PLT FAILS TO COMPLY WITH TWR INSTRUCTIONS FOR CLRING THE RWY AFTER LNDG.

Narrative

FOLLOWING AN IMC DSCNT TO VISUAL CONDITIONS; SIGNIFICANT TURB WAS ENCOUNTERED. APPARENTLY AS I WAS TUNING MY RADIO FROM APCH CTL FREQ TO GND CTL FREQ; I MUST HAVE ACCIDENTALLY BUMPED THE SELECTOR FREQ SWITCH FROM TWR; WHICH HAD PREVIOUSLY CLRED ME TO LAND ON RWY 29. I WAS ABOUT 4 MI OUT WHEN CLRED TO LAND. I HEARD NO FURTHER RADIO TFC. AFTER ROLLOUT; I REACHED TO SWITCH TO GND AND DISCOVERED THAT I WAS ALREADY ON THAT FREQ. I CONTACTED GND PRIOR TO THE INTXN OF RWY 7/25 AND REQUESTED BACK TAXI ON RWY 25. GND APPROVED THAT AND INSTRUCTED ME TO TURN L ON TXWY A. I WAS DISTRACTED BY AN ACFT IN POS ON RWY 25 AND MISSED TXWY A. GND INSTRUCTED ME TO MAKE AN IMMEDIATE L TURN ON THE NEXT TXWY; WHICH I DID. DURING TAXI TO THE FBO; GND INSTRUCTED ME TO CALL THE TWR ON THE TELEPHONE. I CALLED IMMEDIATELY. I WAS ASKED IF I HEARD THE TWR INSTRUCT ME TO ROLL OUT PAST RWY 7/25 AND EXIT AT TXWY G. I SAID; 'NEGATIVE' AND TOLD THEM THAT I HAD SOMEHOW BEEN ON GND CTL FREQ SHORTLY AFTER I WAS CLRED TO LAND. I APOLOGIZED FOR MISSING TXWY A. THIS INCIDENT RESULTED FROM 1) POOR ERGONOMIC DESIGN OF THE RADIO CTL HEAD. IN THIS CASE; I THINK I BUMPED THE FREQ SELECTOR SWITCH DURING TURB OR BUMPED IT AS I SELECTED THE NEXT STANDBY FREQ. 2) AS A RESULT OF BEING OFF FREQ (TWR) I DID NOT HEAR MY POST LNDG INSTRUCTIONS AND DID NOT KNOW AN ACFT WAS IN POS ON RWY 25. IF I KNEW THAT; I WOULD NOT HAVE BEEN LOOKING TO DETERMINE HIS INTENTIONS AND WOULD HAVE MADE MY TURN ON TXWY A. CONTRIBUTING FACTORS: I WAS DISTRACTED BY THE EMT AND NURSE ON THE INTERCOM. SINGLE PLT OPS. CORRECTIVE ACTIONS: FAR 135 AIR AMBULANCE OPS SHOULD BE MANDATED BY FARS TO BE 2 PLT OPS -- PERIOD! I AM A RETIRED MIL PLT WITH 4 JET TYPE RATINGS. THIS TYPE FLYING IS THE MOST DEMANDING/DANGEROUS I HAVE EVER DONE. NORMALLY; I ALWAYS HAVE THE THIRD RADIO TUNED TO THE TWR; TODAY TOO MANY THINGS WERE GOING ON AND I DIDN'T. IT COULD HAVE COST MORE THAN A PHONE CALL TO THE TWR!