4.1. Scenario Assessment
Figure 7 presents the results obtained for Scenarios A–C for the vertical holding stack procedure and the linear hold point merge procedure in terms of
(see Equation (
12)). The results are presented in the range between 50
and 80
in 5
intervals. Environmental noise regulations worldwide are defined based in different metrics and critical noise levels, differing from country to country. As a reference, the federal German government established noise protection zones around civil airports with an air traffic exceeding 25,000 movements per year for residential areas encompassed by
and
noise contours [
47]. The
is computed for the daytime (06 AM–10 PM) and the six months of the forecast year with the largest number of flights. In the UK, the aviation noise policy defines the lowest observed adverse effect level (LOAEL) of
as measured for the average summer day from 07 AM to 23 PM [
48]. Although the aforementioned noise directives are not based on the
, their use as reference values is valid if we assume that the scenarios assessed in this work occur for a 1-hour period of the daytime, as they set critical noise levels based on a similar noise metric, which only differs in terms of the time period on which the A-weighted sound pressure level is averaged.
By inspecting the top row of
Figure 7, one can verify similar shapes of the ground noise isocontour area for all scenarios. From Scenario A (
Figure 7a) to Scenario C (
Figure 7c), the share of heavy aircraft (here: B77W) is increased. Accordingly, the amount of areas with large equivalent sound pressure levels is increased. Moreover, it can be found that the largest equivalent sound pressure levels occur below the flight trajectories shortly before the last turn that aligns the aircraft with the runway center line and in close proximity of the runway. However, when the share of heavy aircraft is increased, an increased sound pressure level also below the holding stacks can be seen. On the other side,
Figure 7d–f show the ground noise for the linear hold point merge procedure. By comparing the contours with the ones of the vertical holding stack procedure, it is possible to observe that the use of more flight tracks distributes the noise over a broader area instead of concentrating it in particular locations, such as beneath the holding stacks. Thus, the linear hold point merge procedure promotes noise reductions by distributing the arrival sequences in space. Therefore, the linear hold point merge concept would be an interesting choice for an airport which has an homogeneously distributed population around it and, thus, concentrating the noise in specific locations with no residential areas is not possible. Even for the noisiest case, i.e., Scenario C, equivalent sound pressure levels larger than 60
only occur within ≈ 30 km from the runway threshold. In general, the results seem plausible from the viewpoint that increasing the share of heavy aircraft affects the ground noise in the direction of higher levels. Moreover, as the flight time for the holding stack procedure is larger, the expected higher sound pressure levels on the ground can be verified as well.
The differences in terms of noise can be verified as well by inspecting the geometric area of the noise contours for each investigated equivalent sound pressure level category.
Figure 8 shows the percentage difference of the isocontour areas for the linear hold point merge procedure,
, and the vertical holding stack procedure,
, relative to the values of the vertical holding stack for all investigated scenarios. Thus, negative values are read as reductions of the isocontour area due to the use of the linear hold point merge procedure. It should be noted here that all calculated contour areas incorporate the areas of the isocontours with higher noise levels as well. It can be seen that for all cases, except from Scenario C and the 80
isocontour area, the employment of the linear hold point merge procedure results in lower noise contour areas. Thereby, for the 50
isocontour area, the area reductions are in the range of ≈30–40%, for the 55
, 60
, 65
and 70
isocontour area, and the affected area is reduced by ≈80–90%. The isocontour areas for 75
and 80
are nearly unaffected by the choice of the holding procedure. This is due to the fact that these high sound pressure levels only occur in the close proximity of the runway, where the investigated holding procedures do not differ, thus no variation of the ground noise is expected.
For the second investigated metric, i.e., the
(see Equation (13)), the isocontour areas are shown in
Figure 9. Again, the top row shows the results for the vertical holding stack procedure and the results of the linear point merge procedure are shown in the bottom row. As expected for the investigated metric, the reported values are higher than for the
, shown in
Figure 7. Apart from the higher values, a general increase in the reported noise levels can be verified from left to right, as the share of heavy aircraft is zero for Scenario A (
Figure 9a,d), average for Scenario B (
Figure 9b,e) and largest for Scenario C (
Figure 9c,f). Nevertheless, the differences between Scenario B and Scenario C seem to be minimal for both cases. This is possibly due to the fact the that the number of operations in Scenario C is slightly smaller than in Scenario B, thus compensating for the increase in heavy aircraft. Therefore, the
does not seem to be highly influenced by the increase in heavy aircraft. This can also be verified in
Figure 10, where no relative differences between the isocontour areas of the Scenario B and Scenario C are observed. Moreover, the effect of the different flight altitudes in the linear hold point merge procedure can be seen when inspecting
Figure 9e,f. Here, the
are higher for the lower level sequencing leg, as this leg is flown at a 1000
lower altitude than aircraft in the higher level sequencing leg (see
Figure A1 in the
Appendix A).
A rather different picture for the
, compared to the
, can be observed for the relative isocontour area reductions shown in
Figure 10. The results show that, contradictory to the results for the
, the
isocontur area of the linear hold point merge procedure are, in general, larger than the ones of the vertical holding stack for the noise levels below and including 60
and for levels equal to and above 80
. For the levels between 65
and 75
, the isocontour areas of the linear hold point merge are reduced in comparison to the vertical holding stack. However, this generally appears to be plausible as well. The linear hold point merge procedure results in a reduced affected area with respect to the equivalent sound pressure level
, and thus, the noise footprints are more condensed and, accordingly, the maximum noise levels are increased. Therefore, the noise characteristics of the two holding procedures may differ depending on the noise metric and noise levels. This has to be carefully addressed when investigating the implementation of one or the other holding procedure in practice.
4.2. Influence of the N1 Values
As discussed in
Section 2.3, one characteristic of the simulation framework used by the software sonAIR is that the engine noise emissions are modeled using the N1 as the main parameter describing the engine settings. The use of N1 to model the engine noise instead of thrust, as commonly used by BPM [
49,
50], is justified by the fact that the N1 can describe the noise generated by the fan and the jet flow, the two dominant noise sources of a turbofan engine, as a single parameter [
16,
37].
In the absence of flight data recorder (FDR) data, the N1 can be estimated acoustically [
51,
52], using a flight-phase-specific modeling approach [
53,
54] or based on jet engine simulations [
19,
55]. Nevertheless, often, only radar data or automatic dependent surveillance–broadcast (ADS-B) data, which do not provide any information regarding the engine settings, are available. In this case, mean N1 values depending on the flight phase are often used [
56,
57]. This simplification implies relying on a single N1 value to model the engine noise emissions for the entire flight trajectory during a specific flight phase. To the best of the authors’ knowledge, a dedicated study on the effect of such simplification on the aircraft noise predictions provided by sonAIR for large air traffic scenarios is not, to this date, available in the literature.
In this section, we aim at verifying the differences brought by the use of mean N1 values along predetermined flight phases and, consequently, the plausibility of such simplification. As no real N1 data are available, it is assumed that the predictions provided by the multi-level simulation framework introduced in this work (see
Section 2) provide realistic N1 values which are, therefore, adopted as the reference N1 values. The investigation is conducted based on the scenario definitions described in
Section 3 and considering the Scenario C (see
Section 3.4), which is the most diverse scenario defined in this work in terms of aircraft fleet mix.
Figure 11 presents the calculated N1 profiles (mean N1 profiles of all flight trajectories considered in each holding procedure case) for each aircraft type and holding procedure, and their corresponding mean N1 values,
, depending on the flight phase (landing, approach and holding). Please note that the N1 values are presented in
Figure 11 as a function of only one dimension (i.e., the altitude above ground level) and, therefore, the mean N1 values depending on the flight phase do not directly correspond to the mean value of the N1 profiles along the altitude but rather to the mean N1 value computed considering the entire three-dimensional flight trajectories (see
Figure 3b and
Figure 4b). Moreover, it should be noted that the dashed lines in
Figure 11b for the holding flight phase of the linear hold point merge are only for illustration purposes. Since the entire holding flight phase is flown on the same altitude of 7000
and 8000
for the lower and upper sequencing legs, respectively, no altitude variation and thus no variation in N1 can be shown. The mean N1 values per aircraft type, holding procedure and flight phase are provided in
Table 5.
As no real N1 values are available to validate the N1 estimation methodology used in this work, a plausibility check is conducted by means of comparison with the mean N1 values provided by Zellmann [
37]. The provided mean N1 values are calculated for the final approach flight phase, when the aircraft is aligned with the runway and stabilized for landing. According to Schwab and Zellmann [
53] and Blinstrub [
58], the stabilization usually occurs at an altitude of 1000
AGL (≈305
AGL), when the landing gears and high-lift devices are fully deployed and the airspeed is kept constant by increasing the thrust accordingly. Therefore, mean N1 values and their corresponding standard deviation are computed considering all the flight trajectories calculated within the context of this contribution for an AGL of 15
. For an Airbus A320 equipped with V2500 turbofan engines, the mean and standard deviation N1 values provided by Zellmann [
37] for the final approach phase are ≈(48 ± 5)%. The mean and standard deviation N1 values of the flight trajectories calculated for the vertical holding stack are ≈(54 ± 1)%, while for the linear hold point merge they are ≈(56 ± 8)%. This corresponds to an absolute difference of ≈8% if only the mean values are considered, which is considered to be an acceptable agreement. Since the N1 values provided in the literature were obtained from a dataset of real-life air traffic data, differences due to factors such as weather conditions and/or operational requirements are expected. Unfortunately, to the best of the authors’ knowledge, no mean N1 values for the Boeing B77W are available in the literature for comparison.
Figure 12 shows the simulated noise contours as well as footprint differences in terms of
and
, which are calculated as
and
where
and
are computed using the mean N1 values as a function of the flight phase and
and
using the reference N1 profiles calculated along the entire flight trajectories. Thus positive delta values indicate that the results obtained using the mean N1 values are higher than the results obtained using the calculated N1 profiles and vice versa. In
Figure 12, the absolute noise contours are presented in the range between 50
and 80
in 10
intervals for all noise metrics, and differences between noise levels below 50
are omitted.
In general, it is possible to observe in
Figure 12 that the relevant differences between the simulations using the different N1 values are mostly on areas close to the runway up to
km, when the aircraft are in the landing flight phase and at an AGL
m. Therefore, for high altitudes, the influence of the N1 seems to be not prominent, and the use of a mean N1 value would suffice to obtain reliable predictions. For
, the simulations conducted using the reference N1 profiles provide higher noise levels than the ones using the mean N1 values. This corresponds to a flight AGL
, for which, according to
Figure 11, the calculated N1 values are relatively higher than the mean N1 values. For this range, the maximum
and
differences observed were ≈−1.7 dB and ≈−2.3 dB, respectively. For
km, this is reversed, as the mean N1 values are higher than the values calculated for the reference N1 profiles, which account for the reduction in the engine power at a close distance from the runway. For this range, the maximum
and
differences observed were ≈
dB and ≈
dB, respectively. As, during the final approach, the increase in the N1 is a requirement to stabilize the aircraft at constant airspeeds, such differences are expected, as the use of mean N1 values instead of considering a full N1 profile along the flight trajectory will introduce relevant N1 differences.
Additionally, the noise contours presented in
Figure 12 for the simulation with constant N1 values are mostly superimposed by the noise contours obtained using the reference N1 profiles. In fact, a quantitative analysis showed that relevant isocontour area differences occurs only for
dBA isocontours. In this case, the predictions obtained using the mean N1 approach provides isocontour areas, which are 24% and 33% larger than the ones obtained using the full N1 profile for the VHS and the LHPM procedures, respectively. This supports the use of mean N1 values as a function of the flight phase for the simulation of noise contours using sonAIR when the full N1 profiles are not available.