Date: 201707
Local time of day: 0601-1200
Location: DEN.Airport, CO
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B DEN
ATC / advisory: TRACON D01
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: On FRNCH 3 RNAV STAR; passing approximately FL190 we were vectored off the arrival; told to descend and maintain 13;000; and given a traffic call for parachuting over Longmont. I (PF) selected the assigned heading; selected HDG SEL; LVL CHNG; and visually acquired and pointed out the traffic. After clear of traffic; we were given a clearance direct ARCHY; descend via FRNCH 3. We also were given a runway assignment different than planned (RWY 34L assigned; RWY 35R planned). Direct ARCHY selected; verified and executed; LNAV selected and confirmed via FMA and incomplete FMA call out; 'Level Change' 11;000 ft selected in the altitude selector window in accordance with FRNCH 3 STAR; but VNAV was not selected. We were still in LVL CHNG. ERROR! There was a discussion between the FO and myself that it was strange that Denver was departing traffic to the South on the Eastern runways and landing North on the runways west of the field. We proceeded to select the new runway; brief the changes; and were prepared for the visual approach.  Another distraction occurred immediately after this when the Lead Flight Attendant called despite being in sterile. I answered the call since I thought we were all caught up.Approximately 8 miles prior to ARCHY; descending through approximately 12;500 ft; Approach Control directed us to climb to 13;000. We were both confused by this radio call and queried approach control twice before realizing our mistake. ARCHY has an AT OR ABOVE 13;000 ft altitude restriction that we flew through because I was still in LVL CHNG. We reset the MCP ALT to 13;000; I turned off the autopilot and climbed back to 13;000. The lowest altitude we reached was approximately 11;800 before climbing back to 13;000.  The rest of the arrival and visual approach/landing was hand flown in compliance with the FRNCH 3 and all other instructions. The approach controller told us he would not pursue the error further but that we need to pay closer attention the altitude restrictions. Bottom line; I failed to reselect VNAV after being recreated to descend via and I allowed multiple distractions to keep me from using good VVM.  We could have caught the error by taking the time to properly call out our FMA changes in their entirety; instead of just those that had changed. Then we would have noticed that we weren't protected in VNAV.




Synopsis: B737 Captain reported that on the FRNCH3 arrival into DEN they descended below the altitude for ARCHY due to improper automation management.