Safety margins degraded for the third consecutive week, continuing the disappointing news of the past few months. By this time of the year, we expected considerable improvement in safety margins based on analysis of previous years. As reported previously by the Aviation Safety Monitor, safety margins typically are lowest in December, then gradually improve into the summer months with longer daylight and better weather. While safety margins have improved from their December low, that improvement is much less than expected and safety margins are significantly degraded compared to one year ago. Mean hourly buffer encroachment durations across the 26 airports we monitor are 63 percent higher for March 1 through May 10, 2025 than 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 indicators of system stress such as the operational problems at Newark and system failures.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

​​

Figure 1. Hourly 7-Day Moving Average Buffer Encroachment Durations for All Conformance Categories 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 10, 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 increased 1.9 percent and the number of encroachment events rose 2.8 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 early spring. Mean daily encroachment durations for April dropped sixteen percent from March but are still above January levels. The first ten days in May show no improvement.

 

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

​​​​

Figure 2. Weekly Buffer Encroachment Metrics

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 jumped 58 percent as there was more inclement weather in some regions. Category A and B duration totals for the week ending May 10 dropped 9.3 percent but remain high. The Category A and B encroachment duration per event remained stable at about 29 seconds. That metric is also well above the level of one year ago when it was around 18 seconds.

 

 

 

 

 

 

 

 

 

 

 

 

 

​​

​

​

​

​

​

​

​​​​​​

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

Figure 4 reports the 24-hour moving average encroachment duration per aircraft of all conformance categories since February 2024. Figure 4 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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

​

Figure 4. Moving Average Hourly Encroachment Duration Per Aircraft From February 18, 2024 Through May 10, 2025

Figure 4 reports data for past thirteen 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 quarter 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.