SFO Ran Two Runways as One for Decades. The FAA Just Made That Math Impossible.

Two runways. Seven hundred and fifty feet apart. And for decades, San Francisco International threaded jets onto both of them simultaneously, trusting pilots' eyes to hold the gap.

That procedure is gone. The FAA has banned simultaneous independent parallel approaches to SFO's runways 28L and 28R — and the capacity math that disappears with it is staggering.

The trick was always visual. Standard FAA rules require parallel runways to be at least 4,300 feet apart before independent IFR approaches are permitted. SFO's runways aren't close to that threshold. What made simultaneous arrivals possible was a visual separation standard — pilots acquiring the adjacent aircraft and maintaining their own spacing. In clear conditions, it works. In fog, it doesn't.

SFO sits at the edge of the Pacific, directly in the path of the marine layer that rolls through the Bay Area from spring through summer. On a typical fog morning, visibility drops below the visual meteorological conditions minimums that the procedure required. That used to mean a temporary slowdown. Now it means a structural one.

In good conditions, SFO handles roughly 60 arrivals per hour by running both runways in parallel. Lose that and single-runway sequencing drops throughput to something closer to 30. That's not a rounding error — it's half the airport.

For United, which banks transpacific widebodies through SFO in tight morning windows, the downstream effects compound quickly. Delayed arrivals ripple into gate conflicts, connection banks stretch or collapse, and ground delay programs — already a regular feature of Bay Area fog season — become both more frequent and deeper.

PRM approaches, the Precision Runway Monitor system that uses high-update radar to police closely-spaced parallel operations, require that 4,300-foot minimum to function in IMC. SFO never qualified. The visual procedure was always a workaround operating at the edge of the rulebook's tolerance.

The fog at SFO isn't new. The runway spacing isn't new. What changed is that the regulatory patience for threading widebodies through both — simultaneously, on instruments, in marine layer — finally ran out.