We finally detected a big improvement in safety margins last week. After two weeks of improving metrics, overall safety margins are now the best recorded since the week ending October 8, 2024. Total buffer encroachment durations in all separation conformance categories decreased 23 percent, primarily due to a 50 percent fall in Category PE and C buffer encroachments. Category A and B encroachment durations also fell 20 percent. We need to wait another couple of weeks to determine whether the good news from the past two weeks is sustained, as the metrics for the full month of June were not any better than April or May. We expect improved metrics at this time of the year as meteorological conditions are much better, and Category A and B encroachment durations have not fallen quite as much.

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 July 5, 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 improved quite a bit last week, finally breaking out from the narrow range in which they moved the past couple of months. Total encroachment durations decreased 49.8 percent and the number of encroachments dropped 42.2 percent.

We are finally detecting a positive trend 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 did not hold this year, as safety margins remained stubbornly low through the month of June. The mean daily buffer encroachment duration for May was unchanged from April, when we expected significant improvement from better weather and longer days. The mean daily encroachment duration for Category A and B events during the month of June were 6.8 percent higher than in May.

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.  

This conclusion is illustrated by Figure 2 that reports mean daily total encroachments durations by separation conformance category. There are fewer Category PE and C encroachments and they peak in the winter months. In previous years, Category A and B buffer encroachments would peak in the winter months and they gradually decline. In 2025, however, the decline has been modest and Category A and B mean daily encroachment durations for April 1 through July 5 are 68 percent higher than the same period one year ago. That number has improved over the past couple of weeks, so the trend is improving.

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 fell 50 percent, while category A and B duration totals decreased 20 percent, for a net decrease of 23 percent.

Figure 4 reports the 7-day moving average of hourly buffer encroachment durations in all conformance categories since February 2024. The pattern is very clear, as the historical behavior of increasing buffer encroachments from the summer low into winter held, but we are not seeing the improvement expected in the first six 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 was high until the decline in the past two weeks.

It is only in the past couple of weeks that we observe a sustained, significant decrease in the buffer encroachment trend since early April 2025. The big drop of the past three weeks leaves the metric at its lowest value since October 6, 2024. However, Category A and B encroachment durations remain well above their values from one year ago.