B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion; Distraction; Situational Awareness
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Human Factors; Procedure
While at an intermediate altitude during arrival into LAX; we set up for a lavatory break prior to landing.  The aircraft was in autopilot control with LNAV/VNAV selected. I left the flight deck and returned promptly. Upon return the Captain advised that we were in a descent to FL290 and had selected vertical speed for the descent. Subsequently; we were issued a clearance to proceed to SALYY and descend via the ANJLL1 arrival. No altitude was issued to cross SALYY so I expected to cross at the assigned altitude. I selected SALYY and confirmed LNAV was selected. We confirmed the correct altitude was set in the FMC for each of the crossings and then set 12000' for the bottom altitude of the arrival. I failed to recognize or process the system was in V/S descent -1000 fpm and the aircraft continued to descend through the initial altitude that had been issued. I recognized the error passing 300' below the assigned altitude and toggled off the autopilot to initiate a climb to our assigned altitude while simultaneously asking the captain to confirm our altitude assignment. I believe we did not descend more than 500' below the assigned altitude and expeditiously corrected the error. Aircraft was hand flown to SALYY and at that point I reengaged the autopilot and commenced the arrival.
10+ miles to SALYY intersection ATC clearance was to descend to FL290 cleared for the ANJLL 1 arrival at SALYY. SALYY does not have crossing restriction so FL290 was not loaded in FMC. 12000 feet dialed in altitude window for bottom altitude of arrival. Reached FL290 before SALYY. Because 12000 feet was set and no altitude protection at SALYY in FMC; Aircraft descended below FL290 by approx. 300 feet. Crew caught error and climbed back to FL290; crossed SALYY within 45 seconds and continued to descend via arrival.
B757 crew reported descending below cleared altitude on the LAX ANJLL 1 RNAV because Vertical Speed (VS) was selected.