Changes in safety margins were mixed last week as total buffer encroachment durations dropped 1.4 percent but the number of buffer encroachments increased by 5.3 percent. That change is a slight reversal of the trend over the past year, which saw buffer encroachment durations increase 72 percent while the number of encroachments rose only (!) 19 percent. That translates into a 42 percent rise in the encroachment duration per aircraft. Phrased differently, that means that when an aircraft pair generate a buffer encroachment, the hazard exposure time was  on average 42 percent longer in May 2025 compared to May of 2024. This is one more indicator that safety margins degraded  significantly over the past year.  Mean hourly buffer encroachment durations across the 26 airports we monitor are 72 percent higher for April 1 through May 31, 2025 than during the same period one year ago. 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 and several infrastructure failures.

​

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

​

Figure 1. Weekly Buffer Encroachment Metrics at 26 Airports

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 May 31, 2025.  

​

​

​

​

​

​

​

​

​

​

​​​

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 remained low last week, as the risk metrics shows little change from their high levels of the past several months. Total encroachment durations decreased 1.4 percent and the number of encroachment events rose 5.3 percent.

​

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 72 percent higher for April 1 through May 31, 2025 than the same period one year ago.

​

​

​

​

​

​

​

 

 

 

 

 

 

 

 

 

 

 

 

​​

Figure 2. Mean Daily Buffer Encroachment Durations By Month

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.  

​

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 43 percent with an increase in relatively poor meteorological conditions. Category A and B duration totals for the week ending May 31 fell 12.9 percent.

 

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

​

​

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.

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

​​​

Figure 4. Seven-Day Moving Average of Hourly Buffer Encroachment Durations From February 18, 2024 Through May 31, 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.