B737-400
Flight Crew Air Transport Pilot (ATP)
Training / Qualification; Situational Awareness; Human-Machine Interface; Confusion; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Procedure; Human Factors; Aircraft
ATC cleared us to descend via the YYYYY RNAV STAR; South transition maintain 250 KIAS until YYYY1. After the Captain read back the clearance she advised me she had never done a descent via on the YYYYY STAR and asked what she needed to do. I explained the procedure and set 6;000 FT into the MCP and 250 speed restrictions into the FMS; selected LNAV/VNAV on the MCP and brought up the descent page on the right side to monitor the arrival. Then the Captain typed something into the FMS and said 'the FMS is not doing what I want.' She then; as the Pilot Monitoring; reached up and hit level change and the aircraft nosed over and started a descent out of 15;000 FT 5 NM prior to YYYY2. I advised we couldn't descend until YYYY2; disengaged the autopilot and climbed back up to 15;000 FT. After doing so I reset the MCP; engaged the autopilot and explained to the Captain the FMS logic; talking through the rest of the arrival to put the Captain's mind at ease. At the lowest point the aircraft had descended to 14;400 FT. The flight continued without incident to ZZZ. The altitude deviation was caused by a lack of CRM when the Captain; as the pilot not flying; adjusted the MCP without conferring with the pilot flying while the autopilot was engaged. The Captain's inexperience with 'Descend Via' arrivals; FMS management and poor situational awareness also contributed to the Altitude Deviation. Increased emphasis placed on Descend Via approaches in recurrent training.
A B737 flight crew descended prior to crossing an RNAV STAR fix at the required altitude. The Captain's inexperience was cited as a contributing factor.