Safety margins improved slightly as total buffer encroachment durations fell by small amounts for the third consecutive week. A small increase in Category PE and C buffer encroachment durations were offset by the decline in Category A and B encroachments, for a net drop of 1.1 percent. Most metrics have been stable over the past couple of months without much overall trend. This does mean that buffer encroachments remain uncomfortably high for this time of year and are at a much higher plateau than one year ago.  Mean hourly buffer encroachment durations across the 26 airports we monitor were 73 percent higher for April 1 through June 14, 2025 than during the same period one year ago. Even more concerning is the 79 percent jump over the same time period in the more severe Category A and B separation conformation buffer encroachment durations. Encroachment duration per event for all separation conformance categories increased 44 percent over the past year. That means that aircraft pairs generating buffer encroachments were exposed on average nine seconds longer in the first week of June 2025 compared to June of 2024. This is one more indicator that safety margins degraded  significantly over the past year.  

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Instead of improving, recent data suggest that encroachment durations have stabilized at a higher plateau over the past several weeks. This risk metric is consistent with other indicators of system stress such as the operational disruptions at Newark, several close calls, and infrastructure failures.

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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 June 14, 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 showed no significant change from their range over the past several weeks. Total encroachment durations decreased 1.1 percent and the number of encroachment events dropped 2.3 percent.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 1. Weekly Buffer Encroachment Metrics at 26 Airports

The NAS is simply not showing the improvements in safety margins that we expect at this time of the year. 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 is not holding this year, as safety margins remain stubbornly low into late spring. The mean daily buffer encroachment duration for May is unchanged from April, when we expected significant improvement from better weather and longer days. Mean hourly buffer encroachment durations are 73 percent higher for April 1 through June 14, 2025 than the same period one year ago.

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Buffer encroachments are classified into 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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This conclusion is illustrated by Figure 2 that reports mean daily total encroachments durations by separation conformance category. There are fewer Category PE and C encroachments and they peak in the winter months. In previous years, Category A and B buffer encroachments would peak in the winter months and they gradually decline. In 2025, however, the decline has been modest and Category A and B mean daily encroachment durations for April 1 through June 14 are 79 percent higher than the same period one year ago.

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Figure 2. Mean Daily Buffer Encroachment Durations By Month

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 rose 23 percent but that increase was offset by a 6.0 percent decline category A and B duration totals, for a net decrease of 1.1 percent.

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Figure 3. Weekly Trends in Encroachment Durations By Separation Conformance Category

Figure 4 reports the 7-day moving average of hourly buffer encroachment durations in all conformance categories since February 2024.

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Figure 4. Seven-Day Moving Average of Daily Buffer Encroachment Durations From February 18, 2024 Through June 14, 2025

Figure 4 displays data for past fifteen months and the pattern is very clear. The historical pattern of increasing buffer encroachments from the summer low into winter held, but we are not seeing the improvement expected in the first five 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 is high. Visual examination of Figure 4 suggests that buffer encroachments increased in October 2024 and have remained at a new, higher plateau ever since.