B737-700
Workload; Human-Machine Interface; Distraction; Confusion; Time Pressure
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Chart Or Publication; Aircraft; Company Policy; Procedure; Human Factors
We were originally cleared to DEN via the MOLTN RNAV STAR. We were cleared to 'Descend Via' and were in VNAV Path.At about 16;000 near the TOTTT Intersection ATC advised us to expect radar vectors to the east side of the airport for a downwind and approach to [Runway] 35R. They then did so and also cleared us to descend to 11;000.The third ATC clearance change took place when they cleared us direct to FFFAT [a waypoint on the ANCHR arrival] and [to fly] the remainder of the ANCHR RNAV STAR. We proceeded direct to FFFAT in LNAV. We were just about over FFFAT and a few hundred feet above our 11;000 cleared altitude when ATC issued the fourth clearance change and told us to 'Descend Via' the ANCHR arrival.As per current technique on complex RNAV arrivals we set 9;000 [the ultimate low/hard altitude at DONTT in the ANCHR STAR] in the MCP Altitude window. However; our next fix after FFFAT; DOGGG; had a hard crossing altitude of 11;000. Because of multiple ATC route clearance changes; [two different] 'Descend Via' clearances and [the associated] high workload we failed to note the FMS was not in VNAV Path and descended below 11;000 to about 10;600 approaching DOGGG. We realized our mistake about the same time that ATC cleared us to level off at 10;000. We didn't conflict with any traffic and never received an ATC traffic call; TA or RA.The cause of this event was multiple ATC arrival clearance changes in a short amount of time combined with the new; overly complex RNAV arrival procedures (too many fixes with too many speed and altitude restrictions on the new procedures; which create a much higher pilot workload). Getting cleared on the MOLTN arrival; then getting vectors with a hard descent altitude; then getting recleared to fly the ANCHR (which is intended for arrivals from the east) and a clearance to 'Descend Via' lead us to make an automation error. When ATC modifies/switches RNAV arrivals in a short amount of time; errors can occur because of the increased workload created by looking up the new arrival; reprogramming the FMC and briefing the new procedure. When we were cleared to 'Descend Via' the ANCHR1 arrival we should have verified that we were in VNAV Path before resetting the altitude alerter from 11;000 to 9;000; the bottom altitude on the arrival. The normal arrival plan into Denver for the last past years (for me); has been to fly an arrival to the west side of the airport and then get a base leg vector to an approach on the west or east side. Making multiple clearance changes and clearing us from one complex RNAV STAR to another complex RNAV STAR [intended for arrivals] from the east (which we never fly) doesn't make sense to me. ATC should have left us on the MOLTN STAR or vectored us.
I set 9;000 in the MCP; the lowest altitude on the new arrival; as we routinely use this technique on RNAV arrivals with many step down altitudes. However; because we were vectored off an arrival; I came out of VNAV path descent mode and selected LVL CHG while on the heading. Because ATC changed our arrival plan multiple times and cleared us on a new RNAV arrival with very little lead time to look up new arrival; reprogram the FMC and brief; we used an inappropriate descent mode [when doing so was combined with] setting the lowest altitude on the arrival. The complexity of many of the new RNAV arrivals--with many step down altitudes and speed restrictions--increase pilot workload and when ATC switches RNAV arrivals with short notice errors can occur.
After receiving multiple ATC changes to their arrival routing; including two different descend via clearances for RNAV STARs intended for arrivals from both the east and west of the airport; the flight crew of a B737-700 failed to comply with the 'at 11;000 FT' crossing restriction at DOGGG waypoint on the ANCHR RNAV STAR to DEN.