Safety margins decreased again last week and remain near the lowest levels since we started reporting last February. Total encroachment durations for the week ending September 28 rose 8.4 percent while the number of encroachment events fell 0.2 percent. The September 28 duration metric is the second highest reading since early March. During the first four weeks of September, the daily encroachment duration average was 12.3 percent higher than the August average. Daily encroachment durations were 10.5 percent higher in August than in July, and the July average was 11.5 percent higher than the June daily average. The time series reflects a seasonal pattern in which safety margins are highest during the spring and summer months and slowly degrade starting in late summer. We are seeing that recurring pattern continue as we move into the autumn months.

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 September 28, 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 article can be found here https://www.robust-analytics.com/measure on the Robust Analytics website.

Weekly Safety Margin Update. Safety margins degraded last week after a short-lived reprieve the previous week. Total encroachments durations for the week ending September 28 increased 8.4 percent and the number of encroachment events dropped a slight 0.4 percent. The weekly encroachment duration per event increased 1.81 seconds to 22.35, the highest average duration per event since the week of April 6. Figure 1 summarizes the weekly data since the Aviation Safety Monitor began reporting on February 24, 2024.

Let’s now take a closer look at safety margins for the seven days ending September 28. Figure 2 displays estimates of encroachment durations per aircraft for each hour over the previous seven days. This offers a detailed look at how safety margins vary over the operating day. Figure 2 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 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 past seven days worsened as the week progressed, particularly during the last three days. There were 16 duration spikes over the 90th percentile, and another 16 readings in the 75-90th percentile range. The number of higher hourly readings is increasing gradually week by week.

Figure 3 reports the 7-day moving average duration per aircraft for the four weeks from August 25 through September 28, 2024. The chart shows that safety margins over the past month were generally better than the historical average as indicated by the percentile scores, until the month of September arrived.

Figure 4 displays the 7-day moving average encroachment duration per aircraft for the past three months. The chart shows a steady deterioration since the end of June. The first four weeks of September averaged daily encroachment durations 12.3 percent higher than the August average. Daily encroachment durations were 10.5 percent higher in August than in July, and the July average was 11.5 percent higher than the June daily average. The time series reflects a seasonal pattern in which safety margins are highest during the spring and summer months and slowly degrade starting in late summer. We are seeing that recurring pattern continue as we move into the autumn months.

We wrap-up this week’s report with updates to charts presented in the previous weekly reports. Figure 5 shows the daily encroachment event rates and durations per aircraft 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. We detect big jumps in both metrics the weeks ending July 27, August 11, and September 14, and September 28, with each peak higher than the previous one.

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. After high readings for two weeks, the encroachment duration metric moderated last week. and is now at the highest level since we started reporting on February 18. Average daily encroachment durations for the first four weeks in September are 1.5 percent higher than the August average, which was 10 percent higher than in July.