Simultaneous approaches to parallel runways were the topic of a recent article that discussed the PRM approach. The precision runway monitoring system allows approach operations to runways as close as 2500 feet apart (as measured by the distance from one runway's centerline to the other). This criterion gets the job done at most airports, but there are a few places where operational needs dictate having approaches to runways that are even closer. For this, the SOIA approach was developed.
SOIA is an acronym for Simultaneous Offset Instrument Approach. The key distinguishing factor between a PRM and SOIA procedure is the "offset" component. SOIA approaches are authorized for runways spaced between 2,500 and 750 feet apart, so there exists a need to offset one of the runway's approach courses to maintain acceptable levels of separation during low visibility days. For context, the wingspan of a Boeing 777-300ER is 212 feet. SOIA approaches enable simultaneous operations on runways separated by only 3.5 wingspans!
The approach that keeps traffic moving at busy airports with close parallel runways.
The simultaneous nature of the SOIA requires that both runways have approved PRM approaches. One of these approaches must be an offset PRM. This means the approach (whether a localizer, ILS, or GPS PRM approach) is misaligned by about 3 degrees with the runway's magnetic orientation. If pilots were to fly an offset approach to the runway, they would find their path severely misaligned with the pavement. Therefore, the visual minimums for an offset approach tend to be a bit higher than an approach that is perfectly aligned with the runway.
When conducting SOIA PRM approaches, controllers vector an aircraft onto the perfectly straight in PRM procedure slightly ahead of the aircraft that is flying the offset PRM approach. The pilots flying the offset approach need to see the proceeding traffic to the parallel runway, as well as the runway they are supposed to land on, to proceed with the approach past the decision altitude. Unlike a standard PRM approach, the SOIA procedure has no non-transgression zone from where the offset pilots begin maneuvering to line up with their runway. The two aircraft need to have had distance built between them by the controllers to avoid being wing-to-wing on short final.
Only a few airports have ever had SOIA procedures built for them. The most common airport to find the approach was San Francisco International. However, the airport hasn't operated SOIA approaches, nor has the FAA published PRM approaches to the airport, since 2020. One of the busiest airports in the US, San Francisco's runway construction was limited by the bay that surrounds it in three directions and the dense San Francisco suburbs to the west. Runways 28L/R must be used for landings during high traffic volume, while 1L/R are used for departures between arrivals.
During the days when SOIA approaches were used, heavy aircraft (as well as Boeing 757s) were required to land on the offset approach to runway 28R and trail the preceding aircraft to 28L for wake turbulence separation. This missed approach point for the 28R approach (when the pilots need the runway and proceeding parallel plane in sight) is much further from the runway than a standard precision approach, featuring an immediate right-climbing turn to avoid conflict.
Nowadays, SFO operates using controller-induced offsets. There is still one approach to runway 28R that has a 3-degree offset to the left, like the old PRM SOIA procedure used (RNAV X 28R, pictured below). Notice that the missed approach point is on a 3.4-mile final and over 1,100 feet above the runway height, well above standard GPS minimums for an approach with vertical and lateral guidance in order to avoid a conflict with parallel traffic.
With SFO no longer using SOIA procedures, it seems the approach category is no longer used anywhere. Chicago O'Hare has PRM approaches to runways 10C and 10R, with the latter being offset by 3 degrees. However, the runways are spaced 3,100 feet apart. This negates the requirement for SOIA criteria to be applied. Instead, a pair of PRMs is sufficient. Though SOIA procedures are currently out of use, their availability remains should the need arise for exceptionally close runways to handle simultaneous landings.
Credit: https://simpleflying.com/simultaneous-offset-instrument-approach-guide/