Buffer encroachment durations and event counts saw little change last week and have stabilized since the big increases in July and early August. The daily encroachment duration average for the first week in September was 7 percent below the August average. That is a welcome improvement, as the August daily average encroachment duration was 10.5 percent above the July mean, and July in turn was 11.5 percent higher than June.

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 7, 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 improved slightly 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 2.5 percent and the number of encroachment events dropped 2.9 percent. Generally good weather kept the number of Conformance Category PE and C buffer encroachments low throughout the week. The weekly encroachment duration per event increased 1.1 seconds.

Let’s now take a closer look at safety margins for the seven days ending September 7. 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 week ending September 7 was quiet, with only seven duration spikes over the 90th percentile and nine hourly reports between the 75th and 90th percentile.

Figure 3 reports the 7-day moving average duration per aircraft for the four weeks from August 11 through September 7, 2024. The chart shows that safety margins over the past month we generally better than the historical average as indicated by the percentile scores, but with sustained high encroachment durations the week of August 11. 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.

Figure 4 displays the 7-day moving average encroachment duration per aircraft for the past three months. After a quiet month for most of June, encroachment durations increased in July and August as shown in Figure 4. The first week of September shows a slight improvement. Overall, 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.

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. The big jump in both metrics the weeks ending July 27 and August 17 suggest that the improvement detected the week ending August 3 was only a temporary reprieve from the negative trend.

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 continued to moderate last week, as total encroachment Category A and B durations dropped 5 percent over the previous week. Average daily encroachment durations were 10 percent higher in August than in July. The continued high level of the more severe Category A and B encroachments remains concerning, with the August daily encroachment average 26 percent higher than the June average.

Both the number of buffer encroachments and their durations for Category A and B are at the highest levels since we began reporting in the Aviation Safety Monitor. 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.