Integrated Noise Model Accuracy for General Aviation Aircraft (2014)

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8 Potential Solutions As shown in Sections 4 through 7, the INM standard departure profiles for GA jet aircraft produce discrepancies in noise levels and have steeper ascents than those observed for actual operations. The general cause for these discrepancies is the use of maximum thrust departures as standard INM input. As shown earlier; by creating user defined INM profiles which empirically lower thrust levels, both altitude and noise levels can be made to match observations. With this empirical observation in place we then sought out a more operationally motivated reason to explain why aircraft would use reduced thrust during departure. A limited survey of manufacturers, operators and pilots was conducted and it was determined that operating the aircraft with reduced thrust has both economical and operational benefits and is often the preferred method for departure. Section8.1 describes how derated takeoff thrust is commonly implemented in the aircraft by using the Assumed Temperature Method (ATM) departure. Section 8.2 describes the two methods used to test incorporating these derated thrust procedures into the INM. The goal of analyzing the two methods presented in this section is to test the procedures, judge the results vis à vis the measured data and the relative difficulty of each, and draw conclusions about whether to recommend one method for implementation. Derated Thrust; Assumed Temperature Method 8.1 The Assumed Temperature Method (ATM) for applying de-related thrust for departure operations is a process where an aircraft Flight Management System (FMS) is asked to compute the thrust required to safely depart the aircraft from a given runway while demanding a decreased level of engine performance. When using the ATM, the FMS computes the required ground roll and engine performance required for operating at an increased airfield temperature. The aircraft is then operated at the lower, actual airfield temperature. The net result is a departure operation requiring a longer takeoff roll and using less engine thrust. ATM departures will overall result in less wear to an engine and airframe and can therefore be economically advantageous to operators. When compared to an INM standard departure profile an ATM departure also has a significant effect on aircraft noise levels. Aircraft using the ATM produce less thrust and noise but also depart at lower altitudes. In some cases these two factors result in lower sound levels on the ground, but they can also “cancel out” with the decreased thrust producing lower noise and the lower altitude producing higher noise. However, the specific trade-offs and on-the-ground results are highly dependent on individual aircraft type. Methods for Applying Derated Thrust in INM to Provide Realistic Results 8.2 Applying the ATM derated thrust to aircraft in the INM can be accomplished through two general methods. These are labelled here for convenience, “ATM1” and “ATM2.” 1) ATM1 – This method requires determining the specific thrust levels for ATM operations from manufacturer or operator surveys then creating custom profiles to match these inputs. 2) ATM2 – This is a more efficient method that first uses the INM’s internal computation processes to determine the aircraft departure profile, including thrust levels and other profile parameters at an assumed elevated temperature. The resulting departure data are then converted into a static “profile points” style profile which is then input into the INM and run at the normal or average airfield temperature. 51

Both of these processes were evaluated using the GIV and Lear35 INM types and used as a check on the validity of each method when compared to the measured results and radar profile data described in the earlier part of this report, Sections 4 through 7. The ATM2 method was also used to evaluate the CNA560E INM type and compared to radar data. Figure 28, Figure 29, and Figure 30 show the resulting altitude / distance plots for the observed radar data as well as the resulting ATM1 and ATM2 profiles. The computation methods are described next in Sections 8.2.1 and 8.2.2. Figure 28 GIV departure altitudes: all radar data, ATM profiles and polynomial fit Figure 29 Lear35 departure altitudes: all radar data, ATM profiles and polynomial fit 52

Figure 30 CNA560E departure altitudes: all radar data, ATM2 profile and polynomial fit 8.2.1 Using Specific Profile Definitions from Operator and Manufacturer Surveys (ATM1 profiles) This method for introducing reduced thrust for an INM aircraft departure involves changing the thrust coefficient given in INM “Thrust Jet”30 database where, as described in INM 7.0 User’s Guide Section 9.11, the thrust for a given jet engine is described as: Equation 1 INM Thrust Jet 𝐹𝑛 𝛿 = 𝐸 + 𝐹𝑉𝑐 + 𝐺𝑎𝐴 + 𝐺𝑏𝐴2 + 𝐻𝑇 Where: • 𝐹𝑛 is the Net thrust per engine in pounds; • 𝛿 is the ratio of the atmospheric pressure to sea-level standard pressure; • E is the coefficient equal to the static net corrected engine thrust at sea level pressure; • F is the airspeed correction coefficient; • 𝐺𝑎 and 𝐺𝑏 are altitude correction coefficients; and • H is temperature correction coefficient. To change the computed thrust due to the ATM, the E coefficient is altered to match thrust levels either provided by the manufacture or derived (using the Thrust General coefficients) for reduced thrust. 30 Thrust Jet (thr_jet.dbf): This is the fundamental database (or table) that contains for each jet, for each operational mode of departure (takeoff, climb, etc.) a set of data (lines) giving the coefficients for computing the net correct lbs. thrust per engine for each mode. For some aircraft, the INM also has a Thrust General (thr_gnr.dbf) database that permits computation of lbs. thrust per engine using alternative thrust related metrics of N1 (percent maximum engine RPM) or EPR (engine pressure ration), see discussion below of Lear35 thrust modifications. 53

Adjustments to the speed coefficients (𝐺𝑎 , 𝐺𝑏 ) in theory are also required; however a new regression analysis using complete performance data for aircraft flying ATM profiles would be required to derive these speed coefficients and is outside of scope of this project. As an approximation, the existing speed coefficients were used; recall that in Section 2.1.6, the E coefficient is the dominant factor in the thrust equation and small discrepancies in the G coefficients will not result in significant discrepancies to calculated thrust. As a validation for this assumption a sensitivity analysis using manufacturer provided thrust data was done for the GIV aircraft and is described in the next Section. GIV Aircraft ATM Thrust jet and profile modifications: Validity of Leaving Speed Coefficient Unchanged for Thrust Determination Gulfstream Aerospace provided four thrust values for the GIV aircraft: ATM thrusts at 0 Kts and at 160 Kts; max thrust at 0 Kts and at 160 Kts. The max thrust at 0 Kts was equal to the max thrust used by the INM and simply validated the INM Standard profile thrust parameter. The other thrusts were used first to examine the effects of not modifying the speed coefficient contained in the INM and second, to develop ATM. First, to examine the effect of the speed coefficient, the standard INM departure profile was run to determine the thrust it calculates at 160 Kts. The result (10,813) is given in Table 21 and compared with the Gulfstream provided max thrust at 160 Kts (10,892). Next, the Gulfstream provided ATM thrust at 0 Kts was used in the INM to compute a resultant thrust at 160 Kts (8,533) and compared with the Gulfstream ATM thrust at 160 Kts (8,791). Table 21 also shows this comparison. The discrepancy between the INM computed thrust and the ATM 160 Kts thrust is larger, however this discrepancy is relatively small compared with the relative thrust change between the max thrust levels and the ATM thrust levels. The result was therefore considered accurate enough to use unchanged speed coefficients for developing ATM1. Final implementation of this method into the INM would require new speed coefficients for fully accurate INM computation of ATM thrust levels and profile. Table 21 Relative discrepancy due to speed coefficients INM calculated Gulfstream Provided %Difference GIV Max Thrust Departure (lbs. 160 Kts at Sea Level) 10,813 10,892 0.7% GIV ATM Thrust Departure (lbs. 160 Kts Sea Level) 8,533 8,791 2.9% Table 22 compares the Thrust General coefficients of the INM standard takeoff parameters with the ATM1 coefficients where the E coefficient is set to the static ATM departure thrust level provided by Gulfstream. The ATM1 value for the E coefficient is what was used for the start of takeoff segment in modeling the ATM1 GIV departure. Table 22 Standard vs. ATM1 thrust general coefficients Mode E F 𝐆𝐚 𝐆𝐛 H Max Thrust (STANDARD) 13725.0 -18.20000 0.318900 -0.00002 0.000 ATM1 (Gulfstream) 11445.0 -18.20000 0.318900 -0.00002 0.000 54

Completion of ATM1 for GIV To complete development of ATM1, the appropriate cutback thrust and associated altitude for the change to climb thrust was required. Standard INM GIV procedure profiles have a significant thrust cutback around 400 ft. as the aircraft transitions from a takeoff to climb segment. When the ATM profile is used, the amount of thrust cutback applied during this Standard transition is no longer as appropriate. From an interview with a Gulfstream jet pilot (telephone interview with Charles Saul) as well as information from the VNY Clay Lacy survey, typical ATM departures do appear to have thrust cutback during this transition however they are not as significant a cutback or as soon as that used during the Standard takeoff. To compensate for this longer acceleration period, the ATM thrust was used up to 2000 ft. before beginning an INM climb segment. For aircraft without significant changes between takeoff and climb thrust segments this adjustment should not be needed. The INM Standard and ATM1 profiles are shown in INM format in Appendix H as tables Table 36 and Table 37 respectively as well as plotted above on Figure 28. LEAR35 Aircraft ATM Thrust jet modifications: In addition to applying this approach to the GIV it was also applied to the Lear35 INM type; however detailed ATM thrust levels were not obtainable so an interpolated approach using data within the INM was used in conjunction with information obtained from an operator survey conducted previously. In addition to the Thrust Jet database, which INM uses to calculate the thrust in lbs. for given operational configurations, speeds and temperatures, as mentioned in Footnote 30 there is also a Thrust General database available for reference use in calculating thrust levels at more discrete settings using other thrust metrics. This database contains thrust coefficients similar to those described in Equation 1, however additional K coefficients are now included which allow for thrust to be calculated as a function of aircraft engine operating parameters EPR and N1. In general terms, N1 and EPR are non-dimensional parameters used by the aircraft flight management systems (FMSs) to monitor the thrust and operational state of an engine. • N1 is proportional to the engine’s rotational fan speed and is measured in values of 0%-100% • EPR is proportional to the engine’s internal to external pressure ratio and generally falls in a range between 1 and 2, however this range is unique to each engine model. Equations for thrust as a function of N1 and EPR are given below:. Equation 2 INM Thrust General for EPR 𝐹𝑛 𝛿 = 𝐸 + 𝐹𝑉𝑐 + 𝐺𝑎𝐴 + 𝐺𝑏𝐴2 + 𝐻𝑇 + 𝐾1𝑎(𝐸𝑃𝑅) + 𝐾1𝑏(𝐸𝑃𝑅)2 Equation 3 INM Thrust General for N1 𝐹𝑛 𝛿 = 𝐸 + 𝐹𝑉𝑐 + 𝐺𝑎𝐴 + 𝐺𝑏𝐴2 + 𝐻𝑇 + 𝐾2(𝑁1𝑐) + 𝐾3(𝑁1𝑐)2 𝑁1𝑐 = 𝑁1 √𝜃 ; 𝜃 = ratio of the temperature at the airplate to the sea− level standard temperature For the Lear35, an operator survey from Clay Lacy Aviation at VNY provided a typical engine N1 setting of approximately 95% for Lear 35 departures. The N1 value can then be used with Equation 3 to 55

determine the corresponding thrust level. We accepted this reduced thrust level as equivalent to an ATM thrust and can be used to create an updated ATM Thrust Jet entry where the E coefficient from Equation 1 is equal to the ATM thrust determined from Equation 3. One additional complication to this approach is that the INM does not include Thrust General coefficients for the LEAR35. To approximate this lack of Thrust General coefficients, an average ratio between the maximum thrust level at N1=100% and the desired N1=95% can be calculated using aircraft in a class similar to the Lear35, but for which Thrust General coefficients are available. Based on general size and engine thrust the CNA55B, CNA560E and CNA525C aircraft were selected to define this surrogate average. These aircraft have Thrust General coefficients in the INM. Table 23 lists these three comparable aircraft and the LEAR35, giving maximum takeoff weight and maximum thrust. Using an N1 of 95% in Equation 3 for the three comparables, the overall average yields 86.3% of the maximum N1=100% thrust. Table 23 Lear35 surrogate aircraft N1 comparisons Aircraft MTOW Lbs. Max Takeoff Thrust, lbs. (=100%) N1=95% in Equation 3 gives, lbs Resulting %Thrust LEAR35 18,300 3,412.2 CNA55B 16,950 2,658.7 2302.4 86.6% CNA560E 14,800 3,316.5 2875.4 86.7% CNA525C 16,300 3,464.1 2968.7 85.7% Average % Thrust 86.3% Using the 86.3% thrust average calculated in Table 23, a new ATM user defined Thrust General E coefficient of 2945.8 is then calculated for the Lear35 ATM departure. As with the GIV, the speed coefficients are left unchanged which may introduce a small percent thrust error, but not of significant level to impact the overall description. Table 24 gives the final Thrust General ATM coefficients. Table 24 Lear35 thrust jet coefficients Mode E F 𝐆𝐚 𝐆𝐛 H Max Thrust (STANDARD) 3412.2 -3.88800 -0.00441 0.00000154350 0.000 ATM (interpolated) 2945.8 -3.88800 -0.00441 0.00000154350 0.000 The resulting profiles are as shown plotted on Figure 29, above. 8.2.2 Using INM to Produce Assumed Temperature Method (ATM2) Profiles In addition to the method described in Section 8.2.1 a separate approach to applying ATM profiles provides a less direct, but more approachable process. Analogous to how the aircraft FMS is given an elevated temperature with which to compute departure profile information, the INM flight profile output using an elevated temperature and standard procedure profile can be used to create an ATM fixed point profile to be run at the actual airfield temperature. The temperature adjustment selected by the FMS for actual aircraft operations is dependent on multiple factors including runway length, obstacle clearance and other safety considerations. As it is not possible to account for all of these considerations in this test case, a temperature adjustment was selected allowing for the best fit altitude distance profile compared to actual operations as presented in the radar data. 56

The base INM case for this study used a Temperature of 59.9○F based on an average of the six study airports where the elevated temperatures evaluated will start with this as their base. GIV Aircraft ATM Case Temperature Adjustment The best fit GIV ATM profile as shown in Figure 28 was achieved with an assumed temperature of 107○F. The fixed point profile used is shone below in Table 38, Appendix I. One failing with this approach is that thrust changes are applied along the entire profile and not just during takeoff. From an interview with a Gulfstream pilot (Charles Saul) as well as information from the VNY Clay Lacy survey, typical ATM operations do appear to have thrust cutback in the initial climb segments after takeoff; however they are not as significant a cutback as used during takeoff. The standard GIV procedure profiles have a significant thrust cutback around 400ft as the aircraft transitions from a takeoff to climb segment. When this ATM approach is applied to the thrust settings above 400 feet, the thrust is therefore reduced more than is appropriate. To compensate for this thrust settings above 400ft, they were scaled up by one third to bring them in line with thrust values calculated in Section 8.2.1 from the data provided by Gulfstream. The resulting profile in INM profile points format in shown in Appendix I, Table 38 and plotted in Figure 28. For aircraft without significant changes between Takeoff and climb thrust segments this adjustment should not be needed. Lear35 Aircraft ATM Case Temperature Adjustment The best fit Lear35 ATM profile as shown in Figure 29 occurred for an assumed temperature of 120○F. . The resulting profile in INM profile points format is shown in Appendix I, Table 39. CNA560E Aircraft ATM Case Temperature Adjustment One significant advantage to this method for generating ATM profiles is that there is not an explicit reliance on data external to the INM to generate inputs. As an example of this process, the ATM2 method was used to generate a reduced thrust CNA560E profile. In discussions with Cessna Aviation as well as a Cessna 560 pilot, no separately defined ATM procedure is in place for any Cessna business Jet aircraft. As show in Figure 30 however, applying this ATM methodology and comparing it to radar provides excellent agreement to the flown profiles. The best fit CNA560E ATM profile as shown in Figure 30 occurred for an assumed temperature of 110○F. The resulting profile in INM profile points format is shown in Appendix I, Table 40. Noise Results for Assumed Temperature Method Profiles 8.3 The ATM profile modifications described in Section8.2 were conducted initially without consideration to noise levels. This was done in an effort to establish that the correct physics was occurring with the derated thrust departures in use. Once the ATM profile was created, the measured vs. modeled noise value at a point 12,000 feet from the runway end was evaluated to determine if the use of derated thrust could obtain a good match for both the altitude profile and noise levels. Where agreement between measured and modeled values was not obtained, the profile was further evaluated to be sure no additional modifications were necessary. GIV noise level comparisons and considerations: As described in Section 8.2 the initial ATM profiles generated were modified to better match the takeoff to climb transition the aircraft are observed to be flying. The parameters by which these modifications were made were evaluated against their noise results to come up with the best match. 57

Table 25 describes the final Thrust, Altitude and noise values for the GIV for the Standard and ATM profiles. With the ATM profiles in place there is now agreement within 20ft of altitude and less than a half dB in Noise compared to greater than 600 ft discrepancy in altitude and a 3dB discrepancy in noise. Table 25 GIV noise measured vs. modeled noise levels Profile INM NPD Thrust lbs. Altitude ft. SEL dB Noise Levels Modeled Measured Difference Observed Radar Data 1,088 87.4 STANDARD TAYGIV 9,097 1,758 84.2 87.4 -3.2 ATM1 TAYGIV 8,722 1,078 87.0 87.4 -0.4 ATM2 TAYGIV 8,721 1,060 87.5 87.4 0.1 Lear35 noise level comparisons and considerations: As described in Section 8.2 no additional profile modifications beyond the thrust reductions were needed for the Lear35. Despite this level of agreement with the altitude profile, using the default Lear35 INM Noise Power Distance (NPD) curve resulted in modeled noise levels approximately 8dB above the measured values. The INM defines the Lear35 as using the Garrett/Honeywell TFE731-2 engines which are assigned to the TF731 NPD curves. With the noise discrepancy of about 8dB of what is observed in the measurements, it appears that these NPD curves are incorrect for the current Lear35 fleet and do not accurately represent the observed noise levels. A possible explanation for the discrepancy could be that the original prototype of the Lear35 used the early stage TFE731-2 engine (the INM TFE7312 NPD) which was later updated to the TFE731-2b engine before the initial production run31. There is no additional information within the INM for the TFE731-2b engine; however the difference in modeled minus measured noise level is possible if the active Lear35 fleet uses a variant of the TFE731 engine different from what was used for the original INM certification of NPD levels. To test this assumption, the TF7313 NPD curves (for the TFE731-3 engine, the successor to the TFE731- 2 engine), were used with the Lear35 and ATM profiles. Table 26 describes the final Thrust, Altitude and noise values for the Lear35 for the Standard and ATM profile. With the ATM profiles in place and using the TF7313 NPD curves there is now a good agreement in altitude and less than a one dB difference in noise compared to a greater than 8dB difference for the INM standard inputs. Table 26 Lear35 noise measured vs. modeled noise levels Profile INM NPD Thrust lbs. Altitude ft. SEL dB Noise Levels Modeled Measured Difference Observed Radar Data 1,041 STANDARD TF7312 2,788 1,524 94.7 86.6 8.1 ATM1 TF7313 2,329 1,024 86.8 86.6 0.2 ATM2 TF7313 2,377 976 87.4 86.6 0.8 31 Janes’ All the World’s Aircraft 1980-81 Page 342 58

CNA560E noise level comparisons and considerations: As described in Section 8.2 no additional profile modifications beyond the thrust reductions were needed for the CNA560E. Table 27 describes the final Thrust, Altitude and noise values for the CNA560E for the Standard and ATM2 profile where with the ATM profile in place there is now a high agreement in altitude and less than a one dB difference in noise. Noise levels for the Standard profiles also agree with the measurement levels however the altitude of the Standard is approximately 500ft above what is observed. Introducing the ATM profiles allow for both the altitude and noise levels to agree. Table 27 CNA560E noise measured vs. modeled noise levels Profile INM NPD Thrust lbs. Altitude ft. SEL dB Noise Levels Modeled Measured Difference Observed Radar Data 1,106 STANDARD 2PW535 2,840 1,596 88.8 88.8 0.0 ATM2 2PW535 2,473 1,030 89.2 88.8 0.4 Similarity to Commercial Jet Operation 8.4 Commercial jet departure operations also use reduced or derated thrust. Another ACRP study, 02-55, Enhanced AEDT Modeling of Aircraft Arrival and Departure Profiles, is exploring developing standard profiles that include reduced thrust procedures. Eventually, some standardization across all jet aircraft reduced thrust departures could be thought desirable, though from the findings of this study, the possibility of consistency is judged remote because of how different the methods of implementation are likely to be. Commercial jet aircraft continuously record all engine and flight parameters, providing the types of data pursued in this study. GA jets rarely have on-board equipment to record such data, and hence the techniques for developing appropriate departure profiles are likely to remain quite different for the two classes of aircraft. 59