Widebody Transport
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 184; Flight Crew Total 12293; Flight Crew Type 4290
Confusion; Situational Awareness; Workload
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
On arrival into DFW we requested R36L from Center and Approach but were initially cleared for R35C; BRDJE3 RNAV. We were taken off the RNAV STAR and given vectors twice; once told to resume the arrival; the second time to plan for R36L as requested. The workload became task saturated when we were re-cleared for R36L on downwind vectors and although we had initially planned for and briefed that approach; we had to switch the FMC to R35C for a short time; then change everything back and double-check everything; in close to the airport environment; morning rush on a [weekday].We handled everything; however; just before being cleared for the visual; while on vectors to final intercept of the localizer; ATC queried our altitude; I looked up to see we were about 2;500 MSL with 2;000 MSL in the MCP. I had thought the last clearance was for 3;000 MSL; but the Captain and pilot flying had heard 2;000 MSL; had set that in the MCP and was about to level there to shoot the visual approach. We had both just confirmed field in sight to each other; but I hadn't been able to report it in sight to ATC because of frequency congestion; before ATC made the call to us about our altitude. As soon as ATC questioned it; we reported the field in sight and he immediately cleared us for the visual and changed us to Tower frequency.During this slight confusion the pilot flying started to make a correction to climb back to 3;000 MSL; but changed back to 1;500 MSL as soon as the visual clearance was received. All the while he was configuring flaps; gear; and speed; and I had to check in with Tower Control. As soon as there was a break I interrupted the pilot flying as to why he was continuing to descend; well below glide path; and he should just level and catch the path; which he eventually did.At 1;000 MSL we were stable; and so we continued to an uneventful landing and taxi in. However; as usual; ATC changed our cleared arrival; vectored us off the approach; then back on; then a runway change; more vectors; speed control; altitude changes; all below 10;000 MSL and within the busy Class B airspace; on a perfectly good RNAV approach; increasing the workload in the cockpit (only a 2 man crew today). While we complied with all these changes; perhaps we should have not made the 3rd request for our landing Runway 36L; because by that time we were close to turning base; there were thunderstorms in the area as well as low cumulus and haze which made it difficult to see the runway until closer in on final. In my effort to confirm the approach was properly loaded; tuned and identified; change minimums and appropriate frequencies and help configure for the landing; my awareness of the altitude selected was compromised for less than a minute; but that was enough to descend about 500 feet below the assigned altitude before I had a chance to declare the field in sight and obtain the visual approach clearance. Had we left R35C in as the landing runway; or had ATC granted our previous requests much earlier; our workload would have been less intense and the confusion on maintaining altitude/cleared for the visual approach would not have been so confusing for the pilot flying and I would not have been so task saturated that I missed his early descent before receiving the actual visual approach clearance.
Air carrier First Officer reported numerous changes to the arrival into DFW; which contributed to an altitude deviation; confusion; and task saturation.