B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 167; Flight Crew Type 20000
Confusion; Distraction; Workload
ATC Issue All Types; Airspace Violation All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Chart Or Publication; Procedure
During descent on the SEFER  2 RNAV Arrival between EPICK and EDDYY; NorCal Approach told us we were outside of Class B airspace and then a few seconds later notified us we were back in. It appeared that when the aircraft was fully coupled in LNAV and VNAV; the FMC continued the descent to 6;000 feet descending below 8;000 feet; 21 miles south of the SFO VOR; which placed us below the Class B for about a mile at 250 knots. The Arrival was fully briefed and loaded properly. This appears to be an anomaly in the arrival and FMC software limitations that allows the descent below Class B to happen.Reworking the FMC software or going out of VNAV until the Class B restriction is made or slowing on the arrival to 200KTS on the arrival prior to going out of class B.
There was no known violation or deviation just confusion. We were told to 'descend via the SERFR 2 Arrival expect the TIPP TOE visual.' If you descend via the SERFR you will exit the SFO Class B airspace. The TIPP TOE visual has a different altitude for MENLO versus the arrival. MENLO starts the TIPP TOE (5;000 feet for the Class B) and it is the last point on the SERFR (4;000 feet). The airspace versus altitude is confusing.
A B737NG crew reported descending below the SFO Class B floor flying the SERFR RNAV STAR. NORCAL alerted about the deviation but the crew had been unaware of the Class B boundary which was not depicted in either the FMS or on the SERFR chart.