C172 INSTRUCTOR AND PLT HAD SPLIT FLAPS ON GAR.

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WHILE COMPLETING VOR RWY 14 TO GAI INST STUDENT TURNED R TO ENTER L BASE TO RWY 14. HE LOWERED FLAPS TO 40 DEGS AND STARTED DSNDING. I ADVISED HIM HE WAS TOO HIGH AND TO GO AROUND. HE RAISED FLAPS (SWITCH UP) FOR GAR. THERE WAS A 'POP' SOUND AND I SAID 'WHAT WAS THAT' AS I SCANNED R THEN L. STUDENT SAID THE CTLS WERE NOT RIGHT AS I SAW THE L FLAP FULLY EXTENDED AND THE R FLAP UP. I TOLD HIM WE HAD A SPLIT FLAP AND I HAD THE AIRPLANE. I MANAGED TO ARREST THE R TURN AND ESTABLISH A GENTLE TURN TO THE L WITH ROLL CTL AVAILABLE. A RADIO CALL ON CTAF TO AN ACFT ON GND WAS MADE REQUESTING THEM TO STAY BY FOR AN EMER LNDG. A GRADUAL DSCNT WAS MADE WHILE IN THE L TURN AND THE FLAPS LEFT IN THEIR EXISTING CONDITION. ACFT WAS LANDED WITHOUT PROB. INVESTIGATION REVEALED A FAILED OUTER TRACK BEARING JAMMING FLAP IN 30-40 DEGS DOWN. THE RETRACTION SYS JACKSCREW HAD WRAPPED PARTS AROUND ITSELF. NTSB AND FSDO ADVISED THE NEXT MORNING AND FSDO TECHNICAL REPRESENTATIVE ASSISTED IN THE CHK. VERY LITTLE OF THE FLAP SYS CAN BE CHKED ON PREFLTS AND THE NEEDLE BEARING FAILED WITHOUT WARNING. ACFT HAS 9000+ HRS (1975 NEW).