Narrative:

We were cleared to descend via the serfr 1 RNAV arrival; except to maintain 8000 ft after epick. We had already discussed that this arrival does not necessarily keep the aircraft at or above the floor of class B airspace and that exceeding 200 KIAS below the class B was possible if ATC gave us a clearance below 8000 before 25 NM from sfo. This was identified as a threat and we mitigated it by using the FMS fix page to draw a ring 25 NM from sfo; and later a ring 30 NM from sfo. Before reaching 8000; ATC then cleared us to 7000 as a 'hard altitude' roughly 35 NM from sfo. I adjusted the descent rate of the aircraft to stay at or above the floor of the class B airspace so that we could maintain the assigned speed on the RNAV arrival (280 KT slowing to 240 KT during this segment). However; when we crossed 30 NM from sfo; I communicated I was continuing the descent to 7000 to the first officer and incorrectly began the descent 5 NM too soon to remain at or above the floor of the class B. The first officer caught my mistake at about 7700 and I leveled the aircraft until reaching 25 NM from sfo before resuming the descent. The rest of the arrival was completed without incident. The root cause was my error with situational awareness in beginning the descent. Contributing to the event was the RNAV procedure design and an ATC clearance that blindly followed would place the aircraft below the class B airspace.I should have explicitly verbalized the class B floor altitude with the first officer before continuing the descent. That would have provided another opportunity to avoid the situation. Also; I should have challenged ATC about the descent clearance instead of trying to 'make it work'. I am very familiar with arriving from the south into sfo and have had several clearances over the years that place the aircraft below class B airspace. My familiarity with operations there has lead to a habit of monitoring the the aircraft's position relative to the airspace and then making adjustments after a clearance is received to comply with fars. Last; the serfr 1 RNAV procedure could be modified to depict and respect the confines of the class B airspace. At this time; this specific procedure is the only arrival I know of into a primary class B airport that does [not] keep the aircraft at or above the floor of class B airspace.

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Original NASA ASRS Text

Title: An Air Carrier on the SFO SERFR ONE RNAV was cleared to 6;000 FT approximately 35NM from SFO which is below the SFO Class B 8;000 FT shelf. The Captain delayed to the descent at 7;700 FT to remain near the Class B before descending to 6;000 FT at the 240KT mandated speed.

Narrative: We were cleared to descend via the SERFR 1 RNAV arrival; except to maintain 8000 FT after EPICK. We had already discussed that this arrival does not necessarily keep the aircraft at or above the floor of Class B airspace and that exceeding 200 KIAS below the Class B was possible if ATC gave us a clearance below 8000 before 25 NM from SFO. This was identified as a threat and we mitigated it by using the FMS FIX page to draw a ring 25 NM from SFO; and later a ring 30 NM from SFO. Before reaching 8000; ATC then cleared us to 7000 as a 'hard altitude' roughly 35 NM from SFO. I adjusted the descent rate of the aircraft to stay at or above the floor of the Class B airspace so that we could maintain the assigned speed on the RNAV arrival (280 KT slowing to 240 KT during this segment). However; when we crossed 30 NM from SFO; I communicated I was continuing the descent to 7000 to the First Officer and incorrectly began the descent 5 NM too soon to remain at or above the floor of the Class B. The First Officer caught my mistake at about 7700 and I leveled the aircraft until reaching 25 NM from SFO before resuming the descent. The rest of the arrival was completed without incident. The root cause was my error with situational awareness in beginning the descent. Contributing to the event was the RNAV procedure design and an ATC clearance that blindly followed would place the aircraft below the Class B Airspace.I should have explicitly verbalized the Class B floor altitude with the First Officer before continuing the descent. That would have provided another opportunity to avoid the situation. Also; I should have challenged ATC about the descent clearance instead of trying to 'make it work'. I am very familiar with arriving from the south into SFO and have had several clearances over the years that place the aircraft below Class B airspace. My familiarity with operations there has lead to a habit of monitoring the the aircraft's position relative to the airspace and then making adjustments after a clearance is received to comply with FARs. Last; the SERFR 1 RNAV procedure could be modified to depict and respect the confines of the Class B airspace. At this time; this specific procedure is the only arrival I know of into a primary Class B airport that does [not] keep the aircraft at or above the floor of Class B airspace.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.