Safety margins at the 26 monitored airports remained at the slightly lower levels of the past two weeks. Overall, safety margins in May are better than the estimated long-term average. Welcome back to the Weekly Aviation Safety Report. 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 18, 2024.

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 can be found here https://www.robust-analytics.com/measure on the Robust Analytics website.

Weekly Safety Margin Update. There was little change from the previous week, with safety margins slightly lower the past three weeks compared to April. Total encroachment durations for the week ending May 18 were 3.9 percent lower than the previous week, but encroachment events were 0.6 percent higher. Average weekly encroachment durations for the month of May are running 2.7 percent above the April average and event counts are 12.3 percent higher. Total encroachment events for the 26 airports are now at the highest level since the week of March 9. These changes can be easily seen in Figure 1 that displays weekly summary data for the 26 monitored airports. The Aviation Safety Monitor team will continue tracking the daily metrics this week to detect whether these past few weeks are a temporary increase or an indicator of a longer negative trend in safety

Let’s now take a closer look at safety margins for the seven days ending May 18. Figure 2 displays estimates of encroachment durations per aircraft for each hour over the seven days. This offers a detailed look at how safety margins vary over the operating day. Figure 2 also indicates the typical range of the data by showing the 25th, 75th, and 90th percentile values of the duration per aircraft metric. The percentiles are calculated from 14 months of data from May 2022 through June 2023.

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. Note that the daily peak increases from Monday to

Figure 3 shows duration per aircraft for the four weeks from April 14 through May 18, 2024. Nearly all observations are below the 90th percentile and most below the 75th percentile. Figure 3 shows that despite the moderate increase in encroachments the past three weeks, safety margins remain very good compared to the historical averages.

Trends are easier to detect visually with the moving average plats shown in Figure 4. This chart reinforces the observation about Figure 3 that safety margins over the past month are better than the historical average.

Figure 5 shows a longer perspective with data back to February 18, 2024. This chart clearly shows the reduced safety margins for several days in early March that we reported on in our initial posting on April 8, 2024. That early March period had the highest durations and the longest extended period with high durations in the past two months, with metrics well above the 90th percentile for many hours.

Figure 6 displays the moving average version for the past three months, which highlights the early March increase in encroachment durations that persisted for ten days. This is the only sustained period of high encroachments that lasted more than a couple of days for the past three months. April and May encroachment metrics are both much lower than March and below the long run average.

We wrap-up this week’s report with updates to the two charts presented in the previous weekly reports. Figure 7 shows the total daily counts of the number of encroachments and their durations across the 26 airports in the 17 metropolitan areas that we monitor. The blue bars report estimates of total daily encroachment durations divided by the number of aircraft in the terminal airspace (approximately 50 miles of the airport center) for all 17 terminal airspaces. The red line reports the daily number of encroachment events per 100 aircraft.

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 8 displays information on the most severe separation conformance categories. The daily durations and event counts for the sum of Category A and B encroachments are shown in Figure 7. The pattern differs from the Figure 1, as there is no obvious trend during the time period. This suggests that Category PE and C encroachments are affected by different factors from Category A and B. We will take a deep dive into those differences in a future weekly report.

Overall, at the NAS level safety margins have been stable for the past several weeks. The industry shows evidence of recovering from the stress of September 2022 to February 2023. We will take a deeper dive into that time period and the subsequent partial recovery in future reports.