DCA airport has two operating patterns: "North Flow" and "South Flow".
When operating in North Flow, planes arrive from the south and depart to the north. In this pattern, they often pass over Accokeek and Fort Washington in a concentrated band, and then take a straight shot down the river to Runway 19.
When operating in South Flow, planes arrive from the north and depart to the south. Arriving in this pattern, they'll follow the twists and turns of the river to the north of the city so as to minimize noise over neighborhoods such as Potomac and Georgetown.
There are multiple factors that go into the FAA's decision to use North Flow or South Flow on a given day, all ostensibly weather related. Generally it's better to land and take off into the wind (so, if the wind is coming from the north, then to land from the south and take off to the north), especially given that DCA has suboptimally short runways. If visibility is low, it's safer to land using instruments rather than a visual approach; DCA's instrument landing system works only for north flow and not for south flow. Finally, because of the twists and turns needed for south flow arrivals, FAA will sometimes operate DCA in north flow even if there's a moderate tail wind.
We collated airport data for August, September, October and November 2016, and correlated it against wind records for those same dates. To do this we recorded the operating direction 3 times a day (at 8AM, 2PM and 8PM) using the public webtrak system, and matched that to weather data made available by Wunderground's archives. We're happy to make our data available for analysis - just message us for access.
The results are as follows.
Percent of DCA Operations in North Flow during late 2016
As you can see from the chart, DCA operated in North Flow a significant majority of the time over these four months. For residents in the worst-affected neighborhoods, this means that for an average 12 out of 16 waking hours of the day planes will be passing overhead.
An interesting aspect of these results is the gap between percentage of wind direction being from the north, and the operation being in North Flow. This is evidence of FAA's strong preference for North Flow, even if it means operating with moderate tail winds. Other factors that play into the decision are visiblity and cloud cover, but even with these taken into consideration FAA seems to feel that North Flow is safer, and therefore defaults to it in neutral weather.
We are unlikely to be able to change this preference. However, we can work to change the route that planes fly in North Flow so that planes don't pass directly overhead so many peoples' houses. A river route is possible, and MWAA is willing to make that recommendation to FAA. We are working through the process to have that recommendation made. If you're interested in being kept informed on this effort, please sign up to our mailing list.
