A B737 flight crew descended prior to crossing an RNAV STAR fix at the required altitude. The captai...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1059700
Time
Date	201301
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

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.