Safety margins improved slightly last week due to good weather throughout most of the NAS, which minimized the Category PE and C buffer encroachments. The mean daily encroachment duration for all separation conformance categories dropped 24 percent after increasing 2.1 percent the previous week. The more severe category (and more common) A and B conformance category durations fell 6.4 percent but remain elevated for this time of the year. Buffer encroachment duration totals remain stubbornly high since November, with only a brief respite in January. There has been some improvement this month, with the mean daily encroachment duration for all conformance categories in April running eight percent below the March daily average.

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Safety margins decreased significantly over the past thirteen months, with the mean daily encroachment duration for March 2025 fifty-seven percent higher than the March 2024 value and the most severe encroachment durations were twice as high. Based on analysis of previous years, we expected encroachment durations to peak in December, followed by a gradual decline into the summer months. Unfortunately, the mean daily February and March encroachment durations were two of the three highest months for the past year, suggesting that conditions are deteriorating rather than improving. The early April data show only a small improvement. Visual examination of the data suggests that buffer encroachments increased sharply in November 2024 and have since remained at a higher plateau ever since.

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Figure 1. Weekly Encroachment Durations for Category A and B Conformance Categories

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 April 19, 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.  

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 24 percent due to a 79 percent drop in Category PE and C buffer encroachment durations as there was good weather all week throughout the NAS. The more severe category A and B separation conformance deviations decreased 6.4 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 this month. Mean daily encroachment durations for March fell just 1.7 percent from February, but only because good weather reduced the number of category PE and C separation conformance encroachments. Category A and B separation conformance deviations rose 17.8 percent in March 2025 and was the worst performing month of the past year on that metric. Early April metrics show little improvement.

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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.  

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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 decreased 79 percent due to better meteorological conditions. Category A and B duration totals for the week ending April 19 dropped six percent from the previous week but remain high. Curiously, the number of Category A and B buffer encroachment events has fallen the past three weeks while total durations remain relatively unchanged, leading to a sharp increase in the encroachment duration per event. The Category A and B encroachment duration per event is now 32.9 seconds, slightly below the high value of 36.7 seconds recorded the previous week. That value was the highest recorded for the past thirteen months.

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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.

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Figure 4. Moving Average Hourly Encroachment Duration Per Aircraft From February 18, 2024 Through April 19, 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 November 2024 and have remained at a new, higher plateau ever since. It is time to conduct a deep dive into the data and determine why safety margins remain degraded. Expect to see those findings soon in a Special Report by the Aviation Safety Monitor research team.