B757 Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 144; Flight Crew Total 5404; Flight Crew Type 2157
Situational Awareness; Confusion
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Airspace Structure; Human Factors; Procedure
Class B airspace is from Ground to 10;000 ft. with caveats to altitude and distance based off of the VOR. We were given an ATC clearance to descend from 6;000 ft. to 1;600 ft. while on the downwind leg of the RNAV arrival. An RNAV arrival waypoint takes you out of Class B airspace at 1;600 ft. if a turn towards final approach is not completed before 20 DME based off of the VOR. We had descended to no lower than 2;700 ft. just passed 20 DME off of the VOR when ATC gave us an approximate turn to final of heading 270 degrees. Almost simultaneously it appeared that ATC realized the mistake in the clearance given and stated (paraphrasing); 'You will exist Class B airspace and will re-enter Class B airspace inbound on approach to final inside of 20 DME off of the.'Additionally; and almost simultaneously; we realized the ATC clearance to descend and maintain 1;600 ft. would put us out of Class B airspace if an ATC clearance to turn toward final was not obtained prior to extending past 20 DME off of the VOR at which time we requested to climb back to 3;000 ft. again as NOT to go below Class B airspace. In the end; again we ended up descending to no lower than 2;700 ft. just outside of the 20 DME. You could tell in the Controller's voice that they understood they had made a mistake so when we requested to climb back to 3;000 ft. to join final approach to Runway 35R the clearance was immediately granted. The remainder of the approach to final and landing on Runway 35R was uneventful.This is a classic setup for a Class B airspace bust based on ATC Controller clearance; and/or if the operating pilots are not aware of the Class B airspace altitude and distance restrictions.
I slowed our descent and asked FO to inquire with ATC if they were planning to gives us a turn to the West to remain in Class B airspace.  The frequency was congested; and I saw we were rapidly approaching the edge of Class B.  Almost; simultaneously we were issued a turn to 270 as we were descending through 3000 ft. (the floor of the Class B airspace beyond ORL20NM). By the time we were in the turn; I felt we were departing Class B airspace; therefore; I initiated a climb back up to 3000 ft. and requested FO to tell ATC we were going to return to and maintain 3000 ft.
B757 flight crew reported that they were issued vectors that would take them outside the Class B airspace and resulted in them deviating from their assigned altitude.