 

• An equivalent procedure is a test or analysis procedure which, while differing from one 

specified in Annex 16 Volume II, in the technical judgement of the certificating authority 

yields effectively the same emissions levels as the specified procedure. 

• The use of equivalent procedures may be requested by applicants for many reasons, 

including: 

c) to make use of previously acquired certification test data for the engine type; and 

d) to minimize the costs of demonstrating compliance with the requirements of ICAO Annex 16, 

Volume II by keeping engine test time, test bed usage, and equipment and personnel costs to 

a minimum. 

Technical Procedures 

• A technical procedure is a test or analysis procedure not defined in detail in ICAO Annex 16 

Emissions Standards but which certificating authorities have approved as being acceptable for 

compliance with the general provisions the Emissions Standards specify. 

3.4 Emissions Compliance Demonstration Plan 

3.4.1 Prior to undertaking an emissions certification demonstration, the applicant is normally required 

to submit to the certificating authority an emissions compliance demonstration plan. This plan contains 

the method by which the applicant proposes to show compliance with the emissions requirements. 

Approval of this plan and the proposed use of any equivalent procedure remains with the certificating 

authority. The determination of equivalency for any procedure or group of procedures must be based upon 

the consideration of all pertinent facts relating to the application. 

3.4.2 Emissions compliance demonstration plans should include the following types of information: 

Introduction:

description of the engine emissions certification basis, i.e. the applicable Annex 16, 

Volume II Amendment and Chapter 

e) engine description: 

type, model number and specific details of the basic configuration to be certified 

f) engine emissions certification methodology: 

test concepts, equivalent procedures and technical procedures 

g) test description: 

test methods to comply with the emissions standards 
 A-7 CAEP/8-WP/18 Appendix 

h) measurement system: 

description of measurement and sampling system components and procedures, 

including calibration procedures, that are intended to be used to demonstrate 

compliance with the emissions standards 

i) data evaluation procedures: 

Emissions evaluation and adjustment procedures (including equivalent and technical 

procedures such as those provided in this manual) to be used in compliance with the 

provisions of ICAO Annex 16, Volume II appropriate to the engine type being 

certificated 

3.5 Emissions Certification Reports 

3.5.1 Following completion of an emissions certification demonstration test, an applicant is normally 

required to submit an emissions certification report. This report provides a complete description of the test 

process and the test results with respect to compliance with the provisions of Annex 16, Volume II. 

3.5.2 These reports should include the following types of information: 

basis for test approval 

the approved emissions certification compliance plan for the engine type and model 

being certificated. 

description of tests 

actual configurations tested and non-conforming items (with justification that they 

are not significant to emissions, or if significant, can be dealt with by an approved 

method), test methodology (including equivalent procedures and technical 

procedures), tests conducted, test data validity, and data analysis and adjustment 

procedures used. 

test results 

data to demonstrate compliance with the provisions of Annex 16, Volume II 

regarding maximum emissions levels for the engine type being certificated. 

references 
CAEP/8-WP/18 

Appendix 18 A-8 

SECTION 2 – GUIDANCE MATERIAL

PART I – DEFINITIONS AND SYMBOLS

CHAPTER 1: DEFINITIONS

Exhaust nozzle. In the exhaust emissions sampling of gas turbine engines where the jet 

effluxes are not mixed (as in some turbofan engines for example) the nozzle considered is 

that for the gas generator (core) flow only. Where, however, the jet efflux is mixed the 

nozzle considered is the total exit nozzle.

Equivalent Procedure 

Defining the exhaust nozzle in this manner, where the fan and core nozzles are not coplanar, is 

problematic. With imperfect knowledge of the fan flow characteristics when discharged upstream of the 

core exit nozzle, the effective nozzle size may be ambiguous. This affects the extreme downstream 

location of the sampling probe or rake and causes problems in terms of a detailed traverse of the total 

exhaust nozzle. In order to obtain equivalent, but more accurate gaseous emissions measurements and 

representative samples, it is considered best practise to arrange the engine configuration in such a way as 

to separate the fan and core flows, without affecting the engine performance, and sample just the core 

flow. The above is equally applicable to both gaseous and smoke emission measurements, however for 

smoke, the dilution and mixing of bypass air also needs to be taken into account and this is covered under 

Appendix 2 paragraph 2.1. 

......................... 

CHAPTER 2. SYMBOLS 

[Reserved] 

......................... 

PART II. VENTED FUEL 

 

[Reserved] 

......................... 
 A-9 CAEP/8-WP/18 Appendix 

PART III – EMISSIONS CERTIFICATION 

Chapter 1. Administration [Reserved]

Chapter 2. Turbo-jet and turbofan engines intended for propulsion only at subsonic speeds 

2.1 General 

2.1.1 Applicability 

Explanatory Information 

Part I of Annex 16 Vol. II defines a derivative version1

 in terms of emissions certification, but this 

definition is only referred to in the Annex in the context of granting exemptions and does not specify how 

the rule should be applied to modifications of already certificated engine types. Many changes to a 

certificated engine, which are considered to be major from an airworthiness perspective, require an 

amendment or supplement to the Type Certificate (TC). However, this same modification(s) may have no 

or very little effect on the emissions characteristics of that engine. If Annex 16 is interpreted literally, 

these small effects would require a full investigation of compliance against the emissions certification 

requirements. This may not be necessary in many cases. The following guidelines have been developed in 

order to help determine whether a modification could be classified as a “no emission change” or if it 

would affect the emissions levels to such an extent that the engine type would need to be re-certificated 

against Annex 16 Vol. II requirements. Of course manufacturers may elect to recertify to the latest 

requirements at any time. 

Technical procedure 

9. 1. No emissions change

The principle of a “no emissions change” criteria is that an engine would not need to be re-certified 

against the emissions requirements if the manufacturer could demonstrate that the modification(s) would 

result in a small cumulative change to the current certified engine emission levels. Cumulative change 

could be as a result of more than one change at a given time or multiple changes from more than one 

derivative version of the same engine being developed. 

The potential determination of a “no emissions change” is limited to the following conditions: 

i) If all of the characteristic levels prior to any modification are greater than or equal to 95% 

of the existing ICAO standard, the manufacturer must provide new engine emissions test 

data to demonstrate that the resulting characteristic levels after the cumulative changes 

since original emission testing will not exceed the existing ICAO exhaust emission 

standard. 

 

1 Derivative version. An aircraft gas turbine engine of the same generic family as an originally type certificated engine and having features which 

retain the basic core engine and combustor design of the original model and for which other factors as judged by the certificating authority, 

have not changed.
CAEP/8-WP/18 

Appendix 18 A-10 

ii) If all of the characteristic levels are less than 95% of the existing ICAO exhaust emission 

standards, then new or related emissions test data and good engineering judgment based 

on substantive analysis may be an acceptable means to demonstrate that the resulting 

emission levels after the cumulative changes since original emission testing will not 

exceed the existing ICAO exhaust emission standard.2

 Analyses should consider areas 

such as cycle changes, combustor and fuel nozzle design, or large changes in combustor 

inlet velocity profile or turbine cooling flows as discussed in paragraph 5 below. 

iii) Any new emissions test data and/or engineering/technical analysis under subparagraphs 

(i) and (ii) must demonstrate that the cumulative changes in absolute emission levels, as 

compared to those of the original certification, are within:3

NOx ± 3 g/kN 

HC ± 1 g/kN 

CO ± 5 g/kN 

Smoke ± 2 SN 

With respect to the tracking of cumulative changes, an applicant should maintain formal documentation 

of the technical basis for all approved “no emissions changes” for an engine model. The tracking list will 

be reproduced in each emissions certification dossier demonstration. 

If a modification is classified as a “no emissions change”, the characteristic levels for the 

derivative version will be considered the same as the parent engine. 

2. Changes requiring new emissions levels

A change, or cumulative set of changes to the type design, which is/are not considered a “no emissions 

change” and thus affects the characteristic levels would require new characteristic levels to be determined 

during the certification program. This would normally be done through a new emissions certification test. 

3. Existing emissions certification basis retained

If a modified engine remains on the existing type certificate, it may retain the existing certification basis 

of the parent engine4

 if the modification(s): 

i. meets the demonstration criteria of 1(i) or 1(ii); 

ii. results in a decrease of the absolute emissions levels; 

iii. results in an increase of the absolute emissions levels below those prescribed in 1(iii);

iv. are necessary for improved safety and continued airworthiness (e.g. ADs).

4. Latest emissions standards applied

An engine type should demonstrate compliance with the latest emissions standards when: 

i) The engine requires a new TC; 

ii) The engine modification(s) involves significant technical modifications and where, in the 

judgment of the certificating authority, the engine would not meet the definition of a 

derivative engine as defined in Annex 16 Vol. II; 

 

2

 Good engineering judgment means judgments made consistent with generally accepted scientific and engineering principles and 

all available relevant information. 

3

 Absolute emission level refers to the average of the measured emission levels corrected to reference conditions. This does not 

include application of the Appendix 6 statistical factors used to determine characteristic levels. 4

 In terms of emissions certification the parent engine must have demonstrated emissions compliance and is not considered to be a derivative of 

an even earlier standard itself. 
 A-11 CAEP/8-WP/18 Appendix 

iii) The engine modification(s) does not meet the demonstration criteria of 1(i) or 1(ii); 

iv) The engine modification(s) results in an increase in any of the absolute emissions levels 

below those prescribed in 1(iii) but which results in an exceedance of the existing ICAO 

standard; 

v) The engine modification(s) results in an increase in any of the absolute emissions levels 

in excess of those prescribed in 1(iii); or 

v) There are significant future environmental impacts5

. 

10. 5. Engineering analysis examples

The basic premise in assessing emissions effects of design changes from an engineering analysis 

perspective is that emissions are mainly affected by cycle changes, combustor and fuel nozzle design, or 

large changes in combustor inlet velocity profile or turbine cooling flows. A number of examples are 

provided below for illustration. 

Cycle change 

Annex 16 Vol. II only requires the measurement of fuel mass flow by direct measurement to an 

accuracy of ±2% (Attachment F to Appendix 3, d.) and thrust to an accuracy of ±1% at take-off 

power and ±5% at the minimum thrust. It may be that no affect on emissions beyond those prescribed 

in paragraph 1(iii) would be expected unless changes in air fuel ratio (AFR) result in a specific fuel 

consumption (SFC) change of more than 1%. 

Regarding combustor inlet temperature (T3), the Annex also states that the combustor inlet 

parameters shall preferably be measured but may be calculated from ambient conditions by 

appropriate formulas. Following a comparison on T3 detailed traverses versus typical certification test 

measurements, it was concluded that measurements are within ±6°F (ca. 3.5°C) of theoretical 

calculations. It may be that no affect on emissions beyond those prescribed in paragraph 1(iii) would 

be expected if the cycle T3 change is within ±6°F.

Combustor and fuel nozzle 

Combustor and fuel nozzle changes include some key design characteristics that can significantly 

affect emissions (e.g. swirl cup flow, primary hole flow, front end cooling flow, injector atomization). 

Changes within or outside of production tolerances or part-to-part variation may indicate when a 

change would be important for emissions or else considered to be not measurable. No affect on 

emissions beyond those in prescribed paragraph 1(iii) might be expected if the changes are within 

current part-to-part variation or prescribed production tolerances. 

Boundary conditions 

Similar limitations to that for key design characteristics can be applied to that resulting from changes 

in boundary conditions. For example, if turbine cooling changes by 2%, one could look at the 

resulting change in combustor flow distribution, and if the change is of the same order as typical partto-part variation, it may be acceptable to conclude that no affect on emissions beyond those 

prescribed in paragraph 1(iii) would be expected. 

 

5

 The application of an old engine type to a new aircraft design could mean a life time of 30 years or more and thereby have a more serious 

impact on the environment than if the engine was produced for spares only. 
CAEP/8-WP/18 

Appendix 18 A-12 

2.1.1.1 The provisions of this chapter shall apply to all turbo-jet and turbofan engines as 

further specified in 2.2 and 2.3, intended for propulsion only at subsonic speeds, except when 

certificating authorities make exemptions for: 

a) specific engine types and derivative versions of such engines for which the type 

certificate of the first basic type was issued or other equivalent prescribed procedure 

was carried out before 1 January 1965; and 

b) a limited number of engines beyond the dates of applicability specified in 2.2 and 

2.3 for the manufacture of the individual engine. 

2.1.1.2 In such cases, an exemption document shall be issued by the certificating authority, 

the identification plates on the engines shall be marked “EXEMPT,” and the grant of 

exemption shall be noted in the permanent engine record. 

 

Technical Procedure 

1. Introduction 

1.1 The current ICAO Annex 16, Volume II contains two different references to applicability 

dates: 

• “date of manufacture for the first individual production model” which refers to the engine 

type certification; and 

• “date of manufacture for the individual engine” which refers to the production date of a 

specific engine serial number. 

1.2 The second reference is used in the application of global engine NOx production cut-off 

standards which specify a date after which all in-production engine models must meet a certain NOx 

emission standard. For example, all engines manufactured after 31st December 1999 must be compliant 

with the CAEP/2 NOx standard. It should be noted that these requirements are applicable to complete 

new “engine units” released into service as spares or for new aircraft installations, as discussed below, 

and not engine components required for maintenance aspects, overhaul, parts, etc. 

1.3 It is recognized that there may be certain circumstances where it is justified to permit 

manufacturers to continue to produce new non-compliant engine units after a production cut-off date. 

These take the form of exemptions against the relevant Annex 16 Vol. II provisions. 

1.4 In order to promote a harmonized global approach to the granting, implementing and 

monitoring of these exemptions, this section provides guidelines on the process and criteria for issuing 

exemptions against a NOx production cut-off standard. 
 A-13 CAEP/8-WP/18 Appendix 

11. 2. Exemption Process 

12.

13. 2.1 Application 

2.1.1 The applicant should submit a formal application letter to the competent authority6

 for the 

manufacture of the exempted engines, signed by an appropriate manager, and copied to all other relevant 

organizations and involved competent authorities. The letter should include the following information in 

order for the competent authority to be in a position to review the application: 

a) Administration 

 Name, address and contact details of the applicant. 

b) Scope of application for exemptions 

 Engine type (model designation, type certificate (TC) number, TC date, emission TC 

basis, ICAO Engine Emissions Databank Unique Identification (UID) Number). 

 Number of engine exemptions requested. 

 Duration (end date) of continued production of exempted engines. 

 Designate whether the proposed exempted engines are “spares” or “new”7

 and to whom 

the engines will be originally delivered. 

c) Justification for exemptions 

In applying for an exemption, an applicant should, to the extent possible, address the 

following factors, with quantification, in order to support the merits of the exemption request: 

 Technical issues, from an environmental and airworthiness perspective, which may have 

delayed compliance with a production cut-off. 

 Economic impacts on the manufacturer, operator(s) and aviation industry at large. 

 Environmental effects. This should consider the amount of additional NOx that will be 

emitted as a result of the exemption. This could include consideration of items such as: 

- the amount that the engine model exceeds the standard, taking into account any 

other engine models in the engine family covered by the same type certificate 

and their relation to the standard; 

- the amount of NOx that would be emitted by an alternative engine for the same 

application; 

- impact of changes to reduce NOx on other environmental factors, including 

community noise and CO2 emissions. 

 Impact of unforeseen circumstances and hardship due to business circumstances beyond 

the manufacturer’s control (e.g. employee strike, supplier disruption or calamitous 

events) 

 Projected future production volumes and plans for producing a compliant version of the 

engine model seeking exemption. 

 Equity issues in administering the production cut-off among economically competing 

parties (e.g. provide rationale for granting this exemption when another manufacturer has 

a compliant engine and does not need an exemption, taking into account the implications 

 

6

 In most cases this will be the Certificating Authority although it may vary depending on individual Member State processes. 

7 In the case that engines are to be “new” (installed on new aircraft), and would thus result in a larger negative environmental impact 

compared to spare engines, greater justification could be required to approve this application. 
CAEP/8-WP/18 

Appendix 18 A-14 

for operator fleet composition, commonality and related issues in the absence of the 

engine for which exemptions are sought) 

 Any other relevant factors 

2.2 Evaluation 

2.2.1 The evaluation of an exemption application should be based on the justification provided and 

the following definitions and criteria: 

a) Use of engines 

“Spare engines” are defined as complete new engine units which are to be installed on inservice aircraft for maintenance and replacement. It can be presumed that applications 

associated with engines for this purpose would be granted as long as the emissions were equal 

to or better than those engines they are replacing. The application should also include the 

other items described in paragraphs 2.1.1 (a) and (b), but it would not need to include the 

items specified in paragraph 2.1.1(c). For spare engines, the evaluation of the exemption 

application would be conducted for recordkeeping and reporting purposes, but it would not be 

done for approval of an exemption. 

 “New engines” are defined as complete new engine units which are to be installed on new 

aircraft. They can only be exempted from a NOx production cut-off requirement if they 

already meet the previous standard (e.g. exemption from a CAEP/6 NOx production cut-off is 

only possible if an engine type already meets the CAEP/4 NOx standard). Also, in order to 

gain approval for this type of exemption the applicant must clearly demonstrate that they 

meet the criteria for an exemption by including items described in paragraphs 2.1.1(a), (b), 

and (c). The competent authority may require additional information regarding the 

appropriateness of the potential exemption. 

b) Number of New Engine exemptions 

Exemptions should be based on a total number of engines and time period for delivery of 

these engines, which would be agreed at time the application is approved and based on the 

considerations explained in 2.1.1.c) above. The number of engines exempted would normally 

not exceed 75 per engine type certificate and the duration would not exceed 4 years from the 

effective date of the production cut-off. Exemptions would only apply to non-compliant 

engine models on an engine type certificate. 

 Exemptions for new engines should be processed and approved by the competent authorities 

for both the manufacture of the exempted engines and the initial operator of the aircraft to 

which they are to be fitted. Given the international nature of aviation, civil aviation 

authorities of member states should attempt to collaborate and consult on the details of 

exemptions. In the case where engine type certification is done through a reciprocity 

agreement between or among member states, the states involved should coordinate on the 

processing of exemptions and concur before approval is granted. 

 As part of the review and approval process for exemptions from any production cut-off 

requirement associated with the CAEP/6 NOx standard, competent authorities may in some 

cases require a form of NOx emissions offsetting, as appropriate within the context of 

aviation and considering an applicant's ability to utilize such measures. However, offsetting 
 A-15 CAEP/8-WP/18 Appendix 

measures should be applied only when necessary in view of the number of engines for which 

exemptions are sought (e.g. greater than 30), duration and/or percent exceedance described in 

an application. 

c) Exceptions 

 Unlimited exemptions should be granted for spare engines having emissions equivalent to or 

better than the engines they replace. Engines for use on state aircraft (e.g. military, customs 

and police) are not covered by the Chicago Convention and therefore excluded from these 

civil aircraft NOx production cut-off requirements. 

2.3 Review 

14.

The competent authority should review, in a timely manner, the application using the information 

provided in para. 2.1 and against the definitions/criteria in para. 2.2. 

The analysis and the conclusions from the review should be communicated to the applicant in a formal 

response. If the application is approved, the response should clearly state the scope of the exemptions 

which have been granted. If the application is rejected, then the response should include a detailed 

justification. 

3. Registration and Communication 

3.1 Oversight of the granted exemptions should include the following elements: 

• The competent authority should publish details of the exempted engines in an official public 

register, including engine model, max. number of permitted exemptions and use of engines. 

• The applicant should have a quality control process for maintaining oversight of and 

managing the production of engines which have been granted exemptions against a NOx 

emissions production cut-off standard. 

• Exempted engine plates should be marked with “EXEMPT [SPARE] or [NEW]”. 

• An exemption should be recorded in the engine release to service document which states 

conformity with the Type Certificate (e.g. EASA Form 1, FAA Form 8130-3). Proposed 

standard text - “[New] or [Spare] Engine exempted from NOx emissions production cut-off 

requirement”. 

• The applicant should provide, on a regular basis and appropriate to the limitation of the 

approval, details to the competent authority on the actual exempted engines which have been 

produced (e.g. model, serial number, use of engine, aircraft type and serial number on which 

new engines are installed).
CAEP/8-WP/18 

Appendix 18 A-16 

2.1.1.3 The provisions of this chapter shall also apply to engines designed for applications 

that otherwise would have been fulfilled by turbo-jet and turbofan engines. 

Explanatory Information 

This sentence anticipates the introduction of future engine technologies. The emissions Standards in 

Chapter 2 would also be applicable to future engine types not categorized as a turbo-jet or turbofan but 

intended for use in international air transport services. The provision above is not directly applicable to 

turbo-prop engines for example. 

2.1.2 Emissions involved 

[reserved] 

2.1.3 Units of measurement 

Explanatory Information 

Smoke level is determined indirectly, by means of the loss of reflectance of a filter used to trap smoke 

particles from a prescribed mass of exhaust per unit area of filter. The result is a dimensionless smoke 

number “SN” which acts as a surrogate for, or indicator of, plume opacity. These smoke sampling and 

measurement procedures standardized in Annex 16 Appendix 2 are derived from SAE Aerospace 

Recommended Practice (ARP) 1179, Aircraft Gas Turbine Exhaust Smoke Measurement. 

The smoke measurement standard was developed for engines that generated smoke at considerably higher 

levels than are seen today. This affects the relative accuracy of the method. The measurement is 

considered (by the SAE E-31 committee that developed the method) to be no more accurate than ±3 SN. 

At smoke levels of SN 50-60 this represents an accuracy of 6%-5%. At regulatory standards of 30 and 

below, relative accuracy becomes 10%-20% or more. 

2.1.4 Reference conditions 

2.1.4.1 Atmospheric conditions 

15.
 A-17 CAEP/8-WP/18 Appendix 

Explanatory Information 

The reference atmospheric conditions to which the gaseous emissions (HC, CO and NOx) are to be 

corrected are the reference day conditions, as follows: Temperature = 15o

C, Humidity = 0.00634 kg 

H2O/kg of dry air, Pressure = 101.325 kPa. 

2.1.4.2 Thrust settings 

[Reserved] 

2.1.4.3 Reference emissions landing and take-off (LTO) cycle 

Explanatory Information 

The exhaust emissions test is designed to measure hydrocarbons, carbon monoxide, carbon dioxide and 

oxides of nitrogen concentrations, and to determine mass emissions through calculations during a 

simulated aircraft landing-takeoff cycle (LTO). The LTO cycle is based on times in mode data during 

high activity periods at major airports for four modes of engine operation: taxi/idle, takeoff, climbout, and 

approach. The mass emissions for these modes are combined to yield the reported emissions certification 

levels. 

2.1.4.4 Fuel specifications 

Explanatory Information 

Aircraft gas turbine engines use a variety of fuels. The specific fuel type and composition can and often 

does have a significant effect on engine emissions. Hence it is an important factor when comparing 

emissions levels from one engine with those from another. It is particularly important in evaluating 

engine emission levels relative to a regulation that was based, in part, on an assumed fuel specification. 

The ICAO fuel specification defined in Appendix 4 is typical, but tighter, than the general Jet A aviation 

fuel specification. The requirement for emissions certification testing with a fuel that meets a particular 

specification provides a fixed point of reference for the engine. It provides for some degree of control 

over the effect of fuel composition on smoke formation and emission. It also helps in the assessment of 

the effects of changing technology. 

2.1.5 Test Conditions 

[Reserved] 

2.2 Smoke 

2.2.1 Applicability 

[Reserved] 
CAEP/8-WP/18 

Appendix 18 A-18 

2.2.2 Regulatory Smoke Number 

[Reserved] 

2.3 Gaseous Emissions 

2.3.1 Applicability 

[Reserved] 

2.3.2 Regulatory levels 

Explanatory Information

The date when a standard in paragraph 2.2 and 2.3 becomes applicable within a certification project is 

related to the “..date of manufacture of the first individual production model..”. As this date is not always 

clear, certificating authorities have used the date of issue of the engine type certificate (TC) as a 

surrogate. The date is transparent as it is recorded on the TC and has therefore proven a useful approach 

in defining the certification basis for an engine type or model. 

The Part 1, Chapter 1 definition of “Date of manufacture” refers to the date of the document issued to an 

individual production engine which attests compliance with the type certificate. 

2.4 Information Required 

2.4.1 General information 

[Reserved] 

2.4.2 Test information 

[Reserved] 

2.4.3 Derived information 

Explanatory Information 

The “maximum Smoke Number” is formally defined as the greatest value of SN measured at any of the 

four thrust levels defined in 2.1.4.2. However, if a higher smoke number is measured at any other test 

condition between 7% and 100% of rated thrust during emissions certification tests, it is recommended 

that the higher value be reported as the “Maximum Smoke Number”. 
 A-19 CAEP/8-WP/18 Appendix 

Chapter 3. Turbo-jet and turbofan engines intended for propulsion at supersonic speeds 

Explanatory Information 

During the CAEP/7 work programme, Working Group 3 reviewed the historic background on the 

development of the emissions standards for turbo-jet and turbofan engines intended for propulsion at 

supersonic speeds and discussed general technology aspects of supersonic engines in comparison to those 

for subsonic applications. The output of this work was reported in CAEP/7-WP/10 in February 2007. 

While further work, taking into account aircraft and engine development, was considered to be necessary 

to give clear recommendations on future changes to Chapter 3, the following preliminary observations 

and conclusions were agreed: 

The current supersonic Standard seems to be outdated. 

The Standard should not be applied for new engine projects. 

Part III, Chapter 3 of ICAO Annex 16, Volume II would need to be revised. 

The timescale for updating should take into account the technological development of any new 

SST engine project and be in line with the work to be undertaken on development of revised 

noise standards. 

Any alleviation compared to the current subsonic Standard would require detailed technical 

investigation. 

In order for these conclusions to become recommendations work needs to be completed on 

whether the current subsonic LTO regulatory approach can be applied to supersonic. 

Effects from cruise emissions from a potential fleet of supersonic business jets require more 

scientific understanding. 

WG3 continues to monitor developments within the aviation industry and scientific community on this 

issue. WG3 has also agreed not to update Chapter 3 until a new SST engine project reaches a sufficiently 

mature level such that it can inform discussions on potential future revisions. 
CAEP/8-WP/18 

Appendix 18 A-20 

Appendix 2: Smoke Emission Evaluation 

Explanatory Information 

The procedure for evaluating smoke emissions is an indirect measure of smoke plume visibility which is 

obtained by using a filter to trap smoke particles contained in a predetermined mass of exhaust gas and 

measuring the loss of reflectance, i.e., degree of staining, of this filter relative to the absolute reflectance 

of the filter when clean or free of stain. The uncertainty of the smoke emission evaluation is estimated to 

be within ±3 SN (smoke numbers). 

1. Introduction and Definitions 

[Reserved]

16.

2. Measurement of Smoke Emissions 

2.1 Sampling probe for smoke emissions 

2.1.a) The probe shall be made of stainless steel or any other non reactive metal. 

If a probe with multiple sampling orifices is used, all sampling orifices shall be of 

equal diameter. 

Equivalent Procedures 

Stainless steel is the preferred probe material but other non-reacting materials may be more suitable under 

specific circumstances, e.g. engine exhaust temperatures which exceed the physical specification limits of 

stainless steel. Inconel 625 and Nimonic 75 alloys have previously been accepted as a non-reactive probe 

material in the context of the regulated species. Other materials may be suitable but need to be approved 

by the certificating authority. 

2.1.b) The probe design shall be such that at least 80 per cent of the pressure drop 

through the probe assembly is taken at the orifices. 

Explanatory Information 

Smoke particles are submicron in size which, for sampling from gas turbine engines, precludes the need 

for isokinetic sampling. Never-the-less good practice would suggest sampling as close to isokinetic as 

possible. Taking an 80 per cent pressure drop at the probe orifices is a reasonable compromise. Further 

information on probe design is provided within the section on Appendix 3, paragraph 5.1.1. 

2.1.c) The number of locations sampled shall not be less than 12. 

[Reserved] 
 A-21 CAEP/8-WP/18 Appendix 

2.1.d) The sampling plane shall be as close to the engine exhaust nozzle exit 

plane as permitted by considerations of engine performance but in any case shall be 

within 0.5 nozzle diameters of the exit plane. 

Explanatory Information 

The definition of the engine exhaust nozzle is contained in Part 1, Chapter 1, Definitions. 

Equivalent Procedure (for mixed flow engine configurations) 

For accurate gaseous emissions measurements and representative samples, it is considered best practise to 

arrange the engine configuration in such a way as to separate the fan and core flows, without affecting the 

engine performance, and to sample just the core flow. This is equally applicable to smoke emission 

measurements, however for mixed flow engine designs, the dilution and mixing of bypass air also needs 

to be taken into account with respect to the exhaust nozzle location. This means that the measured core 

SN would need to be corrected analytically for dilution and mixing in order to compare against the 

original visibility criteria. 

Currently Annex 16 Vol. II does not contain any method or procedure for these corrections which has led 

to inconsistent application of the requirements. Where no specific engine data is available, version 3 of 

the First Order Approximation (FOA) for estimating Particulate Matter emissions, which contains 

correlations between SN and non-volatile PM, is provided as a generic dilution correction in the technical 

procedure below. If it can be shown that an improved correlation is available for a given engine type, or 

the FOA is developed further, then the improved correlation shall be used with the approval of the 

certificating authority. No generic mixing correction procedure has yet been identified, and as such, 

certificating authorities need to address this issue on a project by project basis. Ideally the measured core 

SN would already meet the SN limit. Where this is not the case, further evidence may be required to 

inform a technically based engineering judgement as to whether the plume could still considered to be 

invisible. This could include a detailed traverse at the mixed exhaust nozzle plane in order to perform a 

contour analysis and determine the level of mixing, 

Technical Procedure 

The following procedure is provided as guidance material on how to correct core Smoke Numbers for 

dilution at a mixed flow engine exhaust nozzle: 
CAEP/8-WP/18 

Appendix 18 A-22 

Step 1: Convert measured core smoke number to equivalent carbon mass concentration 

Figure1: SN plotted against carbon concentration 

Based on various research studies, the following FOA carbon correlation equation has been established: 

 CI = 0.0694(SN)1.23357 for SN < 30 

Example: From an engine with a maximum measured core smoke number of 20, the FOA reference 

curve provides an equivalent carbon mass concentration of 2.8 mg/m3 at the core exit plane. 

Step 2: Adjust carbon mass concentration for mixed stream equivalent 

The carbon concentration can now be corrected for the amount of bypass air using the following 

formulae: 

Bypass Ratio

Core Carbon Mass Mixed Carbon Mass

+ = 1

Example: Assuming a bypass ratio of 5, the mixed carbon mass concentration is 2.8/(1+5) = 0.47 mg/m3

at the mixed nozzle exit plane. 

Step 3: Convert mixed stream carbon mass back to a smoke number 

Using the FOA reference curve, the mixed flow carbon mass (calculated in the previous step) is converted 

to an equivalent smoke number. 

Example: The diluted smoke number, accounting for the fan air using the FOA reference curve, is 4.8 at 

the mixed nozzle exit plane. 

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 5 10 15 20 25 30

SN

Carbon Conc. mg/m3 

FOA 3 curve for SN < 30
 A-23 CAEP/8-WP/18 Appendix 

2.1.e) The applicant shall provide evidence to the certificating authority, by means 

of detailed traverses, that the proposed probe design and position does provide a 

representative sample for each prescribed power setting. 

Explanatory information 

Smoke measurements can be performed by means of a single point probe which is traversed through the 

sampling plane in sufficient detail to provide a representative sample. This measurement can also be 

made using a multi-orifice probe which has been demonstrated to provide a representative sample by 

comparison with those of the single point traverse. Work sponsored by the SAE E-31 committee has 

shown that the best agreement between a detailed traverse, used to establish the mean value of smoke 

emissions in the sampling plane, and a multi-point sampling probe is achieved when this probes sampling 

orifices are located on centres of equal area. The most common configuration is that of a cruciform with 

the individual orifices equally distributed and located on centres of equal area. 

2.2 Sampling line for smoke emissions 

17. 2.2.2 Note. Stainless steel or carbon loaded grounded PTFE meet these 

requirements 

Explanatory Information 

If carbon-loaded grounded poly-tetra-fluoro-ethylene (PTFE) is used special care must be taken to 

allow sufficient cooling of the exhaust sample from the probe to the PTFE line to prevent damaging the 

PTFE line and possibly compromising the sample. 

2.3 Smoke analysis system 

18.

a) sample size measurement: 

 [Reserved] 

b) sample flow rate measurement: 

 [Reserved] 

c) filter and holder: 

 [Reserved] 

 d) valves: 

 [Reserved] 

19.

 e) vacuum pump: 
CAEP/8-WP/18 

Appendix 18 A-24 

 [Reserved] 

 f) temperature control: 

[Reserved] 

g) If it is desired to draw a higher sample flow rate through the probe than 

through the filter holder, an optional flow splitter may be located between the probe 

and valve A (Figure 2-1), to dump excess flow. The dump line shall be as close as 

possible to probe off-take and shall not affect the ability of the sampling system to 

maintain the required 80 per cent pressure drop across the probe assembly. The dump 

flow may also be sent to the CO2 analyser or complete emissions analysis system. 

Explanatory Information 

Achieving an 80% pressure drop across the probe assembly can result in an unacceptably high sample 

flow rate through the filter holder due to the pressure drop taken across the filter. In these instances, a 

flow splitter may be required. 

 h) If a flow splitter is used, a test shall be conducted to demonstrate that the flow 

splitter does not change the smoke level passing to the filter holder. This may be 

accomplished by reversing the outlet lines from the flow splitter and showing that, 

within the accuracy of the method, the smoke level does not change. 

Explanatory Information 

Smoke from gas turbine engines, although consisting of sub-micron particles, can be particularly 

sensitive to flow splitter design or other flow elements in the sampling stream due to inertial 

separation at very high flow velocities. This test addresses these concerns and ensures that the 

splitter design does not adversely impact the smoke emissions evaluation. 

i) leak performance: 

 [Reserved] 

j) reflectometer: 

Equivalent Procedure 

ARP 1179 Rev. C requires the use of a green tristimulus filter to adjust for the effect of various light 

sources from different reflectometer manufacturers. While this is not a requirement in Annex 16 Vol. II, it 

is considered best practice to follow this approach. 

2.1 Fuel specifications 

 [Reserved] 
 A-25 CAEP/8-WP/18 Appendix 

2.2 Smoke measurement procedures 

2.5.1 Engine operation 

 [Reserved] 

2.5.2 Leakage and cleanliness checks 

Explanatory Information 

Leakage checks are to ensure clean air does not leak into the system thereby diluting the sample and 

lowering the smoke number. Cleanliness checks ensure that the sampling system is acceptably clean and 

the collecting filter will not be contaminated. If the probe cannot be removed from the sampling stream 

during engine start-up, the probe and lines should be back pressured with a suitably clean gas, such as dry 

nitrogen, to minimize contamination problems. 

2.5.3 Smoke measurement 

Explanatory information: 

It is common practice, while sampling for smoke, to also measure levels of CO2 as an operational 

check of the sampling system. The engine fuel-air ratio is calculated from the measured CO2 and 

compared to the fuel-air ratio obtained from engine performance data. These should be in 

agreement within ±10% at engine power above idle and within ±15% at idle. 

Paragraphs a) through d) provides for adjusting and setting the sample flow rate through the filter 

holder. To duplicate the pressure drop through the filter holder during actual sampling conditions 

a clean filter is clamped into the holder. This filter should be removed and discarded before 

clamping a clean filter into the holder as described in d). 

3. Calculation of Smoke Number from Measured Data 

Explanatory Information 

The absolute reflectance of each clean filter should be determined as well as that of the stained filter. Work 

performed by Dieck, et al, “Aircraft Gas Turbine Smoke Measurement Uncertainty Using the SAE/EPA 

Method”, Journal of Aircraft, Vol. 15, No. 4, April 1978, concluded that “The major instrument-related 

source of error in SAE/EPA smoke measurement is clean-filter reflectance precision. It is a direct result of 

the variability in filter reflectance about the average value used”. 

The backing material should be flat and provide equal pressure across the surface of the filter. 

4. Reporting of Data to the Certificating Authority 

 [Reserved] 
CAEP/8-WP/18 

Appendix 18 A-26 

Appendix 3: Instrumentation and Measurement Techniques for Gaseous Emissions 

1. Introduction 

Explanatory Information 

The sampling and analysis procedures prescribed in ICAO Annex 16 Volume II, Second Edition, 1993 

were adopted from SAE Aerospace Recommended Practice ARP 1256, "Procedure for the Continuous 

Sampling and Measurement of Gaseous Emissions from Aircraft Turbine Engines". The calculation 

procedures were derived from ARP 1533 ‘Procedure for the Analysis and Evaluation of Gaseous 

Emissions from Aircraft Engines”. ARP 1256 and ARP 1533 were developed and are maintained by SAE 

committee E-31, Aircraft Exhaust Emission Measurement. 

2. Definitions 

[Reserved] 

3. Data Required 

3.1 Gaseous Emissions 

a) Hydrocarbons (HC): a combined estimate of all hydrocarbon compounds present 

in the exhaust gas. 

Explanatory information 

Gas turbine engine exhaust gases typically contain a variety of hydrocarbon compounds. The specific 

compounds present and their relative concentrations are usually unknown. Flame Ionization Detectors, 

used to measure hydrocarbons, do not respond equally to all hydrocarbon compounds. Although this 

differential hydrocarbon response is to be held within specific bounds, the resulting measurement is an 

estimate of the hydrocarbon compounds present in the exhaust gas. 

3.2 Other information 

20. [Reserved] 

21.

4. General Arrangement of the System 

Explanatory information 

Water is a major product of combustion. Its removal upstream of the measuring instruments is attractive. 

Removal would minimize possible interference effects where the instrument responds to the water present 

as well as to the gas or vapour being measured. It would also prevent or minimize water condensing in 
 A-27 CAEP/8-WP/18 Appendix 

the instruments which could cause erratic flow and/or contamination. In the worst case, the instrument 

would be rendered inoperable until thoroughly cleaned. However, devices which remove water are 

known to remove hydrocarbons and oxides of nitrogen, and are therefore only permitted for CO and CO2 

measurements. If the sample is dried, an appropriate dry/semi-dry to wet correction must be made. 

For most aircraft gas turbine engines, and most engine running modes, supplemental pumps will be 

needed to meet the probe system pressure drop requirement (80% at the probe entrance orifices), the 

sample line residence time and pressure drop, and the need to remove excess flow from the sampling 

system. Any pump used for the purpose of sample transfer must be heated. Usually, because of the 

sample gas physical properties and the need to maintain temperature and flow control within the Flame 

Ionization Detector used for hydrocarbon analysis, these instruments utilize internal heated, inert sample 

transfer pumps. The use of an upstream flow splitter to dump a portion of the sample is also an 

acceptable procedure to assist in controlling flow to the analytical sampling train. 

If loss of hydrocarbons in the sampling system is a concern, the FID can be, when configured with a 

heated transfer pump, located upstream of the system hot pump as close as physical constraints will allow 

(e.g. temperature, noise, vibration). The necessity for a dump and/or a hot-sample pump will depend on 

the ability to meet the sample transfer time and analysis sub-system sample flow rate requirements. This 

in turn depends on the exhaust sample driving pressure and line losses. Therefore, in general, the size and 

location of the pumps, and the associated flow control devices, are determined from the particular 

sampling system configuration. 

5. Description of Component Parts 

5.1 Sampling system 

5.1.1 Sampling probe 

a) The probe shall be made of stainless steel or any other non-reactive metal. If a 

probe with multiple orifices is used, all sampling orifices shall be of equal diameter; 

Equivalent Procedures 

Stainless steel is the preferred probe material but other non-reacting materials may be more suitable 

under specific circumstances, e.g. engine exhaust temperatures which exceed the physical 

specification limits of stainless steel. Inconel 625 and Nimonic 75 alloys have previously been 

accepted as a non-reactive probe material in the context of the regulated species. Other materials 

may be suitable but need to be approved by the certificating authority. 

Explanatory Information 

A probe design with multiple orifices (mixing probe) could include either several sampling orifices 

leading into a single plenum, or several sampling orifices leading into individual sample lines which are 

mixed external to the probe, as shown in Figure App.3-1. The sampling orifices should be equal in size 

and located on centres of equal area for all mixing probes. If a multi-armed probe is used, then there 

should be an equal number of orifices on each arm. Considerations for probe design leading to these 

criteria can be found in “Gas/Turbine Emission Probe Factors”, SAE Aerospace Information Report 

AIR4068A, 1996.The most common configuration is that of cruciform with individual orifices located on 

centres of equal area. 
CAEP/8-WP/18 

Appendix 18 A-28 

b) the probe design shall be such that at least 80 per cent of the pressure drop through 

the probe assembly is taken at the orifices; 

Explanatory Information 

The pressure drop is needed to isokinetically sample flows at the different orifices. The orifice is 

supposed to be the minimum area right at the probe entrance. To achieve the pressure drop criterion, 

there must be a rapid expansion into the sampling tube. 

The pressure drop refers to the dynamic head, not the total pressure, and is needed to ensure that each 

orifice takes a flow rate that is proportional to the dynamic head present at the sampling orifice. Thus, 

when the samples taken by the individual sampling orifices are mixed together within the probe, the total 

sample is representative of the mass flux of emissions through the engine exhaust sampling plane. 

0.8

0

0 ( ) 〉 −

−

t sout

s n i t

P p

P p o

t sout s n i

Pt − p o 〉 P0 − p 0 ( ) 0.8

Figure App.3-1. Sampling probe designs 

c) the number of sampling orifices shall not be less than 12; 

Explanatory Information 

While 12 orifices is the minimum number for a sampling rake in Annex 16 Vol. II, a more appropriate 

number would be 20 when validated by means of a detailed traverse. 

5mm

Pt0

Ps(n°i)

Psout

Averaging

of individual samplings

Multi-hole non mixing probe

Mixing at the probe exit (before entering the line)

Multi-hole mixing probe

(Mixing in the probe)

Pt0

Ps(n°i)

Psout

5mm

Pt0

Ps(n°i)

Psout

Averaging

of individual samplings

Multi-hole non mixing probe

Mixing at the probe exit (before entering the line)

Multi-hole mixing probe

(Mixing in the probe)

Pt0

Ps(n°i)

Psout
 A-29 CAEP/8-WP/18 Appendix 

d) the sampling plane shall be as close to the engine exhaust nozzle exit plane as 

permitted by considerations of engine performance but in any case shall be within 0.5 

nozzle diameter of the exit plane; 

Explanatory information 

Further guidance on the issue of “Exhaust Nozzle“ is available under the Definitions in Part 1, Chapter 1.

e) the applicant shall provide evidence to the certificating authority, by means of detailed 

traverses, that the proposed probe design and position does provide a representative 

sample for each prescribed power setting. 

Explanatory information 

Detailed traverse measurements, although expensive, could be performed with a single hole probe which 

measures stabilized concentrations at various positions. These individual measurements will then be used 

to derive average values and demonstrate representative sampling. The carbon balance check is also 

derived in the same way. The “80% pressure drop” condition which has been introduced in the past to 

guarantee the sample is representative, is no more justified for the purpose of a single hole probe. 

Technical procedure 

There is no standard definition of a “representative sample” for emissions from aircraft gas turbine 

engines, nor is there a specification for “detailed traverse”. Representative and detailed are, in this 

instance, matters of opinion to be negotiated between the manufacturer and the certificating authority. 

The issue is to do with how much the measured averaged sample can deviate from the true sample mean 

before it is no longer considered to be representative. The most commonly used definition, arrived at 

from decades of testing and collaborative analytical exercises by user groups, is 10% for engine modes 

above idle (i.e., approach, decent, climb and take-off), and ± 15% for idle, as per the carbon balance 

check in section 6.4 of Appendix 3. There is a significant difference however. For the carbon balance 

check a comparison is made between sets of independently related measured values. With knowledge 

about the combustion process and the hydrogen to carbon ratio of the fuel used, an estimate can be made 

from the measured carbon containing species, most of which is CO2, of the engine average fuel to air 

ratio. The actual engine fuel to air ratio can be independently arrived at from measured fuel and airflow. 

These two values can be compared to provide an estimate of how well the exhaust stream was sampled 

for carbon containing compounds. However, as CO2 far outweighs the influence of any other carbon 

bearing species in the calculation of fuel to air ratios, the spatial variability of CO2 will determine how 

many sampling points are required and how these sampling points should be distributed. CO2 has been 

found to consistently exhibit the least variability of all the species of interest, which include CO, HC, NOx

and smoke. This suggests that it would be possible to meet the 10 and 15% criteria for carbon balance 

without having obtained a representative sample, using the same 10 and 15% criteria, of the other species. 

In other words, obtaining a carbon balance, while a necessary pre-condition for a representative sample, 

could not be considered to be a sufficient demonstration of representative sampling on its own. 

Historically engine manufacturers, or testing agencies, have addressed this problem in different ways. 

Recognizing that gas turbine engines are predominately axi-symmetric, one acceptable method (for fixed 

rake designs) has been to sample the exhaust plume, point by point, with a sufficient number of points to 

be able to estimate, by statistical means, the true engine average species concentration for each of the 
CAEP/8-WP/18 

Appendix 18 A-30 

species of interest. Engine exhaust species seem almost normally (Gaussian) distributed, thus making 

simple statistical tools acceptable. Using these sampling points, and the measured concentration values 

for each of the species of interest, contour plots of constant concentration (isopleths) have been 

analytically generated at each power setting tested. There are a number of computer programs available 

for workstations or desktop computers to do this. A probe or rake design is then overlaid on the contour 

plots in order to estimate the average probe/rake values and compare to the estimated true average arrived 

at from the detailed traverse. If the comparisons match the carbon balance criteria, within 10% for engine 

powers above idle and ±15% at idle, the probe/rake can be considered to provide representative sampling. 

This process may have to be repeated several times before an acceptable design is found. 

Equivalent Procedure 

When applying this analytical technique, for core flow only, multi-orifice rake designs which have proven 

most robust seem to have four sampling arms spaced 90 degrees apart with sampling orifices located on 

centres of equal area. The sampling orifices are equally distributed across the sampling arms and the 

contour analysis will determine the minimum number of sampling ports necessary to yield a 

representative sample. 

A detailed traverse was not required for rotating rake designs as they provide, in normal use, as many or 

more sampling points than the typical single point traverse thus ensuring a representative sample. 

However, in order to demonstrate that a fixed rake design is orientated in the right position to collect a 

representative sample, traverse measurements were necessary. The detail (single point, number of rake 

orientations, number of different power settings) of traverse measurements may depend on the number of 

sampling orifices on the rake and existing experience of similar engines or derivatives. These rake design 

requirements were recognized as being complemented by the system check performed in the carbon 

balance criteria. Where a sample has low EIs and relatively large % variations between the detailed 

traverse and the fixed rake measurements, it could be accepted as representative without having 

demonstrated NOx, CO and HC to be within 10 % (15% for idle) as long as there is a sufficient large 

number of orifices and an AFR match within 10% (15% at idle). 

All data used in arriving at a probe/rake design should be made available to the certificating authority. 

5.1.2 Sampling lines 

[Reserved] 

5.1.3 HC Analyser 

[Reserved] 

5.1.4 CO and CO2 analysers 

[Reserved] 

5.1.5 NOx analysers 

[Reserved] 

5.1.6 Smoke analysis system 

[Reserved] 

6. General Test Procedures 
 A-31 CAEP/8-WP/18 Appendix 

6.1 Engine operation 

[Reserved] 

6.2 Major instrument calibration 

[Reserved] 

6.3 Operation 

[Reserved] 

6.4 Carbon balance check 

[Reserved] 

7. Calculations 

Technical Procedure 

This technical procedure provides guidance on the processing of measured data from an emissions 

certification test in the form of a simplified worked example. 

Definitions: 

Reference engine – For this example, the emissions certification values are calculated based on 

combustor inlet conditions (T3, P3, and fuel flow) from a validated engine performance model. 

Test engine(s) – Recommended practice is to use an engine which conforms to the production build 

standard. If differences exist, these differences must be documented for approval by the 

certificating authorities. If any of these non conformances are predicted to impact engine 

performance, gaseous emissions or smoke levels, then an explanation and quantification of the 

impacts will be provided to the certificating authorities for approval. Generally manufacturers will 

keep deviations from the production standard to a minimum. Measured emissions levels will be 

corrected to reference engine (production build standard) and standard day conditions. 

Detailed traverse – For a new engine type, the next step prior to the actual emission certification 

test is to conduct a detailed traverse of the engine exhaust to show that a representative sample is 

being obtained (further guidance in the ETM on detailed traverse see ETM Appendix 3, 5.1.1). 

Emissions tests – Emissions are typically measured at more than the four required thrust levels 

(typically 8-16 conditions) between ground idle and maximum rated thrust. 

Instrument calibration curves for the different analyzers may need to be established in order to translate 

instrument readings to calibrated concentration values. These gas concentrations will be recorded and 

emission indices will be calculated from that using the equations in 7.1.2 of the Annex 16 Vol. II. The 

following simple example shows how to derive EI(CO). Assuming the measured values all on a wet basis: 

n/m = H/C = 2; 

CO = 500 ppm(v)wet = 0,0005; 

HC = 800 ppm(v)wet = 0,0008; 

CO2 = 2,25% = 0,0225; 
CAEP/8-WP/18 

Appendix 18 A-32 

NO2 = 20 ppm(v)wet = 0,00002; 

hamb = 0,0025 volwater/voldryair 

CxHy = CH4 Æ x=1, y=4 

The equation in 7.1.2 for EI(CO) would then read: 

EI(CO) = (CO/(CO + CO2 + HC)) * (103

 * 28,011g / ( 12,011g + n/m* 1,008g) * (1 + (0,0003 * (P0/m))) 

Where: 

(P0/m) = (2*Z - n/m) = 2*Z – 2 

Z = {2 – CO – (2/1 – 4/2*1) * HC + NO2}/ {CO + CO2 + HC} 

 = 2 – 0,0005 – 0 * HC + 0,00002 = 1,99952 

(P0/m) = 1,99904 

Hence, from the equation above: 

EI(CO) = (0,0005/0,0225+0,0005+0,0008) * (28011/(12,011 + 2,016)) * (1 + (0,0003*1,99904)) 

 = 0,021008 * 1996,934483 * 1,000056 

 = 41,98 g/kgfuel 

EI(HC) and EI(NOx) are calculated in a similar manner using the other two equations in 7.1.2. For 

EI(NOx) the NO2/NO converter efficiency must also be taken into account. EIs are calculated for each 

measurement point (thrust condition) and engine run. 

Attachment E of Appendix 3 contains a comprehensive and precise numerical method which is often used 

by engine manufacturers software programs. Further information is contained in SAE ARP 1533 

Procedure for the Analysis and Evaluation of Gaseous Emissions from Aircraft Engines which contains 

two fully worked examples of the Matrix method solving the combustion chemical equation. 

To correct these EIs from measured to reference engine and ambient conditions, a curve fitting technique 

is recommended. One acceptable alternative method of plotting measured test data is to plot the 

following: 

 EI(CO) * P3 v. T3 

EI(HC) * P3 v. T3 

EI(NOx) * P3 ^-0.5 * exp (19 [ hmass – 0.006 34 ] ) v. T3 

A best fit of each of the data curves can then be obtained, typically using a polynomial function. In some 

cases, two curve fit equations are needed, one for low power data and one for high power data. When 

more than one engine test has been conducted on an engine, data may be plotted for each test run or a 

single correlation may be used for the multiple runs. However, if multiple engines are tested, a separate 

set of plots should be made for each test engine. 

The procedure for calculating the corrected EI(CO) at values of Fn corresponding to the four LTO 

operating modes includes the following steps (as shown in Figure A3.1) [Reserved]: 

1. Use validated engine performance model to determine T3ref, P3ref and reference fuel flow 

2. Starting with T3ref, determine EI(CO) * P3 from the EI(CO) * P3 v. T3 curve 
 A-33 CAEP/8-WP/18 Appendix 

3. Divide by the corresponding P3ref to get: Corrected EI(CO) = EI(CO) * P3 / P3ref 

Calculation of corrected EI(HC) follows exactly the same process as EI (CO), as does the calculation of 

corrected EI(NOx) except step 3 involves multiplying by P3ref ^ 0.5 rather than dividing by P3ref. 

Once the Corrected EI(CO), EI(HC) and EI(NOx) have been calculated for each operating mode, Dp is 

calculated using the standard LTO times in mode and corresponding values of reference fuel flow from 

the validated engine performance model. EI (CO) * P3 EI (HC) * P3 EI (NOx) * P3-0.5 * exp(19(hmass-0.00634))

T3

T3

T3ref

P3ref

F00ref F00ref

100% Foo 

100% Foo

Example:

EI (CO)korr = EI (CO) * P3 / P3ref

Step 1

Step 2

Step 3

Step 4

From measured data From performance model

EI (CO) * P3 EI (HC) * P3

EI (NOx) * P3-0.5

* exp(19(hmass-0.00634))

T3

T3

T3ref

P3ref

F00ref F00ref

100% Foo 

100% Foo

Example:

EI (CO)korr = EI (CO) * P3 / P3ref

Step 1

Step 2

Step 3

Step 4

From measured data From performance model

Figure A3.1 – Gaseous Emissions Calculation Procedure 

7.1 Gaseous emissions 

[Reserved] 

7.1.1 General 

[Reserved] 

7.1.2 Basic Parameters 

[Reserved] 

7.1.3 Correction of emissions indices to reference conditions 

[Reserved] 
CAEP/8-WP/18 

Appendix 18 A-34 

7.2 Control parameter functions 

[Reserved] 

7.3 Exceptions to the proposed procedures 

[Reserved] 
 A-35 CAEP/8-WP/18 Appendix 

Appendix 3: Attachment A: Specification for HC Analyser 

1. General 

Precautions: The performance specifications indicated are generally for analyser full scale. 

Errors at part scale may be a significantly greater percentage of reading. The relevance and 

importance of such increases shall be considered when preparing to make measurements. If 

better performance is necessary, then appropriate precautions shall be taken. 

Explanatory Information 

The performance specifications for these analyzers, given in terms of full scale response, can have a 

significant and adverse impact on part scale measurements. In extreme instances, concentrations of HC at 

high power, such as take-off, can differ from concentrations at idle by orders of magnitude. In general it is 

always good practice to use a multi-range instrument and to adjust ranges such as to keep the 

measurement in the upper 30% of the instrument response range. Calibrations should be performed on 

each range used as required. 

The instrument to be used shall be such as to maintain the temperature of the detector and 

sample-handling components at a set point temperature not less than 155oC to a ability of 

±2°C. 

Explanatory Information 

ICAO Annex 16 Volume II previously (Amendment 6 and before) had a set point temperature within the 

range of 155oC to 165oC to a stability of +/- 2°C. This was adopted from SAE ARP1256, “Procedure For 

The Continuous Sampling and Measurement of Gaseous Emissions From Aircraft Turbine Engines - 

1971. ARP1256 specified this range to meet the need for minimising the condensation of hydrocarbons in 

the instrument and in recognition of the operating characteristics of then commercially available total 

hydrocarbon analysers (THAs). Since then the ARP has been revised, and now requires a single 

temperature set point of not less than 160oC but with no temperature stability requirement. Commercially 

available THAs have evolved and the more common detector housing temperatures approach 200oC. This 

increase in temperature does not effect the emissions certification measurements but does place an 

unnecessary constraint on the test procedure. 

a) Total range: 0 to 5 000 ppmC in appropriate ranges. 

Explanatory Information 

A total range of 0 to 5,000 ppmC, while appropriate for the engines in use when Annex 16 Vol. II was 

published in 1981, is broader than needed for today's engines where concentrations are much lower. 

Appropriate instruments should be used to ensure best practice measurements in the upper 30% of the 

range. Thus an instrument with a range upper limit of 5000ppmC may not be necessary and may, in fact, 

negatively affect the ability to ensure suitable range span due to instrument design limitations. 
CAEP/8-WP/18 

Appendix 18 A-36 

b) Resolution: better than 0.5 per cent of full scale of range used or 0.5 ppmC, 

whichever is greater. 

[Reserved] 

c) Repeatability: better than ±1 per cent of full scale of range used. or ±0.5 ppmC. 

whichever is greater. 

[Reserved] 

d) Stability: better than ±2 per cent of full scale of range used or ±l.0 ppmC, 

whichever is greater, in a period of 1 hour. 

Explanatory Information 

Stability, taken to be span stability and sometimes referred to as repeatability or reproducibility, is the 

maximum variation in instrument output over a specified time period and within specified environmental 

conditions when identical concentration samples, near FSD, are passed through the instrument and after 

zero corrections have been made. Stability is the sum of time dependent drift, i.e., the change in output 

under invariant laboratory conditions, and changes in output due to other factors such as environmental 

temperature and/or variations in FID enclosure temperature. Stability is highly dependent on how, and 

under what environmental conditions the analyzer is used. As such it is out of their control and they 

choose to specify a value for time dependent drift along with a range of environmental temperatures, i.e., 

basically under laboratory conditions. Due to improvement of instruments using solid state electronics the 

drift specifications from modern THC Analysers quote better drift performance(< 1% FS over 8 hours in 

laboratory conditions) than the stability requirements of the standard. Errors associated with this factor 

are small to negligible. As measurements are not taken under laboratory condition and as changes in 

environmental conditions are the norm rather than the exception operational procedures, as described in 

6.3.2 d of Appendix 3 are required. 

e) Zero drift: less than ±1 per cent of full scale of range used or ±0.5 ppmC, whichever is 

greater , in a period of 1 hour. 

[Reserved] 

f) Noise: 0.5 Hz and greater, less than ±1 per cent of full scale of range used or 

±0.5 ppmC, whichever is greater, in a period of 1 hour. 

Explanatory Information 

The FID requires fuel and oxidant gases for operation. The fuel gas is typically either a mixture of 

hydrogen/nitrogen, or hydrogen/helium. If the noise specification cannot be met, and a hydrogen/nitrogen 

mixture is being used as the fuel gas, it can be helpful to change to a hydrogen/helium mixture. 

g) Response time: shall not exceed 10 seconds from inlet of the sample to the 

analyser system, to the achievement of 90% of the final reading 

[Reserved] 
 A-37 CAEP/8-WP/18 Appendix 

h) Linearity: response with propane in air shall be linear for each range within ±2 

per cent of full scale, otherwise calibration corrections shall be used. 

[Reserved] 

2. Synergistic Effects 

Oxygen response: 

Measure the response with two blends of propane, at approximately 500 ppmC 

concentration known to a relative accuracy of ±1 per cent, as follows: 

1) propane in 10 ± 1 per cent 02, balance N2 

2) propane in 21 ± 1 per cent 02, balance N2 

If R1 and R2 are the respective normalized responses then (R1 - R2) shall be less 

than 3 per cent of R1. 

Explanatory Information 

The typical range of O2 concentrations in the core exhaust gas is 18% at idle to 15% at take-off. The 

specification for a response of <3% between samples of 10% and 21% is conservative and effectively 

limits the differential response to <1% over the range of interest. If needed the O2 response can be 

minimized by adjusting the FID burner fuel/air ratio. 

Differential hydrocarbon response: measure the response with four blends of 

different hydrocarbons in air, at concentrations of approximately 500 ppmC, 

known to a relative accuracy of ±l per cent, as follows: 

a) propane in zero air 

b) propylene in zero air 

c) toluene in zero air 

d) n-hexane in zero air. 

If Ra, Rb, Rc and Rd are, respectively, the normalized responses (with respect to 

propane), then (Ra - Rb), (Ra - Rc) and (Ra - Rd) shall each be less than 5 per cent 

of Ra. 

Explanatory Information 

While the FID response is assumed to respond in a manner proportional to carbon number, it does vary 

somewhat with the particular hydrocarbon or class of hydrocarbons being measured. For example, three 

molecules of methane (CH4) will not necessarily result in the same instrument response as one molecule 

of propane (C3H8). Due to this differential response, it is useful to think of the FID as responding to an 

“effective” carbon number. It is important that the instrument responses are acceptable for all of the 

hydrocarbons in the engine exhaust. The group of hydrocarbons (propylene, toluene, and n-hexane), with 

propane as a reference, was chosen to represent, in terms of differential response, the range of 

hydrocarbons expected in the engine exhaust. 
CAEP/8-WP/18 

Appendix 18 A-38 

3. Optimisation of Detector Response and Alignment 

3.1 The manufacturer's instructions for initial setting up procedures and ancillary services 

and supplies required shall be implemented, and the instrument allowed to stabilize. All 

setting adjustments shall involve iterative zero checking, and correction as necessary. Using 

as sample a mixture of approximately 500 ppmC of propane in air, the response 

characteristics for variations first in fuel flow and then, near an optimum fuel flow, for 

variations in dilution air flow to select its optimum shall be determined. The oxygen and 

differential hydrocarbon responses shall then be determined as indicated above. 

Explanatory Information 

The FID detector response and alignment can be optimized by adjusting the FID burner fuel and air flow 

while sampling a mixture containing approximately 500 ppmC propane. Care should be taken when 

changing fuel flow that the instrument zero does not shift. If it does, the instrument zero should be reset. 

Response curves illustrating this process are shown in Figures A3-3 and A3-4, and were taken from SAE 

ARP1256, "Procedure for the Continuous Sampling and Measurement of Gaseous Emissions from 

Aircraft Turbine Engines". 

Figure A3-3 Typical Fuel Flow Response Curve 
 A-39 CAEP/8-WP/18 Appendix 

Figure A3-4 Typical Airflow Response Curve 

The objective of this procedure is to select operating flow rates which will give near maximum 

response with least variation for minor fuel flow variations. It may be necessary to repeat this 

operation in an iterative fashion: 

• Adjust the fuel flow to maximize output; 

• Adjust zero if necessary; 

• Adjust the air flow to maximize output; 

• Readjust the fuel flow, if necessary; and 

• Repeat until the burner output is optimized.
CAEP/8-WP/18 

Appendix 18 A-40 

Appendix 4: Specification for Fuel to be used in Aircraft Turbine Engine Emission Testing 

The fuel used during tests shall meet the specifications of this Appendix 4, unless a 

deviation and any necessary corrections have been agreed by the certificating authority. 

Additives used for the purpose of smoke suppression (such as organo-metallic compounds) 

shall not be present. 

Equivalent Procedure 

Appropriate evidence should be provided to the certificating authority to substantiate any deviation from 

the fuel specification of Appendix 4 as early as possible. 

A deviation may be accepted when it can be shown that the locally available fuel does not meet the 

specification. In such a case, use of the available fuel may be acceptable, subject to the substantiation of 

corrections to compensate for the effect of the deviation on the measured emissions levels. The measured 

data should then be corrected to reflect the limiting values of the fuel specification of Appendix 4. 

Corrections will normally be accepted when the magnitude of the correction to the measured data is small 

in relation to the margin to the certification limits. 

The corrections to the declared emission levels resulting from the deviation in test fuel properties should 

be based upon engine or rig test data which can be related to the specific combustor type, supported by 

validated analysis where necessary. The corrections would need to be conservative, particularly when test 

data is not available for the specific combustor type being certificated. Manufacturers should avoid use of 

fuels that have been heavily hydro-treated, or produced using synthetic processes. 

The deviations from the fuel specification, and the associated corrections, require the agreement of the 

certificating authority. 

.............................. 

22.

Appendix 5: Instrumentation and measurement techniques for gaseous emissions from 

afterburning gas turbine engines [Reserved] 

................................ 

Appendix 6: Compliance procedures for gaseous emissions and smoke [Reserved] 

................................ 

— END —