MDT CLBS ABOVE ASSIGNED ALT IN RESPONSE TO A GPWS ALARM.

Narrative

THIS EVENT WOULD BE COMMONLY DESCRIBED AS A 'NUISANCE' GPWS. WE WERE BEING VECTORED IN IMC FOR AN ILS APCH TO RWY 13 AT MDT (HARRISBURG INTL ARPT). WE WERE ON THE N SIDE OF THE LOC; 3-4 NM N OF THE HAR VOR; LEVEL AT 3000 FT; ON A SOUTHERLY HDG. WE WERE BOTH WELL AWARE OF THE HIGH TERRAIN IN THE IMMEDIATE VICINITY -- A RIDGE LINE N OF THE ARPT RUNNING ENE TO WSW; AVERAGE ELEVATION; 1800 FT. WE WERE ALSO WELL AWARE THAT 3000 FT IS THE NORMAL VECTORING ALT IN THIS AREA (MDT IS OUR HOME BASE). JUST AS THE CTLR ISSUED A NEW VECTOR TO INTERCEPT THE LOC; WE GOT A GPWS 'TERRAIN; PULL UP' WARNING. I RESPONDED BY SETTING MAX PWR AND INITIATING A MAX EFFORT CLB. AT ABOUT 3700 FT; THE GPWS WARNING STOPPED AND I LEVELED OFF AT ABOUT 3800 FT. MY COPLT SIMULTANEOUSLY RESPONDED TO THE CTLR THAT WE WERE CLBING STRAIGHT AHEAD DUE TO A GPWS WARNING AND WE WOULD NEED A NEW VECTOR FOR THE INTERCEPT. WE SUBSEQUENTLY DSNDED BACK TO 3000 FT; GOT A NEW VECTOR; INTERCEPTED THE LOC AND GS AND COMPLETED THE APCH UNEVENTFULLY. THE CTLR DID NOT QUESTION OUR ALT EXCURSION; BUT HE COMMENTED THAT TO HIM WE APPEARED WELL CLR OF ANY TERRAIN. (WE DID HAVE MODE C.) WE AGREED WITH HIM BUT CLRLY STATED THAT WE WERE REQUIRED TO RESPOND TO THE GPWS. THERE SEEMS TO BE SOME TECHNOLOGY GENERATED PLT/CTLR FRICTION HERE (SIMILAR TO TCASII RA ALT CHANGES) WHEN CTLRS DON'T 'SEE' THE CONDITIONS THAT SOMETIMES TRIP THE WARNING DEVICES.