Date: 201509
Local time of day: 1801-2400
Location: OAK.Airport, CA
Flight conditions: VMC
Light: Night
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E NCT
ATC / advisory: TRACON NCT
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Distraction
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: On descent we were cleared to descend via the EMZOH 1 arrival to 4100 msl. FO (PF) had briefed the arrival and approach and we had both confirmed all altitude/speed restrictions were in the FMS. We began our descent and were in VNAV PTH. We were then given a speed reduction to 250 KIAS. PF opened the speed window and selected 250. I was looking through the HUD and was switching between EVAS and regular vision when I remarked that we seemed closer to the terrain than I normally thought. We then both noticed that ALL of our altitude restrictions on the arrival had deleted and we were approx. 1000 ft below the profile. We leveled off and informed Norcal who seemed to have no issue with us. They gave us a descent to 5600 MSL and a heading of 280 for the visual at OAK. FO and I extensively debriefed the event and neither of us can come up with an answer of why the restrictions were deleted. I have a call into a Check Airman to discuss this further as I can't figure this one out. Lesson learned is for me to spend less time in the HUD as PM and more time monitoring our flight path.


Second narrative: Approaching MYNEE waypoint aircraft descended below altitude restriction of 7000 ft MSL. All altitude restrictions were somehow deleted during descent and wasn't noticed until descending towards terrain; altitude set in ALT Selector was 4100. Aircraft descent was arrested at approximately 5700 ft MSL and a climb was started. ATC was advised and we were given clearance to 5600 ft and given heading off of STAR and towards the airport. We are not sure why altitude restrictions were removed during the descent. FMC was properly programmed and arrival was briefed. Aircraft was on VNAV PATH and was descending normally. We were given speed restriction of 250 knots and the speed intervene window was opened and set at 250 prior to 10;000 ft MSL. Altitude deviation wasn't noticed until I as flying pilot thought we were getting too close to terrain south of the airport and a discrepancy was noted when altitude was compared to STAR. Autopilot was engaged with LNAV and VNAV selected. Both pilots were looking outside for aircraft and the landing airport; expecting a visual approach. Cross checking altitude restrictions by both pilots would have alerted us to the FMC dropping the restrictions while we descended on the STAR.


Synopsis: B-757 flight crew descended below the prescribed altitude on the arrival after all altitude limits and restrictions were deleted in the FMC for the EMZOH1 STAR being flown. Crew received new clearance from ATC after visually noticing the terrain proximity.