3.1 Summary of statistics
We randomly selected the trajectory data of 120 different flights from January 2020. When we selected the flight, we did not use flight data that had more than five non-flight days, except on January 12 and 13, because we needed to focus on flights that operated under a stable flight plan. Table 2 shows the summary of the data.
Table 2. Summary of data. FSC means full-service carriers; LCC means low-cost carriers.
Total
|
120
|
FSC
|
85
|
LCC
|
35
|
To/from Manila
|
49
|
To/from the Philippines
|
70
|
Intra-Asia
|
96
|
No. of cancellations (January 12, 13, and both 12 and 13)
|
32, 10, and 4
|
Regarding the airline category, 30% of the samples are of low-cost carriers (LCCs). Considering the share of LCCs in Asia-Pacific markets, the rate is almost like the market share, 29% (CAPA, 2019). Most of the samples are of intra-Asia flights, and 51% of them are from/to Manila flights. Thus, many flights of the samples are supposed to be affected by Taal volcano’s eruption. The number of cancellations on January 12, 13, and both days are 32, 10, and 4 flights, respectively. This means that approximately 27% of the samples were cancelled or there was no flight on the day of the eruption.
3.2 Categorizing the flights
Applying the HDBSCAN algorithm can categorize trajectories of each flight. We mainly use a density of five; therefore, we can categorize two or three types of flights. However, some flights have more variable patterns. Consequently, we sometimes use a density of four or three; The choice of the density depends on the case.
Table 3. Example of results (HDBSCAN).
|
1st
|
2nd
|
3rd
|
4th
|
5th
|
6th
|
7th
|
8th
|
9th
|
10th
|
11th
|
12th
|
13th
|
LJ63
|
2
|
0
|
2
|
0
|
2
|
1
|
1
|
1
|
1
|
2
|
X
|
0
|
0
|
JL746
|
2
|
1
|
1
|
1
|
1
|
1
|
1
|
2
|
1
|
1
|
1
|
1
|
1
|
5J459
|
3
|
3
|
3
|
3
|
3
|
3
|
3
|
2
|
3
|
3
|
3
|
1
|
1
|
Note: Each flight has different routes and the number (1, 2, 3…) means the categorized group based on the similarity measured by HDBSCAN. “0” means non-categorized route (noise) and X means the cancellation.
Table 3 shows the results of the HDBSCAN analysis on flight LJ63 (Jin Air), JL746, and 5J459 (Cebu Pacific). X means there was a cancellation/no flight. “0” appears in some columns; this means the trajectory cannot be categorized, and it is treated as “noise.” JL746 has two clusters, that is “1” and “2”, and it has no anomalistic trajectory; 5J459 has three categories; LJ63 has two categories, 1 and 2, but some uncategorized trajectories shown as “0” appear. From the table, LJ63’s trajectories in January 2020 were much various and unstable than other flights.
First, let us see the change in categories from January 11 to 15. Airlines are supposed to have cancellations/no-flight or changing route to avoid accidental events due to ash-contaminated air. There can be several combinations of decisions for flight cancellation, change of route, or flight-as-usual. Table 4 shows the results of the classification of routes.
I: Flight-as-usual from January 11 to 15.
II: Cancel/no-flight or irregular route on Jan 12, and on Jan 13, back to the regular.
III: Cancel/no-flight or irregular route on Jan 13, and on Jan 14, back to the regular.
IV: Cancel/no-flight or irregular route on Jan 13 and keep it by Jan 15.
V: Cancel/no-flight or irregular route on Jan 12 and 13; on Jan 14 back to regular.
VI: Cancel/no-flight or irregular route on Jan 12 and 13; keep it by Jan 15.
VII: Changing the route on 12 and 13, but both are categorized as regular routes.
Table 4. Classification of routes (January. 12-15, 2020).
Category
|
No
|
Category
|
No
|
Category
|
No
|
Category
|
No
|
I
|
21
|
II
|
29
|
III
|
22
|
IV
|
4
|
V
|
20
|
VI
|
10
|
VII
|
14
|
|
|
As Categories I and VII represent flights with “no change” on their flight routes during these days, 70% of samples had some route changes on January 12 and 13. Categories II and IV, which represent flights that took a special operation for one day, occupy 42.5%. From the results, we can say; they consider that a one-day special (anomalistic) operation is enough for avoiding accident risks owing to the ash-contaminated air. Category IV, V, and VI represent airlines that keep their special operation for a couple of days.
Fig. 4 shows the comparison of trajectories of Z29047 (Philippines Air Asia) on January 11, 12, 14, and 15.
The regular route of Z29047 ran through the area where the ash-contaminated air remained. Thus, they changed the route drastically, far from the area of the ash-contaminated air. The next day, they canceled the flight. On January 14, they kept their irregular route to avoid the accident risks due to the ash-contaminated air; however, that route was a bit closer to the regular route (January 11 and 15 routes) because the major part of ash cloud moved out from the are relevant to their regular route. From this behavior, Z29047 seemed to take “more risk-averse” behavior. As Z29047 is an intra-Asia flight, considering the behavior of 96 intra-Asia flights would be meaningful to understand whether the behavior of Z29047 is unique. Sixty-seven flights from 96 samples are classified as “irregular” routes on January 12 and/or 13. Thirty-eight out of 96 flights that had irregular routes took a special operation for one day; its rate was 39.6%. Consequently, the behavior of Z29047 can be a more risk-averse action as an intra-Asia flight.
Another aspect is observed between intra-Asia flights and Manila-based flights. Sixty-six out of the 96 intra-Asia flights are Manila-based flights. Fifty-three flights out of the 66 Manila-based flights are classified as having “irregular” routes on January 12 and/or 13. Thirty flights are classified as Category II or III flights, that is, special operation for one day. The rate of special operation for one day reached 56.6%. In this context, there is no difference in the rate of special operation for one day between intra-Asia flights and Manila-based intra-Asia flights.
Regarding the long-haul flights[3], we have 23 flights in the sample. Seventeen flights were classified as having “irregular” routes on January 12 and/or 13. In 17 flights, 13 flights took special operation for one day, and their rate reached 76.5%; thus, five flights were supposed to take more risk-averse behavior. Fig. 5 shows the comparison of trajectories of KE112 (Korean Air) from January 11 to 14, 2020. On the 13th, KE112 took an irregular route, but the following day it got back to its regular route because the major part of ash cloud moved out (or was forecasted to move out) from the are relevant to their regular route.
Comparing the rate of intra-Asia flights to that of long-haul flights, we can say that intra-Asia flights can more risk-averse behavior than long-haul flights. However, in each case, the trajectory of irregular flights is close to the edge of the ash cloud (or the expected movement area of the ash cloud); this suggests that the flight path of irregular routes could heavily depend on the information provided by the VAAC.
Another discussion point is about the departure time. Fig. 6 shows the comparison of departure times (UTC). Looking at the figure of Category II (left side chart), no flight is found from 0:00-6:00. Conversely, 0:00-12:00 flights occupy the majority in the pattern of Category III (right side chart). This means that “special operation for one day” can be taken for the flights of which departure times were between 9:00 on January 12 to 12:00 on January 13. An interesting point is that there is no observation at the time range 12:00-15:00 on January 13. Fig. 7 suggests its meaning.
In Fig. 7, half of the observations, that is ten flights, are categorized as the flights with a departure time of 12:00-15:00. More importantly, six of the ten flights are bound for the Philippines. Therefore, one hypothesis can be proposed: based on the VAAC information, flights that were destined for the Philippines and had a departure time of 12:00-15:00 could be affected by the ash-contaminated air if one day had passed since the eruption; therefore, they kept their special operation. Because the last eruption was on the 12th of January (UTC), the decision made after 24 h could be critical; in other words, 24 h later, the airlines (or flight operators) should have a decision of “keeping the special operation” or “back to the regular” in the case of the 2020 Taal volcano.
From above discussion, we have the following possible actions about flight cancellation or rerouting in the case of the 2020 Taal volcano.
- The airlines choose flight cancellation or at least rerouting for the flights that fly through or near the heavily ash-contaminated area within 24 h.
- After 24 h, airlines can have two options for flights that depart within a couple of hours; “keeping the special operation” or “back to the regular”, in the case of the 2020 Taal volcano.
- After 27 h, airlines can turn the schedule back to normal and reduce the number of cancellations and irregular routing drastically.
Action 1 would be rational because airlines should focus on the movement of the volcano after the large eruption to avoid further risks from imminent eruptions. After 24 h (Action 2), their decision would be critical. If the airline is (more) risk-averse, they can keep their special operation (cancellation or rerouting); otherwise, they try to get back to “regular.” This depends on the characteristics of the airline and/or route. In subsection 3.2, we suggest that intra-Asia flights would be more risk-averse than flights that take the long-haul routes. In other words, long-haul flights can be regarded as “more optimistic.” The exact reason is unclear so far, but one possible reason may be the travel time. As long-haul Pacific flights have a long travel time (10-12 h), the airlines may expect the “better situation” several hours later judging from the information from VAAC. However, this hypothesis must be proved with other information. This is our next research target.
[3] We define the flight as long-haul when the flight requires more than eight hours.