About the Aviation Safety Monitor

The Aviation Safety Monitor is a service provided by Robust Analytics to deliver timely information on terminal area safety in the National Airspace System (NAS). The safety monitoring and prediction technologies were developed by Robust Analytics over the past several years. Partial funding was provided by the NASA Small Business Innovation Research Program and the NASA System Wide Safety Project.

The Aviation Safety Monitor provides quantitative estimates of safety margins at 26 airports in 17 metropolitan regions in the United States. This information complements data on several safety-related events that are published elsewhere, with the FAA’s Runway Incursion Statistics website a good example. However, the available safety information can be misleading if it only reports the frequency of violations with no insight into how safety buffers may vary minute-to-minute and day-to-day. The Aviation Safety Monitor aims to provide this insight every week.

How Do We Measure Safety Margins?

The Aviation Safety Monitor summarizes output from Risk Tracker, the Robust Analytics in-time terminal airspace hazard and safety metrics monitoring system.

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Aviation Safety Monitor Weekly Report for the Week Ending May 3, 2025

Safety margins decreased last week for the second consecutive week. Total buffer encroachment durations across all categories rose 9.7 percent as the number of encroachment events increased 4.5 percent. The month of April was better than March, as daily encroachment durations dropped 16 percent and the mean daily event count fell six percent. The improvement is welcome but safety margins are significantly degraded from a year ago, as mean daily encroachment durations in April 2025 were 69 percent higher than April 2024. The more severe Category A and B encroachment durations degraded even more, as the mean daily duration total in April 2025 was 81 percent above the level from one year ago.

Based on analysis of previous years, we expected encroachment durations to peak in December, followed by a gradual decline into the summer months. Visual examination of the data suggests that buffer encroachments increased sharply in October 2024 and have since remained at a higher plateau ever since.

Figure 1. Mean Daily Encroachment Durations for All Conformance Categories, 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 May 3, 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 9.8 percent, with the more severe category A and B separation conformance deviations increasing 12.2 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.

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 were unchanged. Category A and B duration totals for the week ending May 3 rose more than 12 percent. The Category A and B encroachment duration per event increased to 29.3 seconds. That metric is also well above the level of one year ago.

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 3, 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.