Air carrier flight crew reported inadvertently going below DEN's Class B after incorrectly leaving the FMGC in navigation mode instead of heading mode after reaching CLIFF.
Synopsis
Air carrier flight crew reported inadvertently going below DEN's Class B after incorrectly leaving the FMGC in navigation mode instead of heading mode after reaching CLIFF.
Narrative
ZZZ to DEN. Briefed the RNAV Z 16R approach since we were filed via the SSKII STAR and the transition from CLIFF was on ATIS as unavailable. Cleared to descend via the assigned STAR. When we were switched over to DEN Approach we were told to plan on the 16R Y RNAV approach. We requested the 16R RNAV Z (as we briefed) and were told to expect the RNAV Y 16R approach. We switched up the FMGC to the appropriate approach and briefed that. I briefed that since the CLIFF transition was not authorized we have to go to heading mode once we got to CLIFF. Listening to the aircraft ahead of us I could hear the controller giving them radar vectors for the approach and I assumed he would do the same. This is when I incorrectly left the FMGC in NAV mode and the plane started making the turn to the next point on the approach. This is when we queried the controller as to what he wanted us to do. He told us to turn immediately to a heading of 340 for downwind." We complied. We were eventually cleared for the RNAV Y 16R approach via vectors to intercept. Controller gave us another heading to fly for a better intercept and advised we were below the Class B at 7000 feet (assigned by ATC) and would re-enter the Class B shortly."
Second reporter narrative
We were on the SSKII 4 Arrival for RNAV Y 16L. We had talked about and briefed that it was not to be done from the CLFFF transition though it loaded that way in the box. We should have put a discontinuity in after CLFFF so it would continue on the 353 track. I thought about it but did not do it. I should have verbalized it. We expected we would get a vector after CLFFF. When we arrived at OTUNY and we started to turn I quickly asked what Approach wanted us to do and he stated to continue on the downwind. We had turned too far at this point and Approach instructed us to turn left immediately. I should have selected a 353 track for the Captain flying before we got to the turn. To add insult to injury I screwed up the inbound course when we went to extend from ORAGE as it was still linked to WAVAP on the CLFFF transition and not FESKA the straight in option. Knowing the final inbound course was 170 degrees when I went to extend ORAGE I saw the 340 outbound and screwed my math up thinking this was the reciprocal of 170 degrees. So we had to turn back again to the correct course of 169 inbound with a 349 radial in. A very experienced crew made some stupid mistakes.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.