FLC OF AN LTT LOST COM DURING FINAL DUE TO THE FO NOT RETURNING TO THE COM RADIO SELECTOR POS AFTER MAKING A 'PA' ANNOUNCEMENT

Narrative

WE WERE INSIDE THE FINAL APCH FIX; CLRED FOR AN ILS APCH AND I BELIEVE CLRED TO LAND. THIS WAS NEXT TO THE LAST LNDG WE WERE TO DO FOR THE COMPLETION OF FAA PROVING RUNS. WE WERE TRYING; OF COURSE; TO DO A THOROUGH JOB AND WE ADVISING OUR 'FAA PAX' ON SHORT FINAL TO DOUBLE CHK SEAT BELTS FOR LNDG. WE HAD ALSO ALWAYS BRIEFED THESE PAX TO FASTEN THIER SEAT BELTS DURING OUR INITIAL DSCNT. ON APPROX A 3 MI FINAL ON THE ILS TO LAS; I; SITTING ON THE R SEAT AND WORKING WITH THE RADIOS; TURNED THE XMITTER SELECTOR SWITCH TO THE 'PAX ADVISORY' (PA) POS TO TELL THE PAX TO DOUBLE-CHK SEAT BELTS. WHEN THE SELECTOR SWITCH IS IN THE PA POS; NO OUTSIDE COM WILL COME OVER MY SYS. IN OTHER WORDS; MY SYS WILL NOT RECEIVE. NORMALLY THIS WOULD PRESENT NO PROB FOR 2 REASONS: 1)THE OTHER PLT'S SYS WOULD PICK UP NORMAL ATC COM ; AND 2) I WOULD NORMALLY BE IN THE PA POS FOR ONLY 3 TO 5 SECONDS. HOWEVER THIS TIME I NEGLECTED TO TURN MY SELECTOR SWITCH BACK TO THE NORMAL POS FOR ATC COM. THEREFORE I WAS NOT RECEIVING ANY OUTSIDE COM ON MY SIDE AND THE OTHER PLT WAS NOT RECEIVING ANY COM ON HIS SIDE EITHER. HIS COM PANEL WAS SET PROPERLY BUT HE WAS NOT PICKING UP ANY RECEPTION EITHER. FOR THE LAST 1000 FEET ON THE ILS WE WERE NOT IN COM WITH ATC. I NOTICED UPON LNDG THAT IT WAS VERY QUIET ON THE RADIO. I CHKED MY SELECTOR SWITCH AND; SURE ENOUGH; IT WAS STILL IN THE PA POS. I TURNED IT TO THE #2 XMITTER AND IMMMEDIATELY I HEARD NORMAL COMS BTWN THE TWR AND OTHER AIRPLANES. I CALLED THE TWR AND TOLD THEM WE WERE EXITING THE RWY TO THE R. NOTHING WAS SAID TO US ABOUT NOT BEING IN CONTACT. THE L COM PANEL HAD HAD A HISTORY OF TROUBLE. IT WAS; I BELEIVE; TURNED IN FOR MAINT BEFORE THIS TRIP AND CORRECTED. HOWEVER THERE WERE SCATTERED INCIDENTS THROUGHOUT THE PROVING RUNS WHERE NOTHING WOULD COME OVER THE CAPT'S SIDE WITHOUT FIDDLING WITH THE SWITCHES. UPON ARR AT HOME BASE; WE RETURNED THE AIRPLANE TO RADIO MAINT TO HAVE THE PROB RE-CORRECTED.