C172 PLT MISSET HIS AUDIO SELECTOR PANEL. HE INADVERTENTLY LOST CONTACT WITH ATC FOR OVER 20 MI. HE WAS BEING VECTORED IN AN AREA OF MOUNTAINOUS TERRAIN.

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THIS WAS AN INST TRAINING/REFRESHER FLT FROM A NON TWRED ARPT (AUBURN; WA; S50) TO PAINE FIELD; ARLINGTON ARPT; PAINE FIELD; AND BACK TO AUBURN ARPT. THE FLT LASTED 2.1 HRS; 1.0 HRS OF WHICH WERE FLOWN IN IMC. THE STUDENT; WHO POSSESSED AN INST RATING AND WAS CURRENT; WAS DOING AN EXCEPTIONALLY FINE JOB ON THE FLT; ESPECIALLY IN REGARDS TO THINKING AHEAD; SETTING UP RADIOS AND FREQS; AND COMS WITH ATC. THE INCIDENT OCCURRED AS WE WERE RETURNING TO AUBURN ON RADAR VECTORS WITH SEA APCH CTL. WE WERE IMC AT THE TIME; APPROX 9 MI E OF SEATAC ARPT AND THE SEA VOR; AT 3000 FT ON A HDG OF 160 DEGS. BECAUSE I'M FAMILIAR WITH THE AREA AND KNEW OF SOME HILLS TO THE E WHOSE TOPS WERE HIGHER THAN WE WERE; I WAS KEEPING CLOSE TRACK OF OUR POS BY MY VFR TERMINAL AREA CHART; VORS; AND DME. HAVING BEEN VECTORED TO AUBURN FROM PAINE MANY TIMES PREVIOUSLY; EVERYTHING SEEMED TO BE PROGRESSING NORMALLY. WE WERE WHERE WE SHOULD HAVE BEEN; AT THE ALT WE USUALLY ARE; AND HEARING RADIO TFC ON THE COM RADIO. WHILE DISCUSSING THE FLT IN PROGRESS WITH MY STUDENT; I HAD NOTICED BY SOME RADIO CALLS THAT MY STUDENT HAD STARTED MONITORING AUBURN UNICOM; SO I LOOKED AT THE AUDIO PANEL TO MAKE SURE THAT HE WAS JUST MONITORING AND HADN'T SWITCHED FROM APCH CTL. SURE ENOUGH; THE 'AUTO' BUTTON WAS PUSHED IN FOR LISTENING ON THE XMITTING FREQ; THE #2 COM BUTTON WAS PUSHED IN TO MONITOR THE #2 COM RADIO; AND THE #2 COM WAS SET TO AUBURN'S FREQ. UNFORTUNATELY; THIS AIRPLANE HAD KING FLIP-FLOP RADIOS; SO WE HAD A TOTAL OF 4 COM FREQS SET IN; 4 NAV FREQS; PLUS DME; ADF; AND XPONDER. WITH SO MANY FREQS DISPLAYED; I DIDN'T NOTICE AT FIRST THAT #1 COM HAD OUR PREVIOUS CTR FREQ IN IT RATHER THAN APCH CTL AND MY STUDENT HAD INADVERTENTLY FLIP- FLOPPED THE APCH CTL FREQ FOR THE LCL AUBURN FREQ. SINCE I PERIODICALLY CHK EVERYTHING I CAN THINK OF WHILE NOTHING ELSE IS GOING ON; AND THINKING THAT WE SHOULD HAVE BEEN TURNED OVER TO ANOTHER CTLR BY NOW BECAUSE OF OUR LOCATION; I FINALLY CAUGHT THE ERROR AND WE SWITCHED BACK IMMEDIATELY TO THE PROPER APCH CTL FREQ. THE APCH CTLR WAS A BIT UPSET. HE SAID THAT HE HAD BEEN CALLING US FOR THE LAST 20 MI; THAT WE WERE HDG TOWARD HIGHER TERRAIN; THAT WE WERE BELOW HIS MVA FOR THAT PARTICULAR AREA; AND THAT WE SHOULD START AN IMMEDIATE R TURN TO 270 DEGS. WE MADE OUR TURN AND WERE THEN TURNED OVER TO ANOTHER CTLR FOR THE REMAINDER OF THE FLT; WHICH WAS UNEVENTFUL. THERE ARE 3 POSSIBLE PROBS HERE: 1) COMPLACENCY; 2) 1 PLT DOING SOMETHING WITHOUT TELLING THE OTHER PLT HE'S DOING IT; AND 3) A PERSON NOT DOUBLECHKING EVERY TIME WHEN HE CHANGES ANY FREQ. SUPPLEMENTAL INFO FROM ACN 405444: THIS DID NOT HAPPEN BECAUSE OF BEING UNFAMILIAR WITH THIS RADIO SYS; AS I FLY THIS ACFT QUITE OFTEN AND AM VERY FAMILIAR WITH THE SYS IN IT. I BELIEVE THE REASON IT DID HAPPEN IS BECAUSE I HAD JUST RECEIVED FROM MY INSTRUCTOR A RATHER LARGE COMPLIMENT WHICH I LET GO TO MY HEAD AND ALLOWED MY VIGILANCE TO DROP A LITTLE BIT.