Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 45; Flight Crew Total 7714; Flight Crew Type 3785
Communication Breakdown; Situational Awareness; Confusion
Airspace Violation All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Airspace Structure; Human Factors
Descending via the NIIXX 2 arrival for the ILS Runway 35R in CAVOK conditions. Around the BSTON Intersection we were given a westerly to northwesterly radar vector and cleared to descend and maintain 10;000. As we got closer to the final approach course (somewhere between DEANE and DRUMM) we were cleared to descend and maintain 8;000 and cleared direct to the FRONZ Intersection (FAF for the ILS). The Controller asked if we had 35R in sight; after a pause; we had the runway in sight and advised ATC. We were then cleared for the visual approach to 35R. The FAF altitude at FRONZ is 7;000 so that is what we set in the MCP while descending through approximately 8;800 in FLCH. We had just gotten the '2;500' call from the Radio Altimeter and made our call outs. As we descended through 8;000 I noticed the Radio Altitude around 1;800 and descending. A quick look at JeppFD PRO showed our aircraft position just south and east of the DRUMM Intersection. I called out the altitude and the fact that we need to go back up to 8;000 (our last cleared altitude prior to the approach clearance) until we are north of DRUMM. I reset the MCP to 8;000 and verbalized that we need to climb back up. The Captain selected a V/S climb back to 8;000. We joined the approach course and followed the G/S and landed without incident.ATC never mentioned anything to us about violating airspace or going below the DEN Class B. Though it is nice to get directs; etc. However; I believe the direct to FRONZ clearance; if not really familiar with the DEN Class B; can set crews up to possibly violate airspace and even lead to an unstable approach.Once parked at the gate with all checklists complete; we debriefed and there certainly was some confusion as to exactly where we were when we descended below 8;000. We had a jump seat observer who thought we might have been a little low as he has a lot of General Aviation experience in the DEN area.
Air carrier First Officer reported an altitude deviation that may have put them below the Class B airspace during a visual approach to DEN airport.