The headline on my recent post about this week’s storm-related delays at Bay Area airports prompted a question.
Why does San Francisco International seem to suffer so many more weather-related delays than airports in Oakland or San Jose?
There are two main reasons:
1>Parallel Runways
SFO is designed to handle up to 60 aircraft arrivals per hour in good 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.”
The problem is that planes are only allowed to land side-by-side on these parallel runways during clear weather. When inclement weather strikes, the FAA forbids parallel landings. So all planes must land in single file, which cuts the airport’s arrivals capacity in half— from 60 down to 30 or 38 per hour, depending on the severity of conditions.
And when planes arrive late, they most likely end up departing late, compounding the frustration level at the airport, and leading to poor on-time performance numbers.
2>Volume
When bad weather strikes and the arrival rate at SFO is restricted to just 30-38 per hour, problems arise when the number of arrivals exceeds that ceiling.
Before 9 a.m., arrival volume at SFO remains 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 over 40 aircraft per hour are scheduled to land at SFO.
So that means that if a storm blows through SFO mid-day, you can expect major delays, which will ease later in the afternoon when arrival volume falls below 30 per hour.
The main reason that airports in Oakland and San Jose don’t face such on-time performance issues is that their volume is low…they rarely exceed their capacity for arrivals in good or bad weather.
One other point about the parallel landings. It’s not just that good weather allows landings on 2 runways at a time (usually 28 Left and 28 Right). Good weather allows a ‘Simultaneous Close Parallel’ approach. Both landing planes fly very close and nearly parallel to each other and land at nearly the same time. This allows departing flights (on the intersecting runways) to launch when there is a gap in landings.
If they didn’t land at nearly the same time, there would be less time (or really no time) available for takeoffs on the intersecting runways.
According to the approach charts, the weather must have a ceiling of at least 2100 ft and 4 miles visibility for these parallel approaches. The runway centerlines are 750 feet apart.
http://204.108.4.16/d-tpp/1001/00375IPRM28L.PDF
http://204.108.4.16/d-tpp/1001/00375IPRM28L_C.PDF
Thanks for publishing the bat!
a reader sent me the following comment via email:
Nice bit of work, but you failed to mention that the problem with SFO’s runways is not so much that the runways are in parallel pairs, but rather, that the pairs are set too close together. The current layout dates to 1955 or so, and was just fine when the Super Constellation was the latest thing in air travel.
The only way around this involves filling in the Bay, which is a solution that carries its own baggage. There was a push to realign the runways back around the turn of the millennium, but it fell apart because of the expense involved. It’s not just the cost of fill and construction that has to be considered, either.
There would be numerous legal challenges, and ultimately, some purchases of bayside land would have be made, with the goal of restoring more wetlands. I believe that some of the old Cargill salt ponds were identified as parcels that might possibly mitigate the SFO expansion, but I don’t have that information immediately at hand.
In other words, it’s probably not going to happen within our lifetimes.
I’m sure I’m not the first to point this out to you, but just to be on the safe side, I thought I’d drop you a line.
Cheers,
PJ Connolly
San Francisco