EXCURSION FROM ALT DURING A NIGHT APCH IN WX AT A MAJOR ARPT.

Narrative

INBOUND FROM SE OF ARPT; BEING VECTORED TO FINAL; WE COULD TELL APCH CTLR WAS BEING OVERWHELMED FROM THE TONE OF VOICE AND HIS RAPID INSTRUCTIONS. THE SUPVR (WE GUESS) WAS COMING ON AND GIVING INSTRUCTIONS TOO. INITIALLY WE WERE 3000 FT HEADING W (LNDG N; RWYS 36L/R AND RWY 27). CTR HAD US MAX SPD INBOUND AND WHEN TURNED OVER TO APCH WERE SLOWED TO 210 KIAS THEN 190 KIAS. ABOUT 3 MI FROM FINAL WE WERE TOLD TO SLOW TO 160 KTS AND GIVEN A NW HEADING TO INTERCEPT LOC FOR RWY 36R. ABOUT 30 SECONDS LATER; I'M NOT SURE BUT MIGHT HAVE BEEN SUPVR; TOLD US TO TURN BACK W FOR VECTORS TO RWY 36L. WE DID AND I WAS WORRIED THAT EVEN AT 15 DEG FLAPS; AT 160 KTS THE AUTOPLT WOULD NOT MAKE THE TURN TO FINAL; SO I DISENGAGED THE AUTOPLT. WE WERE GETTING SET FOR RWY 36L; I WAS BUSY TRYING TO MAINTAIN 160 KTS; I THEN NOTICED I WAS SINKING BELOW 3000 FT (APPROX 100 FT LOW) SO ADDED PWR AND PITCHED UP. THE CAPT SAID WATCH HARD ALT. WE WERE THEN ABOUT 200 FT LOW; I PITCHED UP MORE; AND DID NOT ADD PWR. WE WERE ABOVE 160 KTS; I REMEMBER SEEING 12 DEG PITCH UP SEVERAL TIMES BUT THE ACFT CONTINUED TO SINK TO MAYBE 250-300 FT LOW (2700-2850 FT). THE CAPT THEN TOOK CTL OF THE ACFT AND BROUGHT US UP TO 3000 FT. THE CTLR THEN TURNED US TO A N NW HEADING TO INTERCEPT THE RWY 36L LOC. WE OVERSHOT FINAL; BECAUSE OF THE SLUGGISH AIRPLANE OR LATE TURN; THE CTLR GAVE US. HEADING 010 DEGS TO RECAPTURE LOC; WE INTERCEPTED LOC; WERE GIVEN APCH CLRNC AND WHEN EVERYTHING WAS SETTLED DOWN THE CAPT GAVE ME BACK THE ACFT AND WE LANDED SAFELY. LOOKING BACK; I WAS TOO CONCERNED ABOUT FLYING 160 KTS AND THE OVERWHELMED CTLR VECTORING US INTO OTHER ACFT; I SHOULD HAVE NOT LET ALT DROP FROM XCHK. SUPPLEMENTAL INFO FROM ACN 529345: FO DISCONNECTED THE AUTOPLT JUST AS WE WERE INTERCEPTING THE RWY 36R LOC. BECAUSE THE AUTOPLT WAS NOT TURNING QUICKLY ENOUGH AND THE FO DID NOT WANT TO OVERSHOOT THE LOC. APCH CTL GAVE US A TURN TO 250 DEGS AND APOLOGIZED AND SAID WE COULD EXPECT RWY 36L. AFTER WE TURNED I COULD SEE TFC PASSING UNDER US TO RWY 36R. A SHORT WHILE LATER THE FO STATED THAT WE SHOULD BE AT 3000 FT; WE WERE AT 2800 FT AND DSNDING. AT 2700 FT I TOOK THE ACFT AND RETURNED TO 3000 FT. DURING THIS TIME APCH GAVE US CLRNC FOR THE RWY 36L ILS. THE REST OF THE APCH AND LNDG WAS NORMAL. CONTRIBUTING FACTORS: MARGINAL WX. ATC WAS BEHIND AND WE WERE GETTING MULTIPLE HEADING AND SPD CHANGES. INABILITY OF THE AUTOPLT TO RESPOND FAST ENOUGH. WE WERE DISTRACTED FROM FLYING BY LOOKING OUTSIDE FOR CONFLICTING TFC.