Improperly monitored VNAV performance resulted in an altitude bust and a cftt incident for a B737-ng...

Attributes
ACN	948903
Time
Date	201105
Local Time Of Day	1201-1800
Place
Locale Reference	GEG.Airport
State Reference	WA
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Make Model Name	Medium Large Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Pilot Flying First Officer
Experience	Flight Crew Last 90 Days 205
Person 2
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 198
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Overshoot Inflight Event / Encounter CFTT / CFIT

Narrative:

I was the pilot flying. Our clearance was direct wosax at or above 4;300 ft and cleared for the RNAV (rnp) Z runway 25 approach. In LNAV and VNAV path modes; just prior to wosax and approximately 200-400 ft above 4;300 MSL; I set 3;400 ft in the MCP window. Believing we had not met all the criteria to set zero in the MCP altitude; but wanting the descent to continue after wosax; I set the glidepath intercept altitude at warki. My intention was for the intermediate altitudes to be controlled by VNAV path. A short time later spokane approach indicated we were 400 ft below the published segment altitude. I reset the MCP to the correct altitude and climbed in vertical speed mode. I reengaged VNAV path and we continued the approach uneventfully. I believe VNAV path reverted to vertical speed mode when I set the MCP altitude to 3;400 ft. I failed to notice the mode change and we descended towards 3;400 ft earlier than allowed for that segment of the approach.I need to constantly verify [that] the aircraft is doing what I think I programmed it to do. I need to scan the FMA regularly for mode changes. I [also] need to do more rnp approaches to improve my proficiency.

Original NASA ASRS Text

Title: Improperly monitored VNAV performance resulted in an altitude bust and a CFTT incident for a B737-NG flight crew inbound to GEG.

Narrative: I was the pilot flying. Our clearance was direct WOSAX at or above 4;300 FT and cleared for the RNAV (RNP) Z Runway 25 approach. In LNAV and VNAV path modes; just prior to WOSAX and approximately 200-400 FT above 4;300 MSL; I set 3;400 FT in the MCP window. Believing we had not met all the criteria to set zero in the MCP altitude; but wanting the descent to continue after WOSAX; I set the glidepath intercept altitude at WARKI. My intention was for the intermediate altitudes to be controlled by VNAV path. A short time later Spokane Approach indicated we were 400 FT below the published segment altitude. I reset the MCP to the correct altitude and climbed in vertical speed mode. I reengaged VNAV path and we continued the approach uneventfully. I believe VNAV path reverted to vertical speed mode when I set the MCP altitude to 3;400 FT. I failed to notice the mode change and we descended towards 3;400 FT earlier than allowed for that segment of the approach.I need to constantly verify [that] the aircraft is doing what I think I programmed it to do. I need to scan the FMA regularly for mode changes. I [also] need to do more RNP approaches to improve my proficiency.

Data retrieved from NASA's ASRS site as of April 2012 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.

Improperly monitored VNAV performance resulted in an altitude bust and a cftt incident for a B737-ng flight crew inbound to geg.