Safety margins remained low last week, as the risk metrics generated mixed signals after last week’s sharp decline. Total buffer encroachment durations dropped due to a large decrease in the number of Proximity Events and Category C buffer encroachments. However, the more severe Category A and B separation conformance deviations rose six percent. The Category A and B encroachment durations last week are the highest recorded in the past two months. In fact, last week’s reading was the seventh highest over the past twelve months. 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 February are 26.5 percent above the January average. Those metrics are slightly better than the December peak but February 2025 is the second worst month over the past year. The February daily encroachment average is two times greater than the low value from June 2024. 

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 February 22, 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. The safety trend remains negative this month but at least did not get much worse this past week. Total encroachment durations remain near the highest weekly level reported over the past twelve months. Total encroachments durations for the week were down 24 percent but remain elevated. Safety margins are clearly degrading, when we would expect gradual improvement at this time of the year based on historical seasonal patterns. We are not observing those improved safety margins in 2025.

 

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Figure 1. 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 2 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, primarily reflecting week-to-week variations in meteorological conditions and some seasonal variation. Last week the PE and C buffer encroachment durations dropped more than sixty percent, but that was from the highest level reported since we began reporting 12 months ago. Category A and B duration totals for the week ending February 22 increased 6.2 percent from the previous week. The combined metrics are 2.1 times greater than the low recorded in the first week of May. 

 

 

 

 

 

 

 

 

 

 

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Figure 2. Weekly Trends in Encroachment Durations By Separation Conformance Category

Figure 3 reports the 24-hour moving average encroachment duration per aircraft of all conformance categories since February 2024. Figure 3 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 3. Moving Average Hourly Encroachment Duration Per Aircraft From February 18, 2024 Through February 22, 2025

Figure 3 reports data for past twelve 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 January and February. Variance is also higher with many more disturbingly high encroachment periods. 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.