Date: 201508
Local time of day: 0001-0600
Location: NCT.TRACON, CA
Light: Night
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B SFO
ATC / advisory: TRACON NCT
Aircraft component: FMS/FMC
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Human factors: Time Pressure; Workload; Distraction; Human-Machine Interface
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Chart Or Publication; Human Factors; Procedure
Primary problem: Procedure

Narrative: Our crew thought it was important to alert you about the practices being used by SFO on the BDEGA1 RNAV arrival.  During the initial portion of the arrival; we were assigned the BDEGA1 and told to expect RWY 28R.  Somewhere around LOZIT; we were cleared to fly the BDEGA1 RWY 28L profile; but expect 28R landing.This setup requires us to enter a runway that we do not intend to land on on the FMS to correctly navigate the profile.  Then upon receiving vectors for the approach; we need to modify the FMS back to the runway of intended landing.  This is a lot of extra button pushing; planning; and coordination; and heads-down time that seems unnecessary at this stage of the flight in a high density airspace.  Although no non-complicance was involved in this; I think this adds a level of complexity and therefore safety concerns that should merit a review of this type of practice.Of course; we evaluated the assigned clearance and always had the option to not accept it - with the current weather; traffic; and time; we felt safe in accepting it in this instance.  But; I have seen this practice on this arrival before - so it is a tool ATC uses for traffic management; but I think there can be a better way.  ATC assigned an RNAV arrival that differs course depending on assigned runway clearance; but modifies the procedure to fly one runway specific profile for the opposite runway.Our FMS is not capable of cleanly flying this without having to make modifications after being taken off the arrival.  And being an RNAV arrival; there is no conventional NAV options.While it is easy for me to suggest the disuse of the this practice; I understand that SFO is a highly dynamic airspace environment and I think there are ways to accommodate this safely as it may be as simple as an FMS update that allows this feature; a modification of the BDEGA1; or exercising the authority to refuse this clearance. A few possible solutions to consider:1) Split the BDEGA1 into two different RNAV arrivals that can each can accommodate 28L and 28R.  So if approach wants you on a north-side downwind you fly one; a south-side downwind the other2) Make the courses after CORKK a heading.  This will allow the programming of the FMS for the intended runway much earlier3) Modify software to allow FMS to accommodate this type of practice4) Notify crews ahead of time about this practice; and notify ATC that crews may not accept (gives both ample time or plan B.

Callback: The reported stated that he is fairly certain; not 100%; that NORCAL made the first comment to expect a change to Runway 28R. This anticipation set the crew up to expect the high workload once they were on the heading after BRIXX. The reporter suggested that since this is not the first time he as had this same runway change; that the BDEGA be split into a Runway 28L segment separate from the Runway 28R segment which would allow flight crews time to set up and brief the specific runway while still above 10;000 FT. The late runway change; especially late at night; in the current NEXTGEN atmosphere is counter to the NEXTGEN design philosophy of lowering controller and pilot workloads.



Synopsis: An air carrier First Officer reported that while flying the SFO BDEGA 1 RNAV RUNWAY 28L ATC advised after BRIXX their landing runway would be 28R. The Runway change created a very heavy FMS workload late in the RNAV arrival below 10;000 FT.