SPI OCCUPIES ATA AND ENGAGES IN UNAUTHORIZED TKOF AND LNDGS DURING LOSS OF RADIO CONTACT WITH TWR.

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(SUS) SPIRIT OF ST LOUIS CTL TWR CALLED FBO. SAID THAT SOMEONE (MY STUDENT) HAD NOT BEEN MONITORING/MAINTAINING RADIO CONTACT AND THAT THIS HAD BEEN GOING ON FOR NEARLY 1 HR. STUDENT WAS PRACTICING TOUCH AND GO LNDGS. TWR REQUESTED THAT SOMEONE TRY TO FLAG HIM DOWN OUT BY THE RWY. AFTER REPEATED ATTEMPTS TO FLAG MY STUDENT DOWN (OBVIOUSLY HE WAS IN DEEP CONCENTRATION WITH HIS TOUCH AND GOES) STUDENT FINALLY EITHER SAW MY FURIOUS WAVING OR FIGURED OUT THAT RADIO WAS INOP. STUDENT LANDED ACFT AND TURNED OFF RWY TO MEET ME. I CLBED INTO ACFT AND ASKED WHAT HAD HAPPENED. BOTH RADIOS HAD SEEMED TO BE WORKING NORMALLY. STUDENT CLAIMED THAT AFTER SEVERAL TOUCH AND GOES THAT HE NOTICED THAT THE RADIO WAS NOT RECEIVING. HE THEN FIGURED OUT THAT THE VOLUME HAD BEEN INADVERTENTLY TURNED DOWN BELOW AN AUDIBLE LEVEL. WE HAD PREVIOUSLY TALKED (PRIOR TO HIS FIRST SOLO -- THIS WAS NOT HIS FIRST SOLO -- PERHAPS MAYBE HIS 5TH OR 6TH -- AND HIS FIRST 2 SOLOS WERE SUPERVISED BY MYSELF) ABOUT NORDO PROCS; AND I MAKE IT A POINT ON PRESOLO WRITTENS TO ASK QUESTIONS PERTAINING TO NORDO PROCS AND LIGHT SIGNALS. PERHAPS THE PROBLEM LIES NOT IN KNOWING WHAT TO DO WHEN THERE IS A PROBLEM; BUT RATHER IN RECOGNIZING WHEN THERE IS ONE. IN THE FUTURE I PLAN ON ASKING OTHER STUDENTS HOW THEY WOULD RECOGNIZE THAT THE RADIO IS INOP INSTEAD OF DWELLING ON JUST WHAT THEY SHOULD DO AFTER THE FACT.