B737 Next Generation Undifferentiated
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Deviation - Track / Heading All Types; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Human Factors
LENAR 4 Arrival for ILS DME RWY 07. Cleared to OMAXI descending to 3;000 FT. Cleared for the approach. Intercepted OMAXI on LNAV and selected 1;800 FT for ANAPA crossing. Selected LOC as we crossed OMAXI (waited until between SIA 278 and 218 radials per note on 10-7B-1). Received a warning CAUTION TERRAIN. Disconnected autopilot and verified aircraft position. We were slightly north (left) of localizer and above 1;800 feet for ANAPA crossing. Continued to hand fly a visual approach; intercepted glide slope at ANAPA and landed RWY 07. We are unsure why we received a warning when we were at proper altitudes and north of localizer which should have kept clear of all terrain.
During arrival in MKJS aircraft was on VNAV path and direct to OMAXI. Prior to OMAXI 1;800 feet was selected on MCP to use VNAV guidance until the FAF. During the smart turn to final approach course after OMAXI the Captain selected VOR/LOC to use the LOC lateral guidance. A few seconds later the caution terrain alert sounded for a few seconds. I looked at my EFIS annunciator and realized the aircraft was not in path anymore and was on SPD mode. The altitude was between 3;000 feet (OMAXI altitude) and 1;800 feet (for ANAPA); maybe slightly below the plan path but [I] do not remember exactly at what altitude. Aircraft was on correct course on LOC track. The Captain immediately climbed a little bit and steered the aircraft a few degrees toward the left of course towards the water as a precaution and the alert went away [immediately]. I believe that when VOR/LOC mode was selected the aircraft reverted from path mode to SPD mode and got a little bit lower than the planned descent path prior to the FAF and that triggered the caution terrain a for a few seconds. I realize the importance of the pilot monitoring being on the constant look out for sudden EFIS mode changes and making sure the aircraft is complying with the correct altitudes and courses since the Pilot Monitoring has a broader picture of what is going on with the aircraft as a whole.
B737 flight crew reported receiving a GPWS terrain alert on approach to MKJS.