A J-3 CUB DURING A DUAL INSTRUCTION FLT AT 2100 RPM 14 INCHES OF THE PROP ROCK GUARD AND 4 INCHES OF THE TIP SEPARATED.

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THE DUAL INSTRUCTIONAL FLT DEPARTED FROM ZZZ; A PVT TURF FIELD. A THOROUGH PREFLT INSPECTION OF THE PIPER J-3/C65 WAS CONDUCTED; AND INSTRUCTION GIVEN ON THE PROPER PREFLT PROCS. NO ANOMALIES OR IRREGULARITIES WERE DISCOVERED DURING THE INSPECTION OR SUBSEQUENT PRE-TKOF RUN-UP. AFTER MANEUVERING IN SLOW FLT; THE ACFT WAS RETURNED TO CRUISE CONFIGN AT 2100 FT MSL (APPROX 1850 FT AGL). WITH THE ENG TURNING APPROX 2100 RPM; 14 INCHES OF THE OUTBOARD LEADING EDGE ROCK GUARD AND 4 INCHES OF THE TIP OF THE WOOD SENSENICH PROP SEPARATED. IMMEDIATE; SEVERE VIBRATION WAS OBSERVED AS A RESULT OF THE IMBAL. THE PIC/CFI IMMEDIATELY SECURED THE ENG BY SIMULTANEOUSLY REDUCING THE THROTTLE TO IDLE SETTING AND SWITCHING OFF THE MAGNETOS. THE ACFT WAS PITCHED UP TO SLOW AIRSPD AND STOP THE PROP ROTATION. THE PLT AND STUDENT NOTED THE CONDITION OF THE PROP. THE ACFT WAS TRIMMED FOR BEST RATE GLIDE SPD; AND TURNED IN THE DIRECTION OF A NEARBY FARM FIELD. UPON REACHING THE FIELD; THE PLT NOTED TALL GRASS; ESTIMATED AT 30-40 INCHES IN HT; PRESENTING A PITCH-OVER HAZARD. ALT WAS NOTED AT 1800 FT MSL. ADJUSTING PITCH FOR LEAST SINK GLIDE SPD AND ESTABLISHING A DOWNWIND HDG; THE PIC ELECTED TO GLIDE THE ACFT APPROX 3 MI TO THE E; RETURNING TO THE DEP ARPT. 3 ALTERNATE OFF-FIELD LNDG SITES WERE IDENTED ALONG THE RTE; IN CASE THE PRIMARY FIELD BECAME UNATTAINABLE. EXAM OF THE PROP SHOWED SOME APPARENT DEGRADATION OF THE WOOD INSIDE THE EDGE GUARD. THIS COULD NOT HAVE BEEN DETERMINED PRIOR TO FLT. THE PLT SPECULATES THAT STORAGE OF THE ACFT OUTDOORS PROBABLY CONTRIBUTED TO; OR WAS THE ROOT CAUSE OF THE CONDITION.