A C172 PLT ACCIDENTALLY SETS HIS XPONDER CODE TO 7700 ON A TOUCH-AND-GO LNDG AT PAO.

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RPTR WAS IN CLOSED R TFC FOR RWY 30 AT PAO ARPT. HE HAD JUST EXECUTED A FULL FLAP TOUCH-AND-GO LNDG AND WAS TURNING TO XWIND LEG OF PATTERN WHEN TWR ASKED IF RPTR WAS DECLARING AN EMER (BY MEANS OF XPONDER CODE 7700). A QUICK EXAM OF XPONDER CTL PANEL REVEALED CODE 7700 DISPLAYED! RPTR ADVISED TWR THAT THERE WAS NO EMER AND IMMEDIATELY RESET XPONDER CODE TO 1200. CHAIN OF EVENTS: 1) XPONDER ON RPTR'S FLYING CLUB ACFT HAS 4 LEVERS RATHER THAN KNOBS TO INPUT CODE NUMBERS. LEVERS PROJECT FAR ENOUGH THAT CARELESS OP OF FLAP CTL LEVER TO 'UP' BY RPTR'S HAND APPARENTLY RESET CODE FROM 1200 TO 7700! (THANK GOD IT WASN'T 7500.) 2) RPTR HAD NOT ENCOUNTERED A LEVER-TYPE XPONDER BEFORE AND WILL TAKE GREATER CARE WHEN HIS HAND IS PERFORMING TASKS IN ITS VICINITY. 3) FOLLOWING COMPLETION OF TRAINING FLT; RPTR SECURED CLUB ACFT AND DROVE TO BASE OF ARPT CTL TWR. USING TELEPHONE; RPTR EXPLAINED HOW HE BELIEVED XPONDER HAD BEEN RESET TO 7700. TWR PERSON ASSURED RPTR THAT ATC (BAY APCH) HAD BEEN INFORMED; AND THE SIT HAD BEEN RESOLVED.