Buffer encroachment durations and event counts decreased slightly last week. There was a small drop in the number of lower severity buffer encroachment events after the big jump last week. Of greater concern was the big increase in the more severe Category A and B encroachments that rose over 29 percent during the week. Those events have increased since early June. Overall, daily encroachment durations of all categories were 11 percent higher in July than in June, and the daily average for the first 17 days in August are 14 percent higher than the July average. The degraded trend in safety margins is becoming more pronounced with each week. 

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Welcome back to the Weekly Aviation Safety Report. Every Monday Robust Analytics reports on safety margins at 26 United States airports. With this Aviation Safety Monitor Weekly Report, Robust Analytics offers the aviation community timely assessments of changing safety margins and safety-related events. Dates and times are tracked in UTC and the week ends at midnight every Saturday. This week’s report includes data through 2400 UTC on August 17, 2024.

 

For New Readers: Please read our article “Did Safety Degrade in the National Airspace System in the Winter of 2022-2023?” that applies our methods and data  to examine whether safety margins decreased during the events of winter 2022-2023.  

The Aviation Safety Monitor measures safety margins by estimating the frequency, duration, and severity of buffer encroachments. Our paper “How Do We Measure Safety Margins?” provides a detailed description of the methods and data. That article can be found here https://www.robust-analytics.com/measure  on the Robust Analytics website. 

Weekly Safety Margin Update. Safety margins degraded over the past week. Figure 1 summarizes the weekly data since the Aviation Safety Monitor began reporting on February 24, 2024. Total encroachments durations for the week decreased 7.4 percent as generally good weather decreased the number of Conformance Category PE and C buffer encroachments significantly. That positive outcome was almost offset by the sharp increase in the more severe Category A and B events that rose 29.4 percent. The weekly encroachment duration per event remained steady. 

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Figure 1. Weekly Trends in Encroachment Events and Durations

Let’s now take a closer look at safety margins for the seven days ending August 17. Figure 2 displays estimates of encroachment durations per aircraft for each hour over the previous seven days. This offers a detailed look at how safety margins vary over the operating day. Figure 2 also indicates the historical range of the data by showing the 25th, 75th, and 90th percentile values of the duration per aircraft metric. The percentiles were estimated using data from May 2022 through February 2024.

 

Figure 2 displays the typical time of day pattern that we observe in the data, as encroachment events are highly correlated with traffic density. Even after aggregating the data over 17 metroplexes operating in three time zones the time of day pattern remains prominent. The week ending August 17 had numerous high encroachment duration spikes over the 90th percentile and some of the highest-ever hourly reports on August 13 and 14 (we are checking those data to make sure they were not caused by a data defect). The hourly reports are clearly more erratic than the previous couple of weeks with a continuing upward trend in buffer encroachments.

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Figure 2. Hourly Encroachment Duration Per Aircraft for the Week Ending August 17, 2024

This upward trend is readily observed in Figure 3, which reports the 7-day moving average duration per aircraft for the four weeks from July 7 through August 17, 2024. The chart clearly shows that safety margins over the past month are much better than the historical average as indicated by the percentile scores. However, the upward trend since June is becoming more apparent as we add more data every week. The time series reflects a seasonal pattern in which safety margins are highest during the spring and summer months and slowly degrade starting in late summer, and we are seeing that that general pattern develop with each weekly update.

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Figure 3. Moving Average Hourly Encroachment Duration Per Aircraft, July 21 to August 17, 2024

Figure 4 displays the 7-day moving average encroachment duration per aircraft for the past three months. After a quiet month for most of June, encroachment durations increased in July as shown in Figure 4. Overall, daily encroachment durations were 11 percent higher in July than June, and the daily average for the first 17 days in August are 14 percent higher than the July average. 

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Figure 4. 24-Hour Moving Average Encroachment Duration Per Aircraft for the Three Months Ending August 17, 2024 

We wrap-up this week’s report with updates to charts presented in the previous weekly reports. Figure 5 shows the daily encroachment event rates and durations per aircraft across the 26 airports in the 17 metropolitan areas that we monitor. The blue bars report estimates of total daily encroachment durations divided by the number of aircraft in the terminal airspace (approximately 50 miles of the airport center) for all 17 terminal airspaces. The red line reports the daily number of encroachment events per 100 aircraft. The big jump in both metrics the weeks ending July 27 and August 17 suggest that the improvement detected the week ending August 3 was only a temporary reprieve from the negative trend. 

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Figure 5. Encroachment Duration Per Aircraft and Event Rates for 26 Airports 

February 18, 2024 through August 17, 2024

As data are added to the chart each week, readers can begin to detect some mild day-of-week variation and, more importantly, periods of a few days and even a week or longer in which encroachment durations increase significantly. In the coming weeks, we will examine these fluctuations in more detail.

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How severe are these encroachments? The FAA defines three separation conformance categories based on how far they are from the separation index. (See the description “How Do We Measure Safety Margins?” for details on the conformance categories and how we measure them.) In that classification system, Conformance Categories A and B are the most severe. Under our definition of a buffer encroachment, Category A and B encroachments are counted under all meteorological conditions.

 

Figure 6 displays information on the most severe separation conformance categories. The weekly durations and event counts for the sum of Category A and B encroachments are shown in Figure 6. The upward drift in these more severe buffer encroachments since the start of May continued last week with the largest one-week jump since we began reporting in February 2024. Both the number of buffer encroachments and their durations for Category A and B are at the highest levels since we began reporting in the Aviation Safety Monitor. If readers are interested, we can conduct a deep dive into the conformance category encroachments in a future weekly report. Let us know if that information will be useful.​​