After four weeks of improving safety margins, the trend reversed last week. Total buffer encroachment durations in all separation conformance categories increased 19.2 percent, driven by a jump of 24.4 percent in category A and B encroachment durations. The number of buffer encroachment events increased for the third consecutive week, while the encroachment duration per event remained steady. The 7-day moving average of hourly encroachment durations has been rising for the past 11 days, increasing 33 percent during that short time period. However, the hourly encroachment duration moving average remains well below the levels of May and June. The next few weeks will tell us whether the recent slight deterioration in safety margins is temporary or the beginning of a trend reversal as we approach the end of summer.

Weekly Safety Margin Update. 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 26, 2025. 

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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.  https://www.linkedin.com/pulse/did-safety-degrade-national-airspace-system-winter-kpq8e/?trackingId=dLIUDpIMzA34g3H4GDwrHw%3D%3D

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.

The Weekly Safety Report uses buffer encroachment events and durations to measure changes in safety margins. Safety margins made significant improvements for four weeks until slipping the past couple of weeks.  Total encroachment durations increased 19.2 percent and the number of encroachment events rose 19.6 percent. Duration per event remained flat at 18.6 seconds, about the level of one year ago.

The Aviation Safety Monitor calculates buffer encroachments in four separation conformance categories based on the severity of separation deviation. Category PE and C buffer encroachments identify relatively weak separation conformance deviations and are strongly affected by meteorological conditions. In fact, by definition, during visual meteorological conditions there are no buffer encroachments. The more severe encroachments in Category A and B provide a better indicator of changing safety margins. 

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The seasonal pattern for the previous two winters saw safety margins decline from September to December but then begin to improve steadily through the summer. That pattern was delayed this year, as safety margins remained stubbornly low through the month of June. The mean daily buffer encroachment duration for May was unchanged from April, when we expected significant improvement from better weather and longer days. The mean daily encroachment duration for Category A and B events during the month of June were 6.8 percent higher than in May.

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In previous years, Category A and B buffer encroachments would peak in the winter months and they gradually decline. For most of 2025, however, the decline was modest and Category A and B mean daily encroachment durations for April 1 through July 26 were 55 percent higher than the same period one year ago. That number has improved over the past month and a half, so the trend may be improving.

Figure 3 reports the weekly total durations for the two conformance category groupings. The Category PE and C weekly durations bounce around with little long-term trend since we started reporting in February 2024, primarily reflecting week-to-week variations in meteorological conditions and some seasonal variation. Last week the PE and C buffer encroachment durations dropped 12.8 percent from a low level, while category A and B duration totals increased 24.4 percent, for a net increase of 19.2 percent. The number of Category A and B encroachment events increased 32 percent over the past three weeks

Figure 4 reports the 7-day moving average of hourly buffer encroachment durations in all conformance categories since February 2024. The pattern is very clear, as the historical behavior of increasing buffer encroachments from the summer low into winter held, but metrics did not improve for the first six months of 2025. Buffer encroachment durations rose in September and October, then bumped up again in November and have remained high ever since. Variance improved over the past two months with fewer disturbingly high encroachment periods but the overall level was high until the decline in the past month.

It is only in the past month and a half that we observe a sustained, significant decrease in the buffer encroachment trend since early April 2025. Until the rise the past week and a half, the metric had been at its lowest value since June of 2024. The 7-day moving average of hourly encroachment durations rose steadily over the past 11 days, increasing 33 percent during that short time period.