ACR WDB SPD DEV AND TRACK HDG DEV AS INEXPERIENCED FO TRIES TO FLY THE ACFT USING THE FMS.

Narrative

WE WERE BEING VECTORED FROM THE NW FOR A MOUNT VERNON VISUAL APCH TO RWY 36 DCA. THE WDB WAS BEING FLOWN ON AUTOPLT WITH INPUT THROUGH THE FCU. I WAS EXPECTING A TURN ON THE S SIDE OF P-73 TO JOIN THE DCA 10.1 AT 2500 FT. INSTEAD WE WERE GIVEN A TURN TO THE E A FEW MI N OF P-73 AT A HIGHER THAN NORMAL ALT (A 'SLAM DUNK') TRYING TO DSND AND DIRTY UP WHILE TRYING TO KEEP UP WITH THE AUTOFLT BEGAN TO FALL APART. AT THAT POINT I SHOULD'VE DISENGAGED THE AUTOFLT AND FLT DIRECTOR AND FLOWN THE WDB LIKE A NORMAL AIRPLANE. FOR REASONS I'M NOT COMPLETELY SURE OF; THE ACFT BEGAN TO ADD PWR AND ACCELERATE WHILE I WAS DIALING IN A SLOWER SPD ON THE FCU. (I THINK THE FLT DIRECTOR MAY HAVE GONE INTO ALT CAPTURE THUS CAUSING PWR TO INCREASE IN ORDER TO CLB BACK TO THE DIALED IN ALT.) AT THAT POINT WE WERE HDG E ACROSS THE POTOMAC RIVER AND WERE TOLD TO HEAD N TO REINTERCEPT THE MOUNT VERNON APCH. AT THAT POINT I DISENGAGED ALL AUTOFLT AND FLT DIRECTOR AND FLEW THE AIRPLANE BACK WHERE IT SHOULD'VE BEEN. TRYING TO UTILIZE ALL THE MAGIC OF THE WDB. WHILE INEXPERIENCED IN THE WDB (1 MONTH ON THE LINE) LED TO ME BEING BEHIND THE AIRPLANE DURING THE CRUCIAL APCH PHASE. IN THE FUTURE WHEN I AM 'SLAM-DUNKED' ON A VISUAL APCH I AM GOING TO FLY THE WDB AS A NORMAL PLANE RATHER THAN TRYING TO FLY THE COMPUTER; SO THAT THE COMPUTER CAN FLY THE PLANE.