B757-200 FLT CREW REACTS AGGRESSIVELY TO APPARENTLY FALSE GPWS TERRAIN WARNING WHILE MANEUVERING AT 6000 MSL NEAR IAD.

Narrative

DURING OUR INITIAL DSCNT INTO IAD; WE WERE LEVEL AT 6000 FT WHEN THE CAPT'S RADIO ALTIMETER STARTED 'COMING ALIVE.' WE THEN RECEIVED AN AURAL 'TERRAIN; TERRAIN; PULL UP; PULL UP.' ALTHOUGH WE BOTH ARE FAMILIAR WITH THE AREA AND KNEW WE WERE NOT NEAR TERRAIN; BEING IMC AND NIGHT; THEREFORE; UNABLE TO VISUALLY CONFIRM THE TERRAIN; WE COMPLIED WITH THE 'ESCAPE' MANEUVER AND RAPIDLY CLBED TO 11000 FT WHILE DECLARING AN EMER WITH ATC. ONCE WE COULD CONFIRM (VIA ATC AND A RADICAL CLB) THAT WE WERE ABSOLUTELY NOT GOING TO HIT TERRAIN; WE STABILIZED THE ACFT AND DETERMINED THAT THE LIKELY CAUSE OF THE TERRAIN WARNING WAS THE CAPT'S RADIO ALTIMETER; WE CONTINUED THE FLT. THE CAPT DESIGNATED ME 'PF;' AS WE COULD NOT TRUST HER INSTS. AS WE WERE INTERCEPTING THE LOC AT APPROX 5000 FT; WE AGAIN RECEIVED A 'TERRAIN; TOO LOW; TERRAIN' WARNING AND SELECTED THE GND PROX OVERRIDE SWITCH. WE CONTINUED ON THE APCH FOR AN UNEVENTFUL LNDG AND TAXI TO THE GATE. AS A SIDE NOTE; DURING CLB OUT OF ZZZ; THE CAPT'S EFIS BECAME INTERMITTENT AND SHE SELECTED ALTERNATE; AND ALL INSTRUMENTATION SEEMED NORMAL THROUGH THE ENTIRE FLT UP UNTIL THAT POINT.