An international flight crew reported the aircraft started to descend early on the sfo RNAV 10L appr...

Narrative:

On arrival into sfo we were planning an RNAV approach to runway 10L as was stated on the ATIS. We started briefing the approach about an hour out of sfo using the approach briefing guide. As we approached sfo; norcal changed the approach to the RNAV (rnp) Z 10R approach. We advised norcal that we were unable an rnp approach and we requested the RNAV approach 10L which he granted.norcal gave us a vector to intercept the 10L final approach course (facility) inbound to normm. After intercepting the facility in LNAV and acquiring altitude cap at 3500 feet; we selected 100 feet in the altitude window; opened VNAV (it engaged in VNAV pth); and selected our speed of 180 knots in the speed window. I thought we met [company] requirements so we selected tdze. We were in VMC with the airport in sight the altimeter was 29.32' and the winds were 030/16kts. Prior to arriving over normm the airplane started a slow descent with the autopilot on. We disconnected the autopilot and tried to climb back to 3500 feet at normm to intercept the proper glide slope. We continued the approach in VMC with the airport in sight. Approaching xattu we got low again and received one 'terrain warning'. We immediately got back on glideslope and landed. The approach was just before daylight so we should of executed the proper ground proximity warning system (GPWS) profile starting with 'max thrust; stow speed brakes' when we received the 'terrain warning'.a contributing factor was the PAPI on 10L not being turned on. Tower said it showed on but the only PAPI working was for 10R.

Original NASA ASRS Text

Title: An international flight crew reported the aircraft started to descend early on the SFO RNAV 10L approach after setting 100 feet in the altitude window. After returning to the glideslope; the aircraft descended again followed by a TERRAIN WARNING.

Narrative: On arrival into SFO we were planning an RNAV approach to Runway 10L as was stated on the ATIS. We started briefing the approach about an hour out of SFO using the Approach Briefing Guide. As we approached SFO; NORCAL changed the approach to the RNAV (RNP) Z 10R approach. We advised NORCAL that we were unable an RNP approach and we requested the RNAV approach 10L which he granted.NORCAL gave us a vector to intercept the 10L Final Approach Course (FAC) inbound to NORMM. After intercepting the FAC in LNAV and acquiring ALT CAP at 3500 feet; we selected 100 feet in the altitude window; opened VNAV (it engaged in VNAV PTH); and selected our speed of 180 knots in the speed window. I thought we met [company] requirements so we selected TDZE. We were in VMC with the airport in sight the altimeter was 29.32' and the winds were 030/16kts. Prior to arriving over NORMM the airplane started a slow descent with the autopilot on. We disconnected the autopilot and tried to climb back to 3500 feet at NORMM to intercept the proper glide slope. We continued the approach in VMC with the airport in sight. Approaching XATTU we got low again and received one 'Terrain Warning'. We immediately got back on glideslope and landed. The approach was just before daylight so we should of executed the proper Ground Proximity Warning System (GPWS) profile starting with 'max thrust; stow speed brakes' when we received the 'Terrain Warning'.A contributing factor was the PAPI on 10L not being turned on. Tower said it showed on but the only PAPI working was for 10R.

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	1425191
Time
Date	201702
Local Time Of Day	0601-1200
Place
Locale Reference	NCT.TRACON
State Reference	CA
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	Widebody Low Wing 2 Turbojet Eng
Flight Phase	Final Approach
Component
Aircraft Component	FMS/FMC
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Total 15432 Flight Crew Type 9633
Person 2
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter Weather / Turbulence

An international flight crew reported the aircraft started to descend early on the sfo RNAV 10L approach after setting 100 feet in the altitude window. After returning to the glideslope; the aircraft descended again followed by a terrain warning.