Aviation Safety Monitor Weekly Report for the Week Ending April 5, 2025

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As the month of April 2025 begins, safety margins remain degraded but stable. The mean daily encroachment duration for all separation conformance categories increased 2.5 percent due to a  2.5x jump in category PE and C encroachment durations while the more severe category A and B conformance category deviations fell 23 percent. The net change was small but it is much better to have fewer severe buffer encroachment events. Buffer encroachments have been high since November, with only a brief respite from the poor risk metrics in January. Safety margins decreased significantly over the past twelve 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 twice as high. Based on analysis of previous years, we expect encroachment durations to peak in December, then gradually decline through the summer months. Unfortunately, the mean daily February and March encroachment durations are two of the three highest months for the past year, suggesting that conditions are deteriorating rather than improving. Visual examination of the data suggests that buffer encroachments increased sharply in November 2024 and have remained at a new, higher plateau ever since. The data suggest that the NAS is showing signs of stress similar to the winter of 2022-2023, and the recent incidents are consistent with that degradation in safety margins.

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Figure 1. Mean Daily Encroachment Durations for Conformance Category A and B Deviations By Month

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 5, 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 increased 2.5 percent, while the number of encroachment events decreased by 25 percent. That raised the duration per encroachment event to 31.5 seconds. The more severe Category A and B separation conformance deviations fell 23 percent but remain high.

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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.

 

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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.

 

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 increased 2.5x due to poor meteorological conditions (but not unusual for this time of the year). Category A and B duration totals for the week ending April 5 decreased 23 percent from the previous week, a welcome improvement even if the metric remains uncomfortably high.

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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 5, 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.