AN SMA PLT FAILED TO CLOSE HIS FLT PLAN AFTER LNDG IN IFR CONDITIONS.

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CTLR WAS VERY BUSY AND CLRED ME FOR APCH LATER THAN I WOULD HAVE PREFERRED. SHE CLRED ME AND ALSO ADVISED TO SWITCH TO UNICOM. SHE MADE NO MENTION OF PROC FOR CANCELLING IFR. THE APCH WAS MADE WITHOUT INCIDENT; BUT THE UNICOM WAS NOT WORKING DUE TO TEMPORARY MALFUNCTION. THE COMBINATION OF ALL THESE EVENTS DIVERTED ME FROM MY USUAL SEQUENCE OF CANCELLING AFTER ESTABLISHING CONTACT WITH UNICOM AT UNCTLED ARPTS. I NEEDED TO MAKE A BUSINESS PHONE CALL AS SOON AS I GOT ON GND AND DID SO. FSS CALLED TO INQUIRE IF I WERE ON GND BEFORE I EVEN REALIZED I HAD NOT CANCELLED. THIS CERTAINLY CAUSED A DELAY FOR OTHERS. NO SAFETY PROBLEMS BUT THE ERROR WAS MINE. 2 POINTS ABOUT THIS THAT MAY BE OF BENEFIT TO OTHERS: PLTS WITH 40 PLUS YRS EXPERIENCE CAN BE MORE FORGETFUL THAN YOUNGER ONES. THINGS GET TOO ROUTINE. THUS; WE MUST BE CONTINUALLY ON GUARD TO FOLLOW PROCS. CTR OR APCH CTLRS SHOULD ALWAYS ADVISE PLTS OF CANCELLATION PROC WHEN THEY CLR THEM FROM THEIR FREQ TO CONTACT UNICOM FOR UNCTLED ARPTS.