LGT HAS GPWS ACTIVATE DURING MAP; THEN CLBS ABOVE ASSIGNED ALT.

Narrative

WE WERE CONDUCTING THE LDA-DME APCH TO RWY 18 AT DCA IN VARIABLE IFR CONDITIONS; WITH RAINSHOWERS IN THE VICINITY AND WITH THE POTOMAC RIVER IN SIGHT AS WE PASSED BELOW ABOUT 1500 FT MSL. BY THAT TIME WE WERE IN LNDG CONFIGN WITH ALL CHKLISTS AND SOP'S ACCOMPLISHED AND THE MISSED APCH ALT 1800 FT SET IN THE ALT REMINDER. AT ABOUT 1200 FT MSL THE CAPT DISCONNECTED THE AUTOPLT; AS WE WERE IN WHAT APPEARED TO BE IMPROVED CONDITIONS BELOW THE MAJORITY OF THE CLOUDS; AND WE CONTINUED TOWARD THE 720 FT MSL MDA ON LOC AND GS. AT ABOUT 1000 FT THE TWR CTLR ADVISED THAT A HVY RAINSHOWER HAD JUST MOVED OVER THE FIELD AND ASKED IF WE COULD SEE THE ARPT. I ADVISED THAT WE COULD NOT. AT THE MDA THE CAPT INITIATED THE MISSED APCH WITH TOGA THRUST AND A PITCH UP. WING FLAPS WERE CALLED FOR POS 1 AND I SELECTED POS 1 AND THEN WITH A POSITIVE RATE OF CLB THE GEAR WAS CALLED UP; WHICH I THEN SELECTED UP. I ADVISED THE TWR THAT WE HAD MISSED TO WHICH THE TWR INSTRUCTED US TO FLY THE 185 RADIAL (PUBLISHED MISS-DCA 185). WITHIN SECONDS THE GPWS 'TOO LOW TERRAIN' WARNING SOUNDED. VERT SPD WAS NEAR ZERO WITH THE ALT AT ABOUT 900 FT. THE CAPT APPLIED ADDITIONAL PITCH UP AND SIMULTANEOUSLY THE TWR CLRED US TO CLB TO 3000 FT. BEFORE THAT ALT COULD BE SET AND ACKNOWLEDGED THE ACFT WAS CLBING SO RAPIDLY THAT WE WENT ABOVE 4000 FT. THE TWR TOLD US TO CONTACT DEP CTL AND REITERATED THAT WE WERE TO MAINTAIN 3000 FT. BY THIS TIME THE OVERSHOOT WAS EVIDENT AND PWR AND PITCH ANGLE WERE REDUCED TO RETURN TO 3000 FT. IT IS MY RECOLLECTION THAT A R TURN WAS COMMENCED AT THE TIME OF PULL UP AND WE SHOULD HAVE REMAINED OVER THE ARPT AND WELL W OF THE RIVER'S E BANK. THE 'TOO LOW TERRAIN' WARNING IS THEREFORE A COMPLETE MYSTERY TO ME. WITH TOGA PWR SET AND A PITCH ATTITUDE OF AT LEAST 10 DEGS IT SEEMS INCONGRUOUS THAT WE WERE NOT ALREADY CLBING RAPIDLY AND WELL ABOVE 900 FT. WE WERE IN HVY RAIN AS WE EXECUTED THE MISSED APCH AND EVEN THOUGH WE HAD NO 'WIND SHEAR' ALERT; NO TURB; PERHAPS WE WERE CAUGHT IN A STRONG DOWNDRAFT OR VERT SHEAR THAT CAUSED THE ACFT TO CEASE CLBING FOR A BRIEF PERIOD. BUT THAT DOES NOT EXPLAIN THE 'TOO LOW TERRAIN' EITHER. CLRLY THE ADDITIONAL PITCH APPLIED AT THE ONSET OF THE 'TOO LOW TERRAIN' WARNING WAS EXCESSIVE FOR NORMAL CIRCUMSTANCES BUT WITH THE URGENCY OF THE MOMENT IT WAS THE ONLY APPROPRIATE ACTION. IT IS MY OPINION THAT WITH THAT MUCH THRUST AND THAT MUCH PITCH THAT IT WOULD HAVE BEEN VERY DIFFICULT TO STOP THE CLB AT 3000 FT UNDER A MORE NORMAL CIRCUMSTANCE. AFTER APPROX 12-15 MINS WE WERE VECTORED FOR THE 36 ILS AND LANDED IN BASICALLY VFR CONDITIONS WITH RAINSHOWERS STILL SCATTERED IN THE VICINITY.