*5.2. Traffic Analysis: Historical Traffic under Adverse Weather Conditions at Milano Malpensa and Bergamo Airports*

In order to evaluate the influence of adverse weather conditions on air traffic in the extended TMA around MXP and BGY, located at ca. 77 km distance from each other, FlightRadar24 datasets from three different selected days with severe events—11 May 2019, 7 July 2019, and 6 August 2019—were analyzed.

In the case of MXP, there were two available days—7 July 2019 and 6 August 2019 without airport closure under disturbed weather conditions and one day—11 May 2019 when adverse weather conditions caused temporal closure of the airport. For BGY, there were two days—11 May 2019 and 7 July 2019—in the dataset where adverse weather did not lead to an airport closure and one day—6 August 2019—when the airport was temporarily closed. As the airports are located adjacent to each other, it is of interest to investigate the datasets in pairs in order to analyze the influence of the changing capacity of one airport on the second one. Therefore, the following pairs of data were considered:


The first two datasets recorded on 7 July 2019 contain 322 arrivals at MXP and 144 arrivals at BGY. The travel time of arrivals in the extended TMA and touchdown time are illustrated in Figure 9 in orange for MXP and in blue for BGY. In order to detect peak times while smoothing the influence of outliers, a moving mean value over five neighbor travel times was calculated. These mean values are shown as dark blue and dark orange lines in Figure 9. In the case of MXP, the time until landing in the extended TMA varied between 693 and 2913 s with an average time of 1246 s and five peak periods, the most noticeable of which was between 19:30 UTC and 21:30 UTC (Figure 9). The maximal and the average flown distances in the extended TMA were approximately 302 and 168 km, respectively, whereas the average flown distance at the mentioned peak period was equal to 225 km (i.e., 57 km longer than the average flown distance over the whole day).

**Figure 9.** Visualization of travel time in the extended TMA and touchdown time for historical traffic at MXP (orange color) and at BGY (blue color) on 7 July 2019.

In the same day, there were two outlier arrivals at BGY in terms of travel time in the extended TMA. Taking them into account, the time until landing varied between 817 and 4836 s, with an average travel time of 1186 s. The detectable peak period is between 16:15 UTC and 17:30 UTC (Figure 9). The maximal and the average flown distances in the extended TMA are approximately 718 and 160 km, respectively. The average flown distance at the peak time amounts to 247 km with the outlier arrival and 194 km without it. Even without the outlier, the average flown distance during the peak period is 34 km longer than the corresponding value over the whole day.

Summing up, there are two possible coherent peaks at the considered airports that may have been caused by weather conditions. These peaks are pairwise-marked by the areas bounded by the dotted lines in Figure 9.

The second pair of datasets collected on 6 August 2019 contains 421 arrivals. It includes 99 more arrivals than the traffic scenario on 7 July 2019 for MXP and 144 arrivals for BGY. The travel time in the extended TMA and the touchdown time of arrivals are shown in Figure 10. The time until landing in MXP's extended TMA was between 715 and 2436 s, with an average travel time of 1161 s. The maximal and the average flown distances in the extended TMA were approximately 352 and 161 km, respectively. In spite of the fact that this scenario is larger than the previous one for MXP, it has lower values for maximal and average travel time and, as a consequence, average travel distance.

**Figure 10.** Travel time in the extended TMA vs. touchdown time for historical traffic at MXP (orange color) and at BGY (blue color) on 6 August 2019.

In the same day, the time until landing in the extended TMA around BGY was between 703 and 5679 s, with an average of 1332 s. There are two peaks with respect to travel time in the extended TMA between 19:30 UTC and 23:15 UTC, which are marked by the areas bounded by the dotted lines in Figure 10. The maximal and the average flown distances in the extended TMA were approximately 739 and 185 km, respectively.

The average flown distance during peak time amounts to 310 km and is, through temporal closures of runways, at least 63 km greater than the corresponding average flown distance during the peak time on 7 July 2019, when BGY was not closed. Figure 11 illustrates the accumulated rainfall per ten minutes on 6 August 2019 at BGY, which correlates with the peaks for travel time shown in Figure 10. However, there are no significant peaks between 19:30 UTC and 23:15 UTC at the nearby MXP airport, as illustrated in orange in Figure 10.

**Figure 11.** Histogram of the rainfall (blue bars) and accumulated rain (red line) at the Bergamo airport on 6 August 2019 [46].

The last pair of datasets was recorded on 11 May 2019 and contains 233 arrivals to MXP and 122 arrivals to BGY. On this day, a squall line hit MXP between 14:00 UTC and 16:00 UTC causing intense precipitation and heavy hail formation [46]. These adverse weather conditions (Figure 12) led to the airport's closure for about 40 min and some flight delays. In addition, nine aircraft were diverted to other airports. Figure 13 illustrates travel time at the extended MXP TMA, which was between 719 and 4693 s with an average of 1196 s. This distribution indicates that controllers had to guide many aircraft into holdings. According to Figure 13, the airport was closed at around 16:00 UTC. The maximal and average flown distances in the extended TMA were approximately 676 and 165 km, respectively. The average flown distance during the peak time was equal to 349 km.

**Figure 12.** Apron of a runway at MXP on 11 May 2019 [53].

**Figure 13.** Visualization of travel time in the extended TMA and touchdown time for historical traffic at MXP (orange color) and at BGY (blue color) on 11 May 2019.

Historical traffic at BGY on 11 May 2019 indicated a time until landing in the extended TMA of between 782 and 2985 s, with an average travel time of 1197 s (Figure 13). There was a peak period with respect to travel time between 17:30 UTC and 19:30 UTC, probably as a consequence of the weather conditions at MXP airport described above. The maximal and average flown distances at the BGY extended TMA were approximately 415 and 109 km, respectively, and the average flown distance during the peak period is equal to 215 km. Although the weather conditions at BGY did not cause a runway closure, one can notice the direct influence of the weather conditions at MXP on operations at BGY. Therefore, the historical dataset for MXP on 11 May 2019 was chosen to construct simulation traffic scenarios for investigation of the influence of adverse weather on arrival traffic. Table 1 summarizes the mentioned parameters for the three considered case studies.


**Table 1.** Data summary of available historical traffic.

### *5.3. Traffic Scenario Definition*

The simulations involved three traffic scenarios for the TMA at MXP, which were based on FlightRadar24 datasets of the three days (11 May 2019, 7 July 2019, and 6 August 2019). There were two days when weather conditions did not cause airport closure (7 July 2019 and 6 August 2019) at MXP, which were used as baseline scenarios, and one day with an airport closure for some period of time (11 May 2019), which was used as the disturbed scenario.
