Improperly monitored VNAV performance resulted in an altitude bust and a CFTT incident for a B737-NG flight crew inbound to GEG.
Synopsis
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.
Second reporter narrative
There were several factors that contributed to the situation. When we then requested the RNAV (RNP) Z Runway 25 [approach]; he cleared us direct to ZOOMR and cleared the approach which; at that point; required a hard left turn (which we started) followed by a hard right to WOSAX. When I mentioned that this would be difficult; he asked if we wanted something further down the line. I asked for direct WOSAX and he gave us direct WOSAX; cross WOSAX at or above 9;000 FT! When I looked at the 9;000 FT restriction and mentioned to ATC that I didn't think we could make the airport with that restriction; he gave us WOSAX at or above 4;300 FT (published is 4;200 FT). In addition; throughout all of this; the Approach Controller kept calling us by another carrier's call sign; adding to the confusion.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.