Date: 201508
Local time of day: 1801-2400
Location: PIT.Airport, PA
Flight conditions: VMC
Light: Dusk
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B PIT
ATC / advisory: TRACON PIT
Aircraft component: GPWS
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Airport; Aircraft; Human Factors
Primary problem: Ambiguous

Narrative: Approximately 11-13 miles on downwind; Approach cleared us to follow aircraft abeam us for the visual. We turned base at 4000 FT. In heading mode; Approach said to 'make room for traffic following as well as departing on runway.' A little confusing (non-standard); but no issue. I took it off autopilot and hand flew while still looking for traffic. Overshot the final as a result and the darkness. We maintained 4000 FT until we got back on runway C/L. GS was OTS on ATIS. So we were backing up visual with RNAV/PAPI and Fix page. We started down on RWY C/L while configuring. Got the 'Too Low Terrain' Caution. I briefly strained to look outside while leveling with power. Couldn't make out terrain; and immediately called for a go-around. We told Tower the reason for the go-around. They cleared us to 3000 FT. We couldn't figure out the reason for the GPWS; but briefed the extra precaution we would take on the next approach. Again we turned right base and remained at last assigned 3000 FT (MSA 3100 FT) till assured on C/L. FO backed me up that we were inside the DME to start down. I looked at the VASI; showing all white. We configured and I told him I would stay a one red/three white and high as a precaution. We got a partial call briefly and again we ensured visually that we were safe. We both continued on the high side but stable at 1000 FT and Landing Checklist complete. On landing we debriefed everything; still unable to find the reason for the caution. This runway should not be made 'visual' approach at night time. The terrain on final could easily get a caution GPWS Warning. Strict step-down procedure is required.


Second narrative: Doing a visual to 28R backed up by GPS. Glideslope out of service. Crossing ZAIKO at 3000 FT we started down. Got Terrain Caution. We went around. Came back around and did another visual. Crossed ZAIKO at 3000 FT. PAPI looked good. We started down. Another Caution. We continued. We were on the center line both times.


Synopsis: B737 flight crew experienced a GPWS terrain warning during a night visual approach to Runway 28R at PIT and goes around. On the second approach more care was taken to remain on profile and a second GPWS warning was annunciated while showing three white over one red on the PAPI. The approach was continued to landing.