So what is it about SFO? It all boils down to three things: fog, runways and air traffic.
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While San Francisco might not have blizzards or hurricanes, it turns out a little bit of fog can go a long way to hinder operations at SFO.
“On a clear day, aircraft at SFO can land side by side, because the two pilots on each aircraft are able to look out the window, see each other and maintain separation,” says SFO spokesman Doug Yakel. “The problem is when it becomes foggy, and those two aircraft can no longer see one another.”
When pilots can’t visually maintain distance from other planes, they must rely on instruments. According to the Federal Aviation Administration (FAA), aircraft must maintain a distance of 4,300 feet when landing side by side in periods of low visibility. However, the two runways primarily used for landings at SFO are only 750 feet apart.
A scenic view of SFO. (Molly Samuel/KQED)
So when it’s too foggy, aircraft can’t land side by side at SFO anymore. Instead of landing 60 planes in 60 minutes, like SFO can do on a clear day, they can land about half that number.
The FAA also employs a Ground Delay Program, where flights destined for SFO are held at their originating airports until they are given clearance to take off.
“Think about metering lights going on to a bridge, for example, where you need to meter the traffic that’s going into that area, because there’s more demand than there is space,” says Yakel. “That’s essentially what happens in bad weather at SFO.”
SFO’s runways were built so close together because they were constructed in the 1940s, when there was much less air traffic. As air travel has become more popular, the distance between runways has made weather more of a limiting factor.
Busiest in the Bay
The fog doesn’t delay planes at other local airports nearly as much. In 2018, San Jose International Airport’s (SJC) arrivals were 82 percent on time, while those at Oakland International Airport were 81 percent on time.
SFO is the seventh-busiest airport in the United States, boarding almost 28 million passengers. According to Yakel, two-thirds of the Bay Area’s air traffic goes through SFO, with OAK and SJC essentially splitting the remaining third.
“If SFO had the same level of flight activity as [the Oakland or San Jose airports], weather probably won’t be as much of a factor,” he says.
The Deal with Departures
While Yakel says fog doesn’t directly affect departures, its effect on arrivals does trickle down. At the very beginning of the day, all of the planes departing SFO have been kept there overnight. So even if it’s foggy, their departures shouldn’t be affected.
But for the rest of the day, most departing planes depend on an arriving plane, so it can load up passengers and head to the next destination. If the arriving flight is delayed, perhaps by fog, then the departing flight will be delayed as well.
Improvements on the Horizon
Yakel says that SFO is working with the FAA to address the delays and they are already seeing improvements. On low-visibility days SFO could once land only 27 planes per hour. Recently they’ve gotten that number into the high 30s.
“Twenty years ago it would have taken additional runways,” he says. “In the years to come, technology is going to address this issue.”
