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819 Figure 6-6 shows that the three SAR conditions required by § 38.13(b) are relatively close to each 820 other in terms of Mach and weight/δ because they represent the same optimum aerodynamic 821 conditions for achieving the best (lowest) 1/SAR values. Although these three SAR conditions may not 822 have been specifically flight tested, they are generally well framed by other relevant flight test data in a 823 local Mach and weight/δ range. 824 825 In order to validate the performance model in conditions relevant to demonstrating compliance with § 826
38.17, flight test points around the optimum Mach number and optimum weight/δ would be selected 827 based on the following criteria: 828 829
a) Weight/δ between ±5 percent of the optimum weight/δ; and 830
b) Mach number between -0.02 and +0.015 of the optimum Mach number. 831 832 Note 1.— The ±5 percent value in a) is used to capture flight test points on weight/δ lines above and 833 below the optimum weight/δ included in the airplane manufacturer’s flight test practices. 834 835 Note 2.— The density of test points is generally lower at Mach numbers below the optimum Mach 836 number and higher at Mach numbers above the optimum Mach number. Additionally, the Mach effect 837 on SAR is generally less at Mach numbers below the optimum Mach number than above, and a sharp 838 SAR decrease may occur when approaching high Mach numbers. For this reason, the Mach range for 839 the test point selection is larger at Mach numbers below the optimum Mach number (-0.02) than above 840 (+0.015). 841 842 Note 3.— Testing target weight/δ values non-dimensionalizes the pressure altitude so that weight/δ 843 varies as CL.M². Therefore, weight/δ at a given Mach varies similarly to CL at that Mach number. 844 845
6.4.4.2 SAR model justification 846 847
6.4.4.2.1 Flight test SAR versus SAR model 848 849 In order to use first principle models to determine the SAR values for calculating the FEM, 850 manufacturers are expected to explain their modelling validation principles and show that their 851 modelling correctly matches flight test data acquired in a manner consistent with part 38 Appendix A, 852 as described in paragraph 6.3 of this AC. 853 854 As shown in Figures 6-7 and 6-8, below, one possible method is to present plots of percent ∆SAR 855
(i.e. Measured SAR minus Computed SAR) versus Mach and versus weight/δ (or CL). These should 856 be shown in the Mach and weight/δ (or CL) range of the cruise envelope. 857 858 An increased number of test points, when available, may be used to qualitatively show that the test 859 point selection, described in paragraph 6.4.4.1 above (red points selected for optimum SAR in 860 Figures 6-7 and 6-8 below, which are within the orange dashed circle in Figure 6-6), is not differently 861 scattered or biased than other test points in the cruise envelope (green points that are all the flight 862 test data in Figures 6-7 and 6-8 below, and these are outside the orange dashed circle in Figure 6-6). 863 Figures 6-7 and 6-8 are examples of a set of data that show no significant biases or differences in 864 scatter inside and outside the cruise envelope. 865 866 867 868 869 870 871 872 873
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Figure 6-7 874 875 ∆SAR versus Mach number 876 877 878 879 880 881 882 Figure 6-8. 883 884 ∆SAR versus CL 885 886 887 888
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889 890
6.4.4.2.2 SAR model presentation 891 892 The comparison of flight test points with a model does not validate how that model is built and how 893 the optimum conditions represent the FEM. Accordingly, the SAR model used to demonstrate 894 compliance with § 38.17 must be approved by FAA per § 38.13(a)(2). 895 896 One possible method to present the SAR model is to present plots of computed SAR versus Mach 897 and computed SAR versus CL for the three masses referenced in § 38.13(b). Figures 6-9 and 6-10 898 show that there are no abnormal discontinuities in the Mach and CL ranges selection around SAR 899 conditions used for the FEM determination. 900 901 902 903 904 Figure 6-9 905 906 Computed SAR (model) versus Mach at optimum altitude 907 908 909 910 911 912 913
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Figure 6-10 914 915 Computed SAR (model) versus CL at optimum Mach 916 917 918 919 920
6.4.4.3 SAR model validation – confidence interval determination 921 922 As explained in paragraph 6.4.4 above, the SAR model needs to be validated under the conditions 923 relevant to demonstrating compliance with § 38.17. To be used for validation, flight test points need to 924 be selected using the criteria described in paragraph 6.4.4.1 above. The number of flight test points 925 selected will represent n measurements (number of data points) of SAR (SAR1, SAR2, …, SARn) 926 obtained in approximately the same conditions of Mach and CL. 927 928 Step 1 — Estimate of the mean ∆SAR value and of the standard deviation 929 For each flight test point (i), the SAR difference ∆SAR(i) between the measured SAR and the computed 930 SAR can be determined as follows (in percent): 931 932 ∆SAR(i)(%) = [(Measured SAR(i) - Computed SAR(i)) / Computed SAR(i)] x 100 (%). 933 934 The estimate of the mean ∆SAR value in percent, ∆SARAVG(%), can then be determined for the n 935 measurements as follows: 936 937 𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆𝐴𝐴𝐴𝐴𝐴𝐴(%) = 𝐹𝐹𝑒𝑒𝑒𝑒𝐸𝐸𝑒𝑒𝑣𝑣𝑒𝑒𝐹𝐹 𝑜𝑜𝐸𝐸 𝑒𝑒ℎ𝐹𝐹 𝑒𝑒𝐹𝐹𝑣𝑣𝐸𝐸 𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆 (%) = 1𝑛𝑛 �∑ 𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆(𝑖𝑖)𝑖𝑖=𝑛𝑛𝑖𝑖=1 �(%). 938 939 The estimate of the standard deviation of the mean is found as follows: 940 𝑒𝑒 = 𝐹𝐹𝑒𝑒𝑒𝑒𝐸𝐸𝑒𝑒𝑣𝑣𝑒𝑒𝐹𝐹 𝑜𝑜𝐸𝐸 𝑒𝑒ℎ𝐹𝐹 𝑒𝑒𝑒𝑒𝑣𝑣𝐸𝐸𝑠𝑠𝑣𝑣𝑒𝑒𝑠𝑠 𝑠𝑠𝐹𝐹𝑣𝑣𝐸𝐸𝑣𝑣𝑒𝑒𝐸𝐸𝑜𝑜𝐸𝐸 𝑜𝑜𝐸𝐸 𝑒𝑒ℎ𝐹𝐹 𝑒𝑒𝐹𝐹𝑣𝑣𝐸𝐸 = �∑ �𝐷𝐷𝐷𝐷𝐴𝐴𝐷𝐷(𝑖𝑖)− 𝐷𝐷𝐷𝐷𝐴𝐴𝐷𝐷𝐴𝐴𝐴𝐴𝐴𝐴�2𝑖𝑖=𝑛𝑛𝑖𝑖=1 𝑛𝑛−1 (%). 941
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942 Step 2 — Confidence interval determination 943 944 The low number of test points in a small range of Mach and CL results in a statistical population that 945 follows the Student law, and allows the 90 percent confidence interval to be defined in accordance with 946 the methodology for Direct Flight Testing, as described in appendix 4 of this AC. The objective is to 947 determine an average ∆SAR value (%) and its associated 90 percent confidence interval. The 948 minimum acceptable sample size is twelve points (see § A38.5.3.4.). 949 950 Following this step and using the Student t-distribution, the 90 percent confidence interval (CI90) for the 951 estimate of the mean ∆SARAVG can be determined as: 952 953 𝐶𝐶𝐶𝐶90 (%) = �𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆𝐴𝐴𝐴𝐴𝐴𝐴 ± 𝑒𝑒(.95,𝑛𝑛−1) s √𝑛𝑛� (%) 954 955 where t(.95, n-1) = Student t-distribution (for 90 percent confidence) for the degrees of freedom (i.e., n-1, 956 where n is number of data points). (See Table A4-1 in Appendix 4 of this AC). 957 958 Step 3 — Validity of results 959 960 Section A38.5.3.2 contains the following requirements: 961 962 “A38.5.3.2 If the 90 per cent confidence interval of the SAR value at any of the three reference 963 airplane masses – 964 965 A38.5.3.2.1 Is less than or equal to ±1.5 per cent, the SAR value may be used. 966 967 A38.5.3.2.2 Exceeds ±1.5 per cent, a penalty equal to the amount that the 90 per cent confidence 968 interval exceeds ±1.5 per cent must be applied to the SAR value, as approved by the FAA.” 969 970 Applying those requirements to a use of a first principle model for showing compliance: 971 972
a) If the 90 percent confidence interval remains within the ±1.5 percent limits, then the ∆SARAVG 973 (%) value can be retained as the reference deviation for model correction. 974
b) If the 90 percent confidence interval exceeds the ±1.5 percent limits, then a penalty equal to 975 the amount that the 90 percent confidence interval exceeds ±1.5 percent should be applied to 976 the ∆SARAVG(%) value, subject to the approval of AEE. For example, if the 90 percent 977 confidence interval is 5 percent, then the corrected ∆SARAVG(%) is 3.5 percent (5 percent -1.5 978 percent). The corrected ∆SARAVG(%) value then becomes the reference deviation for the 979 model correction. 980 981 Step 4 — SAR Model correction 982 983 When used to determine the FEM, the SAR values computed with a first principle model at each of the 984 three reference airplane masses must be corrected by a percentage amount that is equal to the 985 reference deviation determined in Step 3. 986 987 988
6.4.4.4. Example of the determination of 90 percent confidence intervals 989 990 The set of 12 flight test points are presumed to meet the selection criteria of paragraph 6.4.4.1 above. 991 The difference between the measured SAR and the computed SAR has been established as follows: 992 993 994 995 996
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Table 6-1 997 998 Test point selection 999 1000 Selected test point number ∆SAR (%) 1 0.08 2 -0.6 3 -0.42 4 0.19 5 -0.43 6 0.23 7 -0.28 8 0.45 9 0.10 10 -0.28 11 -0.80 12 -0.64 1001 1002
a) The number of data points (n) = 12. 1003
b) The degrees of freedom (n-1) = 11. 1004
c) The Student t-distribution for 90 percent confidence and 11 degrees of freedom (𝑒𝑒(.95 ,11) ) = 1.797. 1005 (See examples in Table A4-1 in Appendix 4 to this AC.). 1006 1007 1008 Step 1 — Estimate of the mean ∆SAR (%) and of the standard deviation(s) 1009 1010 𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆𝐴𝐴𝐴𝐴𝐴𝐴(%) = 112 �∑ 𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆(𝑖𝑖)𝑖𝑖=12𝑖𝑖=1 � = -0.200 % 1011 1012 𝑒𝑒 = �∑ �𝐷𝐷𝐷𝐷𝐴𝐴𝐷𝐷(𝑖𝑖)− 𝐷𝐷𝐷𝐷𝐴𝐴𝐷𝐷𝐴𝐴𝐴𝐴𝐴𝐴�2𝑖𝑖=12𝑖𝑖=1 𝑛𝑛−1 = 0.39950 % 1013 1014 Step 2 — 90 percent confidence interval (CI90) 1015 1016 𝐶𝐶𝐶𝐶90 = 𝐷𝐷𝑆𝑆𝑆𝑆𝑆𝑆𝐴𝐴𝐴𝐴𝐴𝐴 ± 𝑒𝑒(.95,11) s √12 = −0.200 ± 0.207 % 1017 1018 Step 3 — Check of confidence interval limits 1019 1020 The confidence interval extends to ±0.207 percent around the mean ∆SAR value (-0.200 percent), 1021 which is within the confidence interval limit of ±1.5 percent. As a result, a ∆SAR value of -0.200 1022 percent can be retained for model correction. 1023 1024 Step 4 — SAR Model correction 1025 1026 The SAR value calculated for each reference mass of the FEM can be determined by computation 1027 with the first principle model and would be corrected by the -0.200 percent value determined in Step 3 1028
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as follows: 1029 1030 SARcorrected = SARcomputed x 0.998 1031 1032
6.5 Validity of Results – Confidence Interval 1033 1034 Section A38.5.3.1 requires the 90 percent confidence interval to be calculated for each of the SAR 1035 values at the three reference masses. Appendix 4 of this AC provides guidance regarding confidence 1036 interval evaluation, including examples. 1037 1038
6.6 Reference Geometric Factor 1039 1040
6.6.1 RGF General Guidelines 1041 1042 The reference geometric factor (RGF) is a non-dimensional parameter used to adjust the value of 1043 (1/SAR)AVG. RGF is based on a measure of fuselage size normalized with respect to 1 m2, and is derived 1044 as prescribed in §38.15. 1045 1046 The basic concept of RGF, and the exponent of 0.24 for RGF in the fuel efficiency metric, is to give 1047 appropriate credit for the size of the cabin. The RGF represents the area that is available for the 1048 transport of passengers or cargo on the main deck, and on an upper deck. §§ 38.15 and A38.3 provide 1049 a description of RGF and list some areas that are to be included in the RGF calculation, including 1050 figures cross-sectional and longitudinal views of notional RGF boundary conditions. This list of included 1051 areas is not comprehensive; RGF also includes, for example, any space in the cabin used for 1052 wardrobes, or for cabinets containing equipment such as passenger entertainment systems or life 1053 rafts. 1054 1055 Most airplanes have a single deck, or floor, for passengers or cargo. Many single-deck airplanes also 1056 have an underfloor cargo bay, but that does not mean that they should be considered as two-deck 1057 airplanes. 1058 1059
6.6.2 Orthogonal Projection and Width Boundary 1060 1061 For a single-deck airplane, the RGF is determined by making an orthogonal projection of the outer mold 1062 line (OML) of the fuselage pressure shell, onto a plane that is parallel to the main deck floor. The term 1063 orthogonal means that the projection is made using theoretical lines of projection which are orthogonal, 1064 or perpendicular, to the main deck floor, and also orthogonal to the plane where the projection is made. 1065 In the case of an airplane in which the main deck is perfectly horizontal when it stands on the ground, 1066 the projection is similar to the ‘shadow area’ on the ground, or the top view area of the fuselage. The 1067 terms ‘shadow area’ and ‘top view’ can be misleading however, and orthogonal projection is the proper 1068 term for obtaining a true projection. The term ‘shadow area’ could imply projection lines emanating 1069 from a single point, effectively like the sun. It is possible to make a projection using lines which emanate 1070 from any single point, but that is not intended, and would not be a true projection for RGF. The term 1071 ‘top view’ can be misleading for an airplane in which the main deck is not perfectly horizontal when it 1072 stands on the ground. Typically the main deck of an airplane is at a constant height (or constant Z 1073 station) in terms of the X, Y, Z coordinate system for the airplane, and in that case the orthogonal 1074 projection is the same as a plan view drawing, viewing either up or down along the Z axis. However, 1075 there is no requirement for any deck to be at a constant Z station. 1076 1077 Typically the maximum width of the fuselage cross section is above the main deck for a single-deck 1078 airplane, and if the fuselage has a constant cross section for part of its length, then the maximum width 1079 will be at the same height above the main deck (i.e. at the same Z station) for part of the fuselage 1080 length. At the aft end of the fuselage however, there will no longer be a constant cross section, and it is 1081 common for the point of maximum width to rise up higher above the main deck towards the tail, with 1082 fuselage upsweep. Towards the nose, typically the point of maximum width descends closer to the main 1083
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deck. For the orthogonal projection, the maximum external width of the OML is always ‘captured’ at 1084 whatever height above the deck that it occurs. 1085 1086 For an airplane that has a main deck and also an upper deck, the RGF for the main deck is determined 1087 using the fuselage OML as the width boundary, as for a single-deck airplane. The upper deck is 1088 considered separately, and the width boundary for the upper deck RGF is defined as the fuselage OML 1089 at or above the upper deck floor level. This means that to determine the upper deck RGF, all of the 1090 fuselage below the upper deck floor can be removed from consideration, and then an orthogonal 1091 projection should be made of the remaining part of the fuselage, the entire upper cabin, onto a plane 1092 that is parallel to the upper deck. This projection will capture the maximum width of the fuselage OML 1093 at any height at or above the upper deck floor. The upper deck may be parallel to the main lower deck, 1094 but there is no requirement for this to be the case. The total RGF is simply the sum of the RGF values 1095 for the two decks. 1096 1097
6.6.3 Forward and Aft Boundaries 1098 1099 The aft boundary to be used for calculating RGF is the aft surface of the aft pressure bulkhead skin. 1100 This bulkhead may be a flat or domed skin to take the pressure loads, and typically has supporting 1101 structure attached either to the forward or aft face of the bulkhead. This aft boundary definition is 1102 consistent with the concept of using the external surface of the pressure shell for the width boundary. 1103 1104 The forward boundary to be used for calculating RGF is the forward surface of the forward pressure 1105 bulkhead skin except for the cockpit crew zone. For all airplanes, a plane will need to be defined for the 1106 aft boundary of the cockpit crew zone. If there is a cockpit door, this aft boundary is the plane of the 1107 forward face of the cockpit door. If there is no cockpit door, a boundary plane will need to be defined 1108 that is aft of the cockpit crew seats. Some airplanes may have optional approved cockpit layouts, for 1109 example to provide for one or two ‘jump seats’ aft of the two standard pilot seats; in that case the RGF 1110 should be based on the smallest approved cockpit layout (see § A38.3). 1111 1112 Some airplanes are certified for single-pilot operations, as illustrated in Appendix A38.3 Figure A38-1113
2. This figure shows that a passenger may be in the right cockpit seat, and the right side of the cockpit 1114 can be included in the RGF, even if a cockpit door is standard equipment. In this case, the aft 1115 boundary of the cockpit crew zone applies to the left side of the fuselage only. 1116 1117 For an airplane with two decks, the cockpit may be accessed from the main deck, or the upper deck, 1118 but the two decks are considered separately. One deck may extend forward, above or below a cockpit 1119 crew zone, and in that case the aft boundary of the cockpit crew zone should only apply to the RGF of 1120 the deck that provides direct access to the cockpit, not to both decks. 1121 1122
6.6.4 Floor Plane 1123 1124 For the floor plane of the main deck, and any upper deck, it is assumed that most, or all, of the floor 1125 area is on the same flat plane. It is common to have some local anomalies, such as a step up in the 1126 floor of the cabin towards the tail, typically in a baggage area. If there is a relatively small, elevated 1127 floor it is usually flat and parallel to the main floor, but there is no requirement for it to be so. Some 1128 airplanes may have other local floor changes, possibly even changes in slope. Some smaller airplanes 1129 have a dropped aisle, with elevated seating areas on both sides of the aisle. There is no requirement 1130 for any deck floor to be parallel with the upper or lower skin of the fuselage, but this will usually be the 1131 case. For each deck, there should be no difficulty in identifying a nominal flat plane which should apply 1132 to it, which then allows for the identification of a true flat plane (with no local anomalies) parallel to the 1133 deck, for the purpose of the orthogonal projection. 1134 1135
6.6.5 RGF Sensitivity 1136 1137 Although RGF is an important component of the CO2 emissions evaluation metric value, the exponent 1138 of 0.24 means that the metric value is less sensitive to a change in RGF as compared to a change in 1139
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the average 1/SAR value. As an example, if the average 1/SAR value is changed by a factor of 1.01, 1140 then the metric value will also change by a factor of 1.01 (a 1% increase). However, if the RGF 1141 changes by a factor of 0.99 (a 1% decrease) then the metric value will increase by only a factor of 1142
1.0024150 (an increase of 0.2415%). The change in metric value as a result of a change in RGF, can 1143 be expressed as the reciprocal of the factor applied to RGF, with an exponent of 0.24 applied. For this 1144 example: ( 1 / 0.99 ) = 1.010101 ; and ( 1.010101 )0.24 = 1.002415. 1145 1146 The RGF sensitivity may lead an applicant to declare positions for the forward boundary and aft 1147 boundary that are slightly conservative in order to avoid lengthy geometric explanations and 1148 justifications. Typically, a small amount of conservatism, by declaring a forward and/or aft boundary that 1149 is displaced by 1 or 2 cm will lead to a very small change in RGF, much less than 1%, and an extremely 1150 small penalty in the metric value of less than 0.1%. 1151 1152 An applicant may also consider the potential for future geometric changes, and the no-FEM-change 1153 criteria. It may be administratively easier to deal with a small geometric change in the future, if the RGF 1154 has been defined in a slightly conservative way. 1155 1156
6.6.6 Floor Plane 1157 1158 For the floor plane of the main deck, and any upper deck, it is assumed that most, or all, of the floor area 1159 is on the same flat plane. It is common to have some local anomalies, such as a step up in the floor of 1160 the cabin towards the tail, typically in a baggage area. If there is a relatively small, elevated floor it is 1161 usually flat and parallel to the main floor, but there is no requirement for it to be so. Some airplanes may 1162 have other local floor changes, possibly even changes in slope. Some smaller airplanes have a dropped 1163 aisle, with elevated seating areas on both sides of the aisle. There is no requirement for any deck floor to 1164 be parallel with the upper or lower skin of the fuselage, but this will usually be the case. For each deck, 1165 there should be no difficulty in identifying a nominal flat plane which should apply to it, which then allows 1166 for the identification of a true flat plane (with no local anomalies) parallel to the deck, for the purpose of 1167 the orthogonal projection. 1168 1169
6.6.7 RGF Sensitivity 1170 1171 Although RGF is an important component of the CO2 emissions evaluation metric value, the exponent 1172 of 0.24 means that the metric value is less sensitive to a change in RGF as compared to a change in 1173 the average 1/SAR value. As an example, if the average 1/SAR value is changed by a factor of 1.01, 1174 then the metric value will also change by a factor of 1.01 (a 1% increase). However, if the RGF 1175 changes by a factor of 0.99 (a 1% decrease) then the metric value will increase by only a factor of 1176
1.0024150 (an increase of 0.2415%). The change in metric value as a result of a change in RGF, can 1177 be expressed as the reciprocal of the factor applied to RGF, with an exponent of 0.24 applied. For this 1178 example: ( 1 / 0.99 ) = 1.010101 ; and ( 1.010101 )0.24 = 1.002415. 1179 1180 The RGF sensitivity may lead an applicant to declare positions for the forward boundary and aft 1181 boundary that are slightly conservative in order to avoid lengthy geometric explanations and 1182 justifications. Typically, a small amount of conservatism, by declaring a forward and/or aft boundary that 1183 is displaced by 1 or 2 cm will lead to a very small change in RGF, much less than 1%, and an extremely 1184 small penalty in the metric value of less than 0.1%. 1185 1186 An applicant may also consider the potential for future geometric changes, and the no-FEM-change 1187 criteria. It may be administratively easier to deal with a small geometric change in the future, if the RGF 1188 has been defined in a slightly conservative way. 1189 1190
6.7 Fuel Efficiency Limits 1191 1192 Section 38.17 provides the maximum permitted fuel efficiency metric values for airplanes defined under 1193 § 38.1(a), as determined pursuant to § 38.11. In accordance with § 38.17(b), the fuel efficiency metric 1194 value may not exceed the values detailed in the table with fuel efficiency limits in § 38.17. Notably, the 1195
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values determined in accordance with the equations in the table with fuel efficiency limits in § 38.17 1196 should be rounded to three decimal places. The maximum permitted fuel efficiency metric values are 1197 based on the type of airplane as defined in § 38.1 (applicability section) and the airplane’s MTOM. A 1198 more detailed discussion on the applicability of § 38.1 can be found in paragraph 5.1 of this AC. 1199 1200
6.8 Change Criteria 1201 1202 A modification requiring compliance with part 38 is determined by the change criteria in § 38.19. The 1203 requirements differ depending on whether an airplane has previously demonstrated compliance with part 1204 38 before a modification is made, or whether an airplane was type certificated before January 1, 2023, 1205 and was not required to demonstrate compliance with part 38. 1206 1207 The change criteria in § 38.19(a) specifies when a compliance demonstration to this new rule is required 1208 if a modification to an airplane, that has been shown to comply with § 38.17, will increase the MTOM of 1209 the airplane as written in § 38.19(a)(1) or increases the FEM value above the thresholds provided in § 1210
38.19(a)(2)(i) through (iii). Figure 6.11, below, illustrates the FEM change threshold limits of § 38.19 for 1211 any MTOM. 1212 1213 Fig. 6.11 1214 1215 FEM change threshold limits with variation in MTOM 1216 1217 1218 1219 Under § 38.19(b), where an airplane has been shown to comply with § 38.17, for a modification that 1220 does not increase either the MTOM or the FEM value, the airplane may retain the same FEM value as 1221 prior to modification. 1222 1223 Section 38.19(c) provides that an airplane, which meets the applicability provisions of § 38.1(a)(4) or (5) 1224 on or after January 1, 2023 and before January 1, 2028, must demonstrate compliance if the 1225 modification exceeds 1.5%. Alternatively, an airplane modification that is shown to have less than a 1226
1.5% increase in FEM does not have to demonstrate compliance. For an airplane whose type design 1227 was not previously certified to part 38, an applicant can estimate the change in the FEM value of the 1228
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modified version of such an airplane by using one of the methods described in this AC38, if approved by 1229 the FAA. For example, for a change to the exterior of the airplane, the change in drag can be estimated 1230 using methods described in paragraph 6.4.3 of this AC, and this estimated change in drag can be 1231 directly related to changes in FEM value. 1232 1233 1234
7. Additional Guidance 1235 1236 If an applicant needs additional guidance, contact the AEE-300 Emissions Division Manager with the FAA 1237 Office of Environment and Energy at ralph.iovinelli@faa.gov. 1238 1239
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Appendix 1: Definitions 1240 1241 1242 The following terms are defined in § 38.3, which is reproduced here for convenience: 1243 Amphibious Airplane means an airplane that is capable of takeoff and landing on both land and water. 1244 Such an airplane uses its hull or floats attached to the landing gear for takeoff and landing on water, and 1245 either extendable or fixed landing gear for takeoff and landing on land. 1246 1247 ICAO Annex 16, Volume III means Volume III of Annex 16 to the Convention on International Civil 1248 Aviation (incorporated by reference in § 38.7). 1249 1250 Maximum takeoff mass (MTOM) is the maximum certified takeoff mass, expressed in kilograms, for an 1251 airplane type design. 1252 1253 Performance model is an analytical tool (or a method) validated using corrected flight test data that can 1254 be used to determine the specific air range values for calculating the fuel efficiency metric value. 1255 Reference geometric factor (RGF) is a non-dimensional number derived from a two-dimensional 1256 projection of the fuselage. 1257 1258 Specific air range (SAR) is the distance an airplane travels per unit of fuel consumed. Specific air range is 1259 expressed in kilometers per kilogram of fuel. 1260 1261 Subsonic means an airplane that has not been certificated under 14 CFR to exceed Mach 1 in normal 1262 operation. 1263 1264 Type certificated maximum passenger seating capacity means the maximum number of passenger seats 1265 that may be installed on an airplane as listed on its type certificate data sheet, regardless of the actual 1266 number of seats installed on an individual airplane. 1267