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.
Weekly Safety Margin Update. Safety margins degraded over the past week. Figure 1 summarizes the weekly data since the Aviation Safety Monitor began reporting on February 24, 2024. Total encroachments durations for the week increased 15.8 percent, a one-third increase since July 20. The number of buffer encroachments rose 13.0 percent, driven by a greater number of Category PE and C encroachment events. The more severe Category A and B event counts was steady and total encroachment durations dropped six percent. The weekly encroachment duration per event rose slightly from 20.85 seconds to 21.35 seconds. As our research team becomes more familiar with the data, we are beginning to focus more on the Category A and B encroachment data. The events are more indicative of possible eroded safety margins and the data are less erratic as the events are less affected by meteorological conditions.

Figure 2 displays the typical time of day pattern that we observe in the data, as encroachment events are highly correlated with traffic density. Even after aggregating the data over 17 metroplexes operating in three time zones the time of day pattern remains prominent. The week ending August 10 had numerous high encroachment duration spikes over the 90th percentile and many other above the 75th percentile. The hourly reports are clearly more erratic than the previous week with a clear upward trend in buffer encroachments.

As data are added to the chart each week, readers can begin to detect some mild day-of-week variation and, more importantly, periods of a few days and even a week or longer in which encroachment durations increase significantly. In the coming weeks, we will examine these fluctuations in more detail. How severe are these encroachments? The FAA defines three separation conformance categories based on how far they are from the separation index. (See the description “How Do We Measure Safety Margins?” for details on the conformance categories and how we measure them.) In that classification system, Conformance Categories A and B are the most severe. Under our definition of a buffer encroachment, Category A and B encroachments are counted under all meteorological conditions. Figure 6 displays information on the most severe separation conformance categories. The weekly durations and event counts for the sum of Category A and B encroachments are shown in Figure 6. The upward drift in these more severe buffer encroachments since the start of May stabilized the past few weeks, which means that encroachment durations for Categories A and B remain at the highest levels since we began reporting in the Aviation Safety Monitor back in February. If readers are interested, we can conduct a deep dive into the conformance category encroachments in a future weekly report. Let us know if that information will be useful.​