Simultaneous parallel approaches to San Francisco International Airport's runways 28L and 28R represent one of the most technically demanding arrival procedures in routine commercial service in the United States. The runways are separated by only 750 feet centerline to centerline — far below the 4,300-foot standard that would normally permit independent parallel ILS operations — making SFO one of a small number of airports in the world where Simultaneous Offset Instrument Approach (SOIA) and Precision Runway Monitor (PRM) procedures are employed to sustain acceptable arrival rates under instrument meteorological conditions. Footage of a United and Delta aircraft touching down on adjacent runways nearly simultaneously captures a procedure that is operationally routine at SFO but aerodynamically and procedurally anything but trivial.
The PRM system underpinning these approaches uses high-update-rate radar with approximately one-second refresh cycles — compared to the standard 4.8-second ASR sweep — and dedicated controller monitoring of each approach corridor. Pilots flying PRM approaches must acknowledge a special briefing, monitor a secondary tower frequency simultaneously, and remain prepared to execute an immediate breakout maneuver if directed. The No Transgression Zone (NTZ), a protected buffer between the two approach courses, is continuously monitored; any aircraft deviation into the NTZ triggers an immediate automated alert and breakout instruction to the adjacent aircraft. For crews, this means a higher-than-normal cockpit workload during what is otherwise a stabilized visual or instrument final, with an emphasis on precise tracking, sterile cockpit discipline, and immediate compliance with ATC instructions.
The United-versus-Delta framing of the Reddit post speaks to a broader competitive reality at SFO. Both carriers maintain significant hub or focus-city operations there, and each is acutely sensitive to delay performance at an airport that consistently ranks among the most delay-prone in the country. SFO's notorious marine layer frequently drives ceilings and visibility below visual approach minima, at which point the airport's arrival capacity drops sharply unless PRM and SOIA operations are in effect. When conditions deteriorate below even those minimums, SFO can lose a full parallel stream entirely, producing cascading delay impacts felt system-wide. The ability to sustain two arrival streams, even at reduced spacing, is therefore not merely a procedural curiosity but a core capacity management tool that directly affects block times, fuel planning, and schedule recovery for every operator serving the Bay Area.
For Part 91 and 135 operators using SFO, the PRM environment introduces specific training and currency considerations. Pilots must be familiar with PRM procedures through ground or online training before being cleared for PRM approaches, and operators should ensure that flight crews have reviewed the specific SFO approach plates for the LDA/PRM and ILS/PRM procedures, including the visual segment breakout criteria for the offset runway. Corporate and charter crews who operate into SFO infrequently may find the dual-frequency monitoring and breakout drill less instinctive than their airline counterparts who fly the procedure routinely, making pre-departure review a meaningful risk mitigation step. As NextGen and RNAV-based approach technology continues to evolve, SFO's PRM program remains a benchmark for what high-density terminal environments require when geography and weather converge to constrain capacity.