Regional Jet 200 ER/LR (CRJ200)
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action
Aircraft
We were receiving vectors and altitudes from ATC for runway 5. Our final instruction from ATC was to descend to 3000 ft on a downwind heading. We reached 3000 ft and noted an ALTS CAP message on the FMA. During the descent; I briefed that an altitude of 3000 ft would keep us at least 1000 ft above the terrain from our approach direction. I referenced both the approach plate for the RNAV RWY 5 and the company reference page. After reaching 3000 ft; we reported the airport in sight and were cleared for the visual approach. The aircraft was slowing to flap extension speed and was in clean configuration. I initiated an approximately 20 degree right turn to stay inside the final approach fix. Approximately 15-30 seconds after beginning the turn we received a GPWS message and instructions to pull up. I was confused because I had visual reference to the terrain and was given an ATC descent to 3000 ft shortly prior. I disconnected the autopilot and initiated a shallow climb to approximately 3300 ft. The conflict was resolved. I continued a right base and began to configure for landing. We received an additional 2 terrain cautions that did not instruct to pull up. They each lasted a brief couple seconds. I continued the approach because the aircraft was stable and terrain clearance was assured visually and reinforced by my prior approach brief. The aircraft was stable and landed uneventfully.After talking with the Captain; we discovered that the CRJ GPWS system is triggered within 1250 ft while in flaps 0 configuration. This tolerance is reduced once flaps are lowered in preparation for landing. I believe that our final assigned altitude of 3000 ft put us within this 1250 ft range for some of the peaks of the surrounding terrain. It also explains the short duration of the GPWS alerts and the lack of GPWS alerts once the flaps were lowered out of 0.A shallow climb was initiated and power added. I did not accomplish a maximum performance escape maneuver because visual confirmation of terrain clearance was assured.I believe that there may be a discrepancy between the minimum altitude Charleston Approach can assign; the height of the terrain on downwind to runway 5; and the CRJs GPWS alerting range while in flaps 0. We received descent instructions to 3000 ft; which put us within as little as 1030 ft above the terrain.
CRJ-200 First Officer reported a GPWS alarm at an assigned altitude during an approach.