Buffer encroachment durations and event counts were little changed last week. The number of lower severity buffer encroachment events increased 5.6 percent and the August daily average is now 13.5 percent above the July mean. The continued high level of the more severe Category A and B encroachments remains concerning, with the August daily encroachment average 11.5 percent higher than July and 27.7 percent higher than in June. The degraded trend in safety margins is becoming more pronounced with each week.

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 24, 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 1.9 percent but the number of encroachment events increased 5.6 percent. Generally good weather kept the number of Conformance Category PE and C buffer encroachments low throughout the week. The weekly encroachment duration per event dropped by one and a half seconds.

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 24 was much quieter than the previous week, with only five duration spikes over the 90th percentile. However, there were eighteen hourly reports between the 75th and 90th percentile, supporting the interpretation of a continuing upward trend in buffer encroachments.

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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 a small increase in the total encroachment durations. Average daily encroachment durations were 11 percent higher in July than June, and the daily average for the first 24 days in August are 1.5 percent higher than the July daily average.

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.