Flight delays at San Francisco International Airport regularly exceed two hours when it’s rainy, windy or foggy, while airports in San Jose and Oakland report few if any delays. As I write this, the FAA is reporting delays averaging 3 hours 40 minutes at SFO.

Here’s why:

As most frequent travelers know, delays mount at SFO almost every time a storm blows in off the Pacific. Just look at these sad numbers from the U.S. Bureau of Transportation Statistics: SFO ranked 27th out of 29 major airports in on-time arrivals in the first 10 months of 2014. It ranked 18th out of 29 in on-time departures (an improvement over 2013 when it ranked 23rd).

Why does SFO seem to suffer so many more weather-related delays than other airports in the Bay Area?

Of course, the answer is endlessly complicated, but for the most part, the main reason is capacity. SFO’s runways are too close together to allow simultaneous operations during wet weather. Due to environmental concerns, there are no current plans to further separate the runways, which would require expansion into the Bay.

SFO runways are designed to handle up to 60 aircraft arrivals per hour in dry weather. That’s because the airport operates two sets of parallel runways– one set for takeoffs, the other for landings. These parallel runways intersect at their midpoint forming a giant “X.” (See figure) During dry weather, two streams of planes can land and take off from these parallel runways.

The problem is that planes are only allowed to take off and land simultaneously on these parallel runways during clear, dry weather.

When storms blow in, air traffic control changes up the formation in which planes land, from the dry weather “West Plan” (with aircraft arriving on runways 28L or 28R and departing on runways 01L or 01R– see above) to the stormy weather “Southeast” plan (when aircraft arrive on 19L & 19R and depart on 10L & 10R– see below).

When air traffic controllers switch to the Southeast plan, simultaneous operations on parallel runways is forbidden, which cuts the airport’s arrivals capacity in half– from 60 down to 30 or 38 per hour, depending on the severity of conditions. All planes must land in single file, which causes delays when there are 60 planes scheduled to land, but air traffic control only allows 35. And when planes arrive late, it’s likely that they will also depart late.

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If you live in San Francisco, you can tell when planes switch to the Southeast Plan because arriving aircraft whistle and moan as they descend through the clouds over the city as they approach SFO.

What can you do to avoid this? Fly early during storm season. Before 9 a.m., arrival volume at SFO is below 30 per hour. But after 9 a.m., just over 30 aircraft are scheduled to land. The arrivals rate peaks between 12 noon and 2 pm when 40+ aircraft per hour are scheduled to land at SFO. The situation usually does not improve until later in the afternoon when arrival volume falls below 30 per hour.

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The main reason that airports in Oakland and San Jose don’t face such on-time performance issues– even when it’s raining—  is that their volume is low. They rarely exceed their capacity for arrivals in good or bad weather. (For example, neither airport is included in the BTS’s top 29 airports cited above.)

Late last year, SFO announced that new landing procedures could help reduce delays– but those new procedures only apply during periods of low ceilings (fog) but not rain.

Would you consider switching your flying to Oakland or San Jose due to delays at SFO? Why or why not? Please leave your comments below. 

— Chris McGinnis

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