A recent day with foggy weather in the Puget Sound area produced a few examples of weather-related delays. In the example presented below, Jazz Air 8089, a Dash-8, departed Vancouver [CYVR] on a short 30-minute flight to SeaTac [KSEA]. The flight departed at 8:55am, just as the KSEA visibility was reducing to a half mile. The crew was turned toward the Olympic Peninsula and issued turns to delay their arrival.
Here is a screen-cap of the METAR weather sequence, reading from bottom to top; thin red boxes have been added, marking the departure time at CYVR and the arrival time at KSEA. The column in the middle is most critical, showing visibility deteriorating from 10-miles to a half-mile; the magenta text to the right, reading BKN001 and VV001 is also significant, indicating low clouds and fog obscuring the sky at 100′ above the surface.The flight altitudes and times at points on the JZA8089 route have been added to this enlarged map view of the delay portion of the flight, over the Olympic National Park:In normal weather conditions, the flight is routine, even boring to both pilots and ATC. To the left are screen-caps for the same flight on days before and after… on 11/9, 11/11, and 11/12. In all cases, KSEA is in a South Flow, so the minor variations in these three flights are almost entirely due to other arrival traffic.
In an extreme case, if traffic volume is sufficiently large, ATC may need to issue a holding loop, or multiple turns to achieve even 20+ minutes of delay. Note on these screen-caps, the busiest day of the week for air travel (Friday) shows the most extreme excess turns to final; the slowest day of the travel week (Saturday) shows essentially no added delays.
One way that FAA fails to prevent excessive delays is by refusing to manage capacity. Especially at hub airports, arrival rates are set too high, so as not to restrict the airlines. In their NextGen studies, FAA has repeatedly referred to maximizing ‘runway throughput’. The problem, though, is that when arrival rates are set too high, it takes just one minor weather glitch to create a cascade of delays, one airplane after another, often for hours. In the worst cases, typical at LaGuardia Airport, cascaded delays can cause arrivals to finish well after midnight, even more than two hours beyond their schedule times. And these delays nearly ALWAYS result in continuous arrival streams, with repetitive noise patterns impacting residential neighborhoods, a problem being exacerbated under NextGen.
(All graphics & flight data from FlightAware)