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IX. ICAO MANUAL APPENDIX 6 ICAO MANUAL APPENDIX 6. COMPLIANCE PROCEDURE FOR
GASEOUS EMISSIONS AND SMOKE
1. GENERAL The following general principles shall be followed for compliance with the regulatory levels set forth in Volume II, Part III, 2.2, 2.3, 3.2 and 3.3 of this Annex:
a) the manufacturer shall be allowed to sele ct for certification testing any number of engines, including a single engine if so desired;
b) all the results obtained during the certificat ion tests shall be taken into account by the certification authority;
c) a total of at least 3 engine tests shall be co nducted, so that if a single engine is presented for certification it must be tested at least 3 times;
d) if a given engine is tested several times, the arithmetic mean value of the tests shall be considered to be the mean value for that engine. The certification result (x) is then the mean of the values (X) obtained for each engine tested;
e) the manufacturer shall provide to the certific ating authority, the information specified in Volume II, Part III, 2.4 or 3.4 of this Annex as appropriate;
f) the engines submitted for testing shall hav e emissions features representative of the engine type for which certification is sought. However, at least one of the engines shall be substantially configured to the production standard of the engine type and have fully representative operating and performance characteristics. One of these engines shall be declared to be the reference standard engine. The methods for correcting to this reference standard engine from any other engines tested shall have the approval of the national certificating authority. The methods for correcting test results for ambient effects shall be those outlined in 7 of Appendix 3 or 7 of Appendix 5, as applicable.
2. COMPLIANCE PROCEDURES The certificating authority shall award a certif icate of compliance if the mean of the values measured and corrected (to the reference standard engine and reference ambient conditions) for all the engines tested, when converted to a characteristic level using the appropriate factor which is determined by the number of engines tested (i) as shown in the table below, does not exceed the regulatory level.
Note.- The characteristic level of the Smok e Number or gaseous pollutant emissions is the mean of the values of all the engines te sted, measured and corrected to the reference standard engine and reference ambient conditions divided by the coefficient corresponding to the number of engines tested. as shown in the table below Page 38
Number of engines tested (i) CO HC NOX SN 1 0.814 7 0.649 3 0.862 7 0.776 9 2 0.877 7 0.768 5 0.909 4 0.852 7 3 0.924 6 0.857 2 0 944 1 0.909 1 4 0.934 7 0.876 4 0.951 6 0.921 3 5 0.941 6 0.889 4 0.956 7 0.929 6 6 0.946 7 0.899 0 0.960 5 0.935 8 7 0.950 6 0.906 5 0.963 4 0.940 5 8 0.953 8 0.912 6 0.965 8 0.944 4 9 0.956 5 0.917 6 0.967 7 0.947 6 10 0.958 7 0.921 8 0.969 4 0.950 2 more than 10 − 0130 59.1 i 02 4 7 2 4 .1− i 0 096 78 .1− i 0157 36 .1− i
3. PROCEDURE IN THE CASE
OF FAILURE Note.- when a certification test fails, it does not necessarily mean that the engine type does not comply with the requirements, but it may mean that the confidence given to the certificating authority in compliance is not sufficiently high, i.e. less than 90 per cent. Consequently, the manufacturer should be allowed to present additional evidence of engine type compliance.
3.1 If an engine type fails a certification test, the certificating authority shall permit the manufacturer, if he so wishes, to conduct additional tests on the ce rtification engines. If the total results available still show that the engine type fails the certif ication requirements, t he manufacturer shall be allowed to test as many additional engines as desired. The resulting test results shall then be considered with all previous data.
3.2 If the result is still failure, the manufacturer shall be allowed to select one or more engines for modification. The results of the tests already made on the selected engine(s) while unmodified shall be inspected, and further testing shall be done so that at least three tests are available. The mean of these tests shall be determined for each engine and described as the "unmodified mean"
3.3 The engine(s) may then be modified, and at least three tests shall he conducted on the modified engine(s), the mean of which shall be described as the "modified mean" in each case. This "modified mean" shall he compared to the "unmodified mean" to give a proportional improvement which shall then be applied to the previous certification test result to determine if compliance has been achieved. It shall be determined before testing of any modified engine is begun that the modification(s) comply with the appropriate airworthiness requirements.
3.4 This procedure shall be repeated until co mpliance has been demonstrated or the engine type application is withdrawn.
4. REPORTING OF DATA TO THE
CERTIFICATING AUTHORITY The measured data shall be reported to the certificati ng authority. In addition the following data shall be reported for each test:
a) sample temperature;
b) sample pressure;
c) actual sample volume at sampling conditions;
d) actual sample flow rate as sampling conditions; and
e) leak and cleanliness checks substantiation (see 2.5.2) Page 40
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X. ICAO MANUAL APPENDIX 3
INSTRUMENTATION AND MEASUREMENT TECHNIQUES
FOR GASEOUS EMISSIONS
1. INTRODUCTION
Note. —- The procedures specified in this appendix are concerned with the acquisition of representative exhaust samples and their transmission to, and analysis by, the emissions measuring system. The procedures do not apply to engines employing after burning. The methods proposed are representative of the best readily available and most established practice.
Variations in the procedure contained in this append ix shall only be allowed after prior application to and approval by the certificating authority.
Supplemental Information The methods recommended 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". ARP 1256 was first issued 10-1-71, by SAE committee E 31, Aircraft Exhaust Emission Measurement. This ARP was developed by user groups in the aircraft industry who were interested in measuring and understanding emissions from aircraft gas turbine engines. Subsequent to the issuance of ARP 1256, in 1971, the methods pr oposed were, with the exception of NOx, adopted by the EPA in 40 CFR part 87, Part II, December 12, 1972.
The exception in NOx measurement involved the adoption of chemiluminescence by the EPA in 40 CFR 87 and NDIR/NDUV by SAE E-31 in ARP 1256. Chemiluminescence, although a new technique at the time, was clearly the superior method and quickly adopted by the us er community, incorporated into ARP 1256, replacing NDIR/NDUV, and recommende d by ICAO in Annex 16, Volume II, first edition, 1981. The methods of measurement recommended in ICAO Annex 16, Volume II, first edition, 1981 were incorporated, by reference, into EPA 40 CFR 87, in 1982, Federal Register, Vol. 47, No. 251. Th ese recommended methods represent the best readily available and most established practice.
2. DEFINITIONS Where the following expressions are used in this appendix, they have the meanings ascribed to them below:
(1) Accuracy . The closeness with which a measurement approaches the true value established independently.
(2) Air/fuel ratio . The mass rate of airflow through the hot section of the engine divided by the mass rate of fuel flow to the engine.
(3) Calibration gas . A high accuracy reference gas to be used for alignment, adjustment and periodic checks of instruments.
(4) Concentration . The volume fraction of the component of interest in the gas mixture - expressed as volume percentage or as parts per million.
(5) 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.
(6) Flame ionization detector. A hydrogen-air diffusion flame detector that produces a signal nominally proportional to the mass-flow rate of hydrocarbons entering the flame per unit of time - Page 42
generally assumed responsive to the number of carbon atoms entering the flame.
(7) Interference . Instrument response due to presence of components other than the gas (or vapour) that is to be measured.
(8) Noise. Random variation in instrument output not associated with characteristics of the sample to which the instrument is responding, and distinguishable from its drift characteristics.
(9) Non-dispersive infra-red analyser . An instrument that by absorption of infra-red energy selectively measures specific components.
(10) Parts per million (ppm ). The unit volume concentration of a gas per million unit volume of the gas mixture of which it is a part.
(11) Parts per m illion carbon (ppmC). The mole fraction of hydrocarbon multiplied by 106 measured on a methane-equivalence basis. Thus, 1 ppm of methane is indicated as 1 ppmC. To convert ppm concentration of any hydrocarbon to an equivalent ppmC value, multiply ppm concentration by the number of carbon atoms per molecule of t he gas. For example, 1 ppm propane translates as 3 ppmC hydrocarbon, 1 ppm hexane as 6 ppmC hydrocarbon.
(12) Reference gas . A mixture of gases of specified and kn own composition used as the basis for interpreting instrument response in terms of the concentration of the gas to which the instrument is responding.
(13) Repeatability . The closeness with which a measurement upon a given, invariant sample can be reproduced in short-term repetitions of t he measurement with no intervening instrument adjustment.
(14) Resolution . The smallest change in a measurement which can be detected.
(15) Response . The change in instrument output signal that occurs with change in sample concentration. Also the output signal corresponding to a given sample concentration.
(16) Stability. The closeness with which repeated measurements upon a given invariant sample can be maintained over a given period of time.
(17) Zero drift . Time-related deviation of instrument output from zero set point when it is operating on gas free of the component to be measured.
(19) Zero gas . A gas to be used in establishing the zero, or no-response, adjustment of an instrument.
3. DATA REQUIRED
3.1 Gaseous emissions Concentrations of the following emissions shall be determined:
(a) Hydrocarbons (HC): a combined estimate of all hydrocarbon compounds present in the exhaust gas.
Supplemental 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 (FID), used to measure hydrocarbons, do not respond equ ally to all hydrocarbon compounds - differential hydrocarbon response. Although the differential hydrocar bon response is to be held within specific bounds, the resulting measurement is an estimate of the hydrocarbon compounds present in the exhaust gas.
(b) Carbon monoxide (CO).
(c) Carbon dioxide (CO2).
Note.— CO2 is not considered a pollutant but its concent ration is required for calculation and check purposes.
(d) Oxides of nitrogen (NOX) an estimate of the sum of the two oxides, nitric oxide (NO) and nitrogen dioxide (NO2).
(e) Nitric oxide (NO).
Supplemental information The measurement of oxides of nitrogen, using an NO specific device, requi res the conversion of NO 2 in the exhaust gas to NO. The conversion efficiency of the device used must be known and must be greater than 90%. Measurements of NO x are adjusted for conversion efficiency. The resulting values are considered to be estimates of the NOx concentration in the exhaust gas.
3.2 Other information In order to normalize the emissions measurement data and to quantify the engine test characteristics, the following additional information shall be provided:
- inlet temperature;
- inlet humidity;
- atmospheric pressure;
- hydrogen/carbon ratio of fuel;
- other required engine parameters (for example, thrust, rotor speeds, turbine temperatures and gas-generator air flow).
This data shall be obtained either by direct measurement or by calculation as presented in Attachment F to this appendix.
4. GENERAL ARRANGEMENT OF THE SYSTEM No desiccants, dryers, water traps or related equipment shall be used to treat the exhaust sample flowing to the oxides of nitrogen and the hydrocarbon analysi s instrumentation. Requirements for the various component sub-systems are given in 5, but the following list gives some qualifications and variations:
a) it is assumed that each of the various individual sub-systems includes the necessary flow control, conditioning and measurement facilities;
b) the necessity for a dump and/or a hot-sample pump will depend on 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. It is considered that these pumps usually will be necessary at certain engine running conditions; and
c) the position of the hot pump, relative to the gas analysis sub-systems, may be varied as required.
(For example, some HC analysers contain hot pumps and so may be judged capable of being used upstream of the system hot pump.)
Note.- Figure A3-1 is a schematic drawing of the exhaust gas sampling and analytical system and typifies the basic requirements for emissions testing.
Figure A3-1 Sampling and Analysis System Supplemental information Water is a major product of combustion. Its removal up stream of the measuring in struments is desirable.
Removal would minimize possible interference effects, where the instrument responds to the water present as well as to the gas or vapor being measured. It would also prevent or minimize water condensing in the instruments, which could cause errati c flow and/or contamination. In t he worst case the instrument would be rendered inoperable until thoroughly cleaned. Some devices which remove water are known to remove hydrocarbons and oxides of nitrogen as well.
For most aircraft gas turbine engines and most engine running modes, supplemental pumps will be needed to match the engine exhaust flow with the probe system pressure drop mandated requirement (80% at the probe entrance orifices), the sample line residence time a nd pressure drop, and the need to remove excess flow from the sampling system. In general the size and lo cation of the pumps and the associated flow control devices are determined from the particular sampling system configuration.
Pumps used for the purpose of sample transfer must be heated. Their location is determined by the particular test configuration. Usually, because of the samp le gas physical properties and the need to maintain temperature and flow control within the FID used for hydrocarbon analysis, some instruments utilize internal heated, inert sample transfer pumps. If loss of hydroc arbons in the sampling syst em is a concern, the FID can be located upstream of the system hot pump as close as physical constraints will allow (temperature, noise, vibration).
5. DESCRIPTION OF COMPONENT PARTS Note.— A general description and specification of the principal elements in the engine exhaust emissions measurement system follows. Greater detail, where necessary, will be found in Attachments A, B and C to this appendix.
5.1 Sampling system
5.1.1 Sampling probe
a) The probe shall be made of stainless steel. If a mixing probe is used, all sampling orifices shall be of equal diameter;
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;
Supplemental information While there is no need to approximate isokinetic conditions when sampling for non-particulates, good practice requires minimum disturbance to the sampled stream as practical. This is especially true when there is concern over spatial variability in gas species concentration, which is the case for the gas turbine engine exhaust plume.
c) the number of sampling orifices shall not be less than 12;
Supplemental information 12 sampling orifices, for an averaging probe, or 12 samp ling points for a single point traversing probe, should be considered acceptable only in extreme circumstances or when this number of sampling points has clearly been demonstrated to yield a representative sample.
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;
Supplemental information For non-coplanar fan/core exhaust nozzles, care must be taken in locating the sampling plane within 0.5 nozzle diameters of the exit plane inas much as there can be two exit planes. The exit plane, from which this distance is measured, is the plane of the final exhaust nozzle – usually the core. The distance allowed would be determined from the larger, fan exhaust nozzle. Further, for these engines, locating the sampling probe as close to the engine exhaust nozzle exit plane as per mitted could compromise the requirement to make measurements of the 'total exit nozzle' as defined. Given this ambiguity and the requirement to determine the concentration of species generated by the engine – restricted to the core flow until mixed out – every effort should be made to sample core flow only. This may r equire a special extension to the core nozzle which will require FAA approval.
and
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.
Supplemental information:
There are no standard definitions of representative sample for emissions from aircraft gas turbine engines nor is there a specification for detailed traverse. The most commonly used definition, arrived at from decades of testing and collaborative analytical exercises by user groups, seems to be the following: for engine modes above idle, (i.e., approach, climb-out and take-off) the integrated average of the measured species should agree with the true average within 10 %. For idle this agreement is ± 15%.
This is similar to the values quoted in section 6.4 of this Appendix, for the carbon balance check. However there is a significant difference. 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 ma de from the measured carbon containing species – most of which is CO 2, – of the engine average fuel-to-air ratio. The engine fuel-to-air ratio can be independently arrived at from measured fuel and airflow. These two values can be compared and an estimate made of how well the exhaust stream was sampled for carbon co ntaining compounds. However, as mentioned, CO 2 far outweighs the influence of any other carbon bearing species in the calculation of fuel-to-air ratios. The spatial variability of CO 2 will determine how many sampling points ar e required and how these sampling points should be distributed. CO 2 has been found to consistently exhibit t he least variability of all the species of interest – CO, HC, NO x, and smoke. This suggests that it would be possible to meet the 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 condition for a representative sample, can not be considered to be a sufficient condition.
Historically, each engine manufacturer or testing agency appears to have addressed this problem in unique and sometimes novel ways. The simplest method is as follows:
• Recognizing that gas turbine engines are, for most emissions sampling purposes, axi-symmetric, sample the exhaust plume, point by point, with a sufficient numb er of points to be able to estimate, by statistical means, the true engine average species concentration fo r each of the species of interest. Engine exhaust species are close enough to normally (Gaussia n) distributed to make simple statistical tools acceptable.
• Using these sampling points and the measured concentrat ion values, for each of the species of interest, analytically generate contour plots of constant concentration (isopleths) at each power setting tested.
There are a number of computer programs available for workstations or desktop computers to do this.
• For any given probe or rake design analytically ov erlay the design on the contour plots, estimate the average probe/rake values and compare to the esti mated true average arriv ed 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 be representative. This process may be have to be repeated until an acceptable design is found.
When applying this analytical technique (for core fl ow only), multiport rake designs which have proven most robust have four sampling arms spaced 90 degrees apart with sampling orifices located on centers of equal area. The sampling ports are equally distributed across the sampling arms . The analysis will determine the minimum number of sampling ports necessary to yield a representative sample.
All data used in arriving at a probe/rake design should be made available to the certificating agency.
5.2 HC analyser Page 47
The measurement of total hydrocarbon sample content shall be made by an analyser using the heated flame ionization detector (FID), between the electrodes of which passes an ionization current proportional to the mass rate of hydrocarbon entering a hydrogen flame. The analyser shall be deemed to include components arranged to control temperature and flow rates of sample, sample bypass, fuel and diluent gases, and to enable effective span and zero calibration checks.
Note. — An over-all specification is given in Attachment A to this appendix
5.3 CO and CO2 analysers Non-dispersive infra-red analysers shall be used fo r the measurements of these components, and shall be of the design which utilizes differential energy absorption in parallel reference and sample gas cells, the cell or group of cells for each of these gas constituents being sensitized appropriately. This analysis sub-system shall include all necessary functions for the control and handling of sample, zero and span gas flows. Temperature control shall be that appropriate to whichever basis of measurement, wet or dry, is chosen.
Note.—- An over-all specification is given in Attachment B to this appendix
5.4 NOX analyser The measurement of NO concentration shall be by the chemiluminescent method in which the measure of the radiation intensity emitted during the reaction of the NO in the sample with added O3 is the measure of the NO concentration. The NO2 component shall be converted to NO in a converter of the requisite efficiency prior to measurement. The resultant NOX measurement system shall include all necessary flow, temperature and other controls and provide for routine zero and span calibration as well as for converter efficiency checks.
Note. — An over-all specification is given in Attachment C to this appendix
6. GENERAL TEST PROCEDURES
6.1 Engine operation
6.1.1 The engine shall be operated on a static test facility which is suitable and properly equipped for high accuracy performance testing.
6.1.2 The emissions tests shall be made at the power settings prescribed by the certificating authority.
The engine shall be stabilized at each setting.
6.2 Major Instrument calibration Note. — The general objective of this calibration is to confirm stability and linearity.
6.2.1 The applicant shall satisfy the certificating authority that the calibration of the analytical system is valid at the time of the test.
6.2.2 For the hydrocarbon analyser this calibration shall include checks that the detector oxygen and differential hydrocarbon responses are within the limits specified, as laid down in Attachment A to this appendix. The efficiency of the NO2/NO converter shall also be checked and verified to meet the requirements in Attachment C to this appendix.