Safety margins improved last week although some missing data make the results ambiguous. During the month of September, the daily encroachment duration average was 8.9 percent higher than the August average. Daily encroachment durations were 10.5 percent higher in August than in July, and the July average was 11.5 percent higher than the June daily average. Average daily durations have now increased for five consecutive months, with the September daily average 36 percent higher than in April. 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. We are seeing that recurring pattern continue as we move into the autumn months.

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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 October 5, 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 improved last week after a short-lived reprieve the previous week. Total encroachments durations for the week ending October 5 decreased 20.8 percent and the number of encroachment events dropped by 22.2 percent. The numbers this week suffer from a server failure on September 29-30 and we will strive to fill in the gap before next week’s report. The weekly encroachment duration per event increased 0.4 seconds to 22.74, the highest average duration per event since the week of March 2. Figure 1 summarizes the weekly data since the Aviation Safety Monitor began reporting on February 24, 2024.

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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 October 5. 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.

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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 past seven days worsened as the week progressed, particularly during the last three days. There were fewer values over the 90th percentile than the previous week, although missing data for September 30 make those counts unreliable.  

 

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Figure 2. Hourly Encroachment Duration Per Aircraft for the Week Ending October 5, 2024

Figure 3 reports the 7-day moving average duration per aircraft for the four weeks from August 25 through October 5, 2024. The chart shows that safety margins over the past month were generally better than the historical average as indicated by the percentile scores, until the month of September arrived. 

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Figure 3. Moving Average Hourly Encroachment Duration Per Aircraft, September 8 through October 5, 2024

Figure 4 displays the 7-day moving average encroachment duration per aircraft for the past three months. The chart shows a steady deterioration since the end of June. Average daily encroachment durations in September were 8.9 percent higher than the August average. Daily encroachment durations were 10.5 percent higher in August than in July, and the July average was 11.5 percent higher than the June daily average. Average daily durations have now increased for five consecutive months, with the September daily average 36 percent higher than in April.  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. We are seeing that recurring pattern continue as we move into the autumn months. 

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Figure 4. 24-Hour Moving Average Encroachment Duration Per Aircraft for the Three Months Ending October 5, 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. We detect big jumps in both metrics the weeks ending July 27, August 11, and September 14, and October 5, with each peak higher than the previous one.  

 

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Figure 5. Encroachment Duration Per Aircraft and Event Rates for February 18, 2024 through October 5, 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. After falling for the previous two weeks, the encroachment duration metric resumed its seasonal increase last week. Average daily encroachment durations for September were 1.6 percent below the August average, which was 10 percent higher than in July. 

 

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