Air carrier flight crew reported receiving an aircraft terrain alert during approach. Flight crew corrected aircraft altitude and continued approach.
Synopsis
Air carrier flight crew reported receiving an aircraft terrain alert during approach. Flight crew corrected aircraft altitude and continued approach.
Narrative
Arriving in to BOI; in VMC conditions we cross IF DIKAC (RNAV 28L Zulu) expecting the visual approach to 28L. ATC asks if we have visual contact with Company aircraft ahead that was on left downwind for 28L. Both Captain and I agree that we have the aircraft to follow in sight. ATC clears us for visual approach; additionally to follow the aircraft ahead. I loaded 4;000 feet into MCP (Mode Control Panel) altitude preselect (I was expecting to intercept FAC outside FAF HOBSI); I began descent and adjusted pattern to follow aircraft ahead. For a moment I lost contact with traffic ahead and verbalized to that fact. Both Captain and I reacquired visual contact with the aircraft. Reacquiring the traffic cause a distraction. Aircraft was descending towards 4;000 feet; at approximately 4;200 feet GPWS issued 'caution terrain' one time. I disengaged automation and corrected altitude. TDZE is lower than general close in surrounding terrain. I had visual terrain contact at all times; however; 4;200 feet MSL at about 3-5 NM SE of HOBSI FAF resulted in a GPWS caution. I corrected altitude; maintained appropriate following distance; intercepted the approach course. We configured per Company SOP; established stable flight for descent and landed the aircraft. Captain and I debrief the situation during post flight discussion. The approach plate possibly led me to an inaccurate understanding of terrain risks outside of the final approach fix HOBSI. The profile view only presents the minimum crossing altitude at HOBSI whereas the plan view issues multiple crossing restrictions at fixes along the approach course. Hindsight: looking at the approach plate again; knowing I was going to be extended VFR downwind for traffic; I would have looked at those waypoint's further out and their associated crossing altitudes. I would have selected a higher altitude in the MCP.
Second reporter narrative
Arriving into BOI; in VMC conditions we cross IF DIKAC (RNAV 28L Zulu) expecting the Visual Approach to 28L. ATC asked if we had visual contact with Company. Cleared visual approach and instructed to follow the aircraft ahead. PF loaded 4;000 feet into MCP (Mode Control Panel) Altitude preselect (I was expecting to intercept FAC outside FAF HOBSI). PF began descent and adjusted pattern to follow aircraft ahead. Visual contact with traffic ahead was lost by PF. PM started to help PF re obtain the visual. Both Captain and PF re-acquired visual contact with the aircraft. The task of re-acquiring the traffic cause a distraction. Aircraft was descending towards 4;000 feet; at approximately 4;200 feet GPWS issued 'Caution Terrain' one time. PF disengaged automation and corrected altitude. TDZE is lower than general close-in surrounding terrain. Aircrew had visual terrain contact at times. The approach was continued after responding to the terrain caution. The approach plate possibly led Aircrew to an inaccurate understanding of terrain risks outside of the Final Approach Fix HOBSI. The Profile View only presents the minimum crossing altitude at HOBSI. The Planview issued multiple crossing restrictions at fixes along the approach course. Hindsight: there was a notice sent out months prior alerting Aircrews of this possible problem.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.