B757 crew reported descending below cleared altitude on the lax anjll 1 RNAV because vertical speed ...

Narrative:

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/south 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.

Original NASA ASRS Text

Title: B757 crew reported descending below cleared altitude on the LAX ANJLL 1 RNAV because Vertical Speed (VS) was selected.

Narrative: 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.

Data retrieved from NASA's ASRS site 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	1443073
Time
Date	201704
Local Time Of Day	0001-0600
Place
Locale Reference	ZLA.ARTCC
State Reference	CA
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Direct STAR ANJLL 1
Flight Plan	IFR
Component
Aircraft Component	Altitude Alert
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Overshoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

B757 crew reported descending below cleared altitude on the lax anjll 1 RNAV because vertical speed (vs) was selected.