When the visual approach is compromised, the "West Plan" is maintained with a modification to allow aircraft landing on 28L to use Instrument Landing System (ILS) while the aircraft landing on 28R takes an offset course, monitored via high scan rate ground radar, to maintain a lateral spacing greater than 750 ft until the aircraft can maintain visual separation. Visual separation typically occurs once the aircraft has descended below the cloud deck at an altitude of 2,100 feet (640 m). This is known as the Precision Runway Monitor/Simultaneous Offset Instrument Approach and reduces the capacity to 36 arriving aircraft per hour. In poor visibility conditions, FAA instrument approach rules require aircraft to maintain lateral separation of 4,300 feet (1,300 m), meaning only one runway may be used, reducing the capacity of SFO to 25-30 arriving aircraft per hour.