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5 Detailed AnalysisâMeasured versus Modeled Discrepancies Departures 5.1 For all jet aircraft types evaluated, Table 5 presents the measured and modeled departure SEL values, the discrepancies due to the difference between measured and modeled SEL, the decibel effect of altitude differences and the resultant discrepancy if the altitude could be corrected.21 Figure 1 and Figure 2 present the measured and modeled SEL values and the altitude corrected discrepancy, respectively. The measured minus modeled discrepancy is the metric of how well the INM as normally used for modeling differs from the measured reality for that aircraft. The altitude discrepancy shows how the measured minus modeled discrepancy would change if the modeled altitude matched the measured reality, assuming all other variables remained unchanged. However, as will be demonstrated in Section 7, because an aircraftâs altitude at any location is determined by the specific values of the aircraftâs thrust, speed, weight, drag over lift coefficient and other physical variables, correcting for altitude can be accomplished only by altering some or all of these values. Altering some of these, especially thrust, will alter the aircraft noise level so that the measured minus modeled SEL will change, possibly increasing or decreasing, depending on the specific values assigned to these factors. Additionally, all values used must lie within the acceptable envelope of the aircraftâs performance capability. Minimizing both the measured discrepancy and the altitude discrepancy thus requires a balancing of thrust, speed, flap management, climb/descent rate, and changes to these during the aircraftâs ascent or descent. This process was followed in Section 7.1 for departures of three aircraft types. 21 Altitude effects were determined flight by flight from differences between measured and modeled slant distances from the relevant monitor. The resultant effect shown in the table was the change in total energy, by aircraft type. Note that some flights may not have flown directly over the monitor so that the change in slant distance from the monitor may not have exactly equaled the change in altitude. However, significant monitors were relatively close to the airport so that aircraft tracks had not dispersed much at the monitor location. Additionally, only tracks that were no more than 30 degrees off vertical were used. This 30 degree restriction ensured that ground attenuation effects were not incorporated in either the measured or modeled results. Expand more on altitude correction. 25
Table 5 Departure discrepancies computed for all aircraft types Order Designator Measured SEL (dB) Modeled SEL (dB) Discrepancies Measured minus Modeled (dB) Effective Altitude Discrepancy (dB) Discrepancy if Altitude Corrected22 (dB) 1 H25B* 88.9 94.3 -5.3 -3.1 -8.5 2 BE40* 89.2 93.7 -4.5 -1.8 -6.3 3 FA50* 93.3 92.7 0.7 -2.4 -1.8 4 C560 88.8 93.3 -4.5 -2.4 -6.9 5 GLF4 87.4 83.5 3.9 -3.5 0.4 6 C56X 83.6 84.5 -0.9 -2.1 -3.0 7 LJ45* 85.2 94.2 -9.0 -2.8 -11.7 8 C550 88.8 88.1 0.7 -2.1 -1.4 9 C525 86.3 85.1 1.2 -4.3 -3.1 10 C650 92.2 91.0 1.2 -2.3 -1.1 11 F2TH* 86.8 88.9 -2.1 -2.7 -4.8 12 CL60 84.3 87.6 -3.3 -0.8 -4.1 13 LJ35 86.6 94.1 -7.5 -2.5 -10.1 14 F900 89.9 92.0 -2.2 -2.4 -4.6 15 GLF5 86.8 87.9 -1.1 -2.9 -3.9 16 LJ31* 86.1 93.6 -7.5 -2.4 -9.9 17 PRM1* 86.1 93.4 -7.4 -3.1 -10.4 18 C750* 81.9 83.9 -1.9 0.3 -1.7 19 GALX* 88.0 88.4 -0.4 -2.3 -2.7 20 C25A* 87.9 84.7 3.2 -4.3 -1.0 21 CL30* 85.8 86.2 -0.4 -1.1 -1.5 22 C680 82.6 86.2 -3.6 -2.1 -5.7 23 ASTR 90.1 92.5 -2.4 0.0 -2.4 24 G150* 90.8 92.4 -1.6 0.0 -1.6 25 C25B 83.5 84.6 -1.1 -3.2 -4.3 26 LJ60* 82.5 85.6 -3.1 -2.1 -5.1 27 C501* 87.2 88.9 -1.6 -1.6 -3.2 28 C510 84.5 84.3 0.2 -2.7 -2.5 29 EA50 80.0 76.3 3.7 -3.9 -0.2 22 This correction is the total sound level discrepancy that would occur if the airplane flight track were as close to the noise monitor as were the actual flights. It assumes that there are no changes in thrust or speed. 26
Figure 1 Measured and modeled departure SEL values for the evaluated aircraft Figure 2 Altitude corrected departure discrepancies for the evaluated aircraft 27
Arrivals 5.2 For all aircraft types evaluated, Table 6 presents the measured and modeled arrival SEL values, the discrepancies including the difference between measured and modeled SEL, the decibel effect of altitude differences and the resultant discrepancy if the altitude could be corrected. Figure 3 and Figure 4 graphically present the measured and modeled SEL values and the altitude corrected discrepancy, respectively. Table 6 Arrival discrepancies computed for all aircraft types Order Designator Measured SEL (dB) Modeled SEL (dB) Discrepancies Measured minus Modeled (dB) Effective Altitude Discrepancy (dB) Discrepancy if Altitude Corrected (dB) 1 H25B* 85.4 85.1 0.3 1.0 1.3 2 BE40* 84.5 84.2 0.3 0.7 0.9 3 FA50* 87.0 88.1 -1.1 1.8 0.7 4 C560 82.5 84.3 -1.9 0.9 -1.0 5 GLF4 85.4 85.6 -0.2 1.2 1.0 6 C56X 85.3 87.8 -2.6 1.0 -1.5 7 LJ45* 84.8 85.0 -0.2 1.2 1.0 8 C550 81.7 85.2 -3.5 0.9 -2.6 9 C525 83.4 82.8 0.6 1.3 1.9 10 C650 84.2 82.1 2.1 0.5 2.6 11 F2TH* 85.2 84.5 0.7 1.2 1.9 12 CL60 84.6 85.4 -0.7 1.0 0.2 13 LJ35 85.4 85.3 0.2 1.1 1.3 14 F900 83.9 88.1 -4.3 1.5 -2.8 15 GLF5 83.0 85.2 -2.2 0.9 -1.3 16 LJ31* 82.2 84.2 -2.0 1.1 -0.9 17 PRM1* 80.5 84.2 -3.7 2.1 -1.6 18 C750* 82.3 87.3 -5.0 1.0 -4.0 19 GALX* 83.7 83.7 0.0 0.8 0.8 20 C25A* 84.4 82.1 2.3 1.3 3.6 21 CL30* 82.6 84.6 -2.0 1.0 -1.0 22 C680 81.7 82.4 -0.7 1.1 0.4 23 ASTR 83.3 81.2 2.1 0.9 2.9 24 G150* 83.4 81.1 2.3 1.0 3.3 25 C25B 82.4 82.2 0.2 1.0 1.2 26 LJ60* 81.6 86.3 -4.7 1.1 -3.5 27 C501* 82.6 81.9 0.7 0.8 1.5 28 C510 79.3 81.8 -2.6 1.3 -1.3 29 EA50 77.9 75.9 2.0 0.9 2.9 28
Figure 3 Measured and modeled arrival SEL values for the evaluated aircraft Figure 4 Altitude corrected arrival discrepancies for the evaluated aircraft 29
Observations 5.3 The INM modeling for almost all aircraft types computes departure SEL values higher than the measured levels (measured minus computed discrepancies are almost all less than zero, Table 5). Correcting for altitude, if it were possible, generally makes the discrepancy more negative because for most aircraft, the INM departure altitudes for the aircraft are higher than actually occurs; see Section 7.1. Arrival discrepancies are more mixed, with some modeled louder, some quieter. This is not surprising because the INM standard approach is a single descent angle of three degrees. Most descents include level portions and frequent changes of thrust. 30