FLC OF A BEECH 1900 AIRLINER OVERSHOT LOC DUE TO FAILING TO SELECT THE ILS FREQ DURING VECTOR TO FINAL.

Narrative

WE WERE VECTORED FOR AN ILS RWY 1R APCH INTO DULLES. AS WE WERE FLYING TO JOIN THE LOC; THE FO AND I BOTH NOTICED THAT THE NEEDLE WAS SLOW TO 'COME ALIVE.' ABOUT THIS TIME WE BOTH REALIZED THAT THE NAV RADIOS WERE STILL TUNED TO THE ARMEL VOR (AML) AS THE PRIMARY FREQ. THE ILS WAS IN THE STANDBY FREQ POS. WE QUICKLY CHANGED FREQS AS WE MADE A HARD R TURN TO RE-INTERCEPT THE ILS COURSE. WE UNINTENTIONALLY FLEW THROUGH THE LOC COURSE. AT THAT TIME TWR ADVISED US TO TURN R AND POINTED OUT TFC FOR THE PARALLEL RWY. WE HAD TFC IN SIGHT AND WERE ABLE TO MAINTAIN VISUAL SEPARATION. A NORMAL LNDG WAS MADE. THIS ERROR RESULTED FROM US; THE FLC; BEING UNDER A HIGH WORKLOAD. WE WERE BUSY LOOKING FOR OTHER TFC THAT WE WERE FOLLOWING ALONG WITH THE USUAL DUTIES IN A HIGHLY CONGESTED AREA THAT WE FAILED TO REALIZE THAT WE HAD NOT TUNED IN THE ILS FREQ. ALSO A CONTRIBUTING FACTOR TO THE INCREASED WORKLOAD COULD BE DUE TO THE FACT THAT WE WERE FLYING AN AIRPLANE WHERE THE CAPT'S PUSH TO TALK SWITCH HAD BEEN DEFERRED FOR MAINT. THIS REQUIRED ME TO COMMUNICATE USING A HAND-HELD MIKE. IF THE MIKE WAS PLUGGED IN I COULD NOT COMMUNICATE WITH THE FO VIA THE CREW INTERPHONE. SWITCHING BACK AND FORTH BTWN THE HAND- HELD MIKE AND THE BOOM MIKE INCREASED OUR WORKLOAD DURING THE APCH PHASE. THIS MAY HAVE BEEN A CONTRIBUTING FACTOR IN OVERLOOKING THE FACT THAT THE ILS HAD NOT BEEN TUNED IN. IF IN THE FUTURE I HAVE AN AIRPLANE WHERE THE PUSH TO TALK IS INOP AND HAS BEEN DEFERRED; I WILL DO ALL THE FLYING AND LET THE FO DO ALL THE TALKING. A BETTER SOLUTION MAY BE TO CHANGE THE PLT ACTIONS IN THE MEL OR NOT DEFER AN INOP PTT ON THE CAPT'S SIDE.