Buffer encroachment durations and event counts increased significantly during the past week and were the highest reported since the week of March 9. Encroachment durations per event remain elevated.

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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 July 27, 2024.

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​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 improved slightly over the past week, continuing the high safety margins that we typically observe during the summer months. Total encroachment durations for the week ending July 27 increased by 20.1 percent and the number of events rose a whopping 25.4 percent from the previous week. The increase in buffer encroachments was driven entirely by a rise in the number of conformance category PE and C events, as the more severe Category A and B events were stable. The weekly encroachment duration per event dropped slightly to 20.7 seconds, down from 21.6 seconds the previous week. 

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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 July 27. 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 weekly average increased, as did the volatility in hourly metrics, with many more spikes above the 75th and 90th percentiles.

 

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Figure 2. Hourly Encroachment Duration Per Aircraft for the Week Ending July 27, 2024

Figure 3 shows duration per aircraft for the four weeks from June 16 through July 27, 2024. We report the 7-day moving average to better highlight the trend over the four weeks. The chart clearly shows that safety margins over the past month is much better than the historical average but the jump last week is worrisome. The time series reflects a seasonal pattern in which safety margins are highest during the spring and summer months and that general pattern continues this month.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 3. Moving Average Hourly Encroachment Duration Per Aircraft, June 30 to July 27, 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 per aircraft started to rise at the end of the month and jumped last week. The metric remains below the average of the past two years. 

 

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Figure 4. 24-Hour Moving Average Encroachment Duration Per Aircraft for the Three Months Ending July 27, 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 last week is readily detected. The Aviation Safety Monitor team will continue to track these metrics to determine whether they are a temporary aberration or early start to the expected decline in safety margins in the autumn and winter months.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

February 18, 2024 through July 27, 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.

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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 durations since the start of May stabilized the last couple of weeks with little change in either event counts or total durations. Weekly total encroachment durations for Categories A and B remain at the highest levels since we began reporting in the Aviation Safety Monitor. What we had been observing is overall encroachment durations remaining relatively flat, but with a shift from the mild Proximity Event and Category C events to the more severe Category A and B events. However, this trend changed last week with a large jump in Category PE and C encroachment events and durations. This shift highlights the importance of calculating several metrics monitoring all event categories. The Aviation Safety Monitor research team will continue to monitor this negative trend over the coming weeks. We will take a deep dive into the conformance category encroachments in a future weekly report.

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Figure 6. Weekly Encroachment Durations and Event Counts for Conformance Categories A and B February 24, 2024 through July 27, 2024