A HILLER 12B ROTOR TACH GEN CABLE BECAME UNPLUGGED GIVING THE APPEARANCE OF ROTOR FAILURE. A SAFE LANDING WAS ACCOMPLISHED.

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MY STUDENT WAS FLYING THE ACFT; APCHING THE ARPT FOR PATTERN WORK. THE STUDENT DIRECTED MY ATTN TO THE ROTOR RPM; WHICH WAS BEGINNING TO DECAY. I IMMEDIATELY TOOK THE CTLS; LOWERING THE COLLECTIVE TO THE FULL DOWN POS AS THE ROTOR RPM DROPPED BELOW THE GREEN ARC. AT THE SAME TIME I BANKED THE ACFT TO THE L IN AN ATTEMPT TO INCREASE ROTOR RPM AND HEAD TOWARDS A SUITABLE LNDG AREA. I CALLED OUT MY LOCATION TO TWR AT THE SAME TIME AND TOLD THEM I WAS MAKING AN EMER LNDG. A POLICE HELI WAS IN THE AREA AND IMMEDIATELY HEADED IN OUR DIRECTION. MY ATTEMPT TO REGAIN ROTOR RPM WAS UNSUCCESSFUL DESPITE CORRECTIVE ACTION AND IT BECAME EVIDENT THAT THE INSTRUMENT HAD FAILED. I EXITED THE AUTOROTATION AND BEGAN A NORMAL APCH WITH PWR. AT THIS TIME WE WERE APPROX 300 FT AGL AND I WAS UNABLE TO JUDGE EXACTLY WHERE THE ROTOR RPM WAS WITHOUT A WORKING TACH. THE ENG RPM SHOWED THE ENG WAS PRODUCING PWR AND WAS ABOVE THE RED LINE. WITHOUT A GOVERNOR EQUIPPED ON THE ACFT IT WAS UNKNOWN IF THE ROTOR RPM WAS STILL CORRELATED WITH THE ENG. I REDUCED ENG RPM TO BRING IT BACK TO THE GREEN ARC AND FELT A LIGHT BUFFET BEGIN TO DEVELOP SO I RETURNED THE THROTTLE TO THE SETTING WHICH HAD BEEN WORKING WELL. I THEN LANDED THE ACFT IN AN UNOCCUPIED LIVESTOCK FIELD USING A RUN-ON LNDG AT APPROX 15 KTS. THERE WAS NO DAMAGE TO THE ACFT; FIELD OR OCCUPANTS. AFTER SHUTTING DOWN WE FOUND THAT THE PLUG FOR THE ROTOR RPM TACH HAD COME LOOSE AND DISCONNECTED. THE ROTOR RPM NEEDLE DECAYED SLOWLY; AS IF THERE WAS AN ACTUAL MECHANICAL PROB. THE ONLY CORRECTIVE ACTION POSSIBLE WAS TO LOWER THE COLLECTIVE AND THEN IDENT IF IT WAS AN INSTRUMENT ERROR OR NOT. WITH THE ENG PRODUCING PWR; A DRIVE SHAFT FAILURE WAS STILL A POSSIBILITY. THE STUDENT HELPED ME BY POINTING OUT ENG INSTS THAT WERE EXCEEDING LIMITATIONS. HIS ATTN TO THE INSTRUMENTATION WAS EXCELLENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THIS PLT ERROR IS AS COMMON AMONG ACR PILOTS AS IT IS WITH GA PILOTS. THE RPTR STATED AN OPERATIONAL ERROR OCCURRED AFTER A PILOT DEPARTING WEST CONTACTED TRACON ON AN EAST FREQ. THE CTLR NOT LISTENING TO THE CALL SIGN CLOSELY HEARD WHAT HE WANTED TO HEAR AND TURNED THE WEST BOUND TRAFFIC TOWARD THE EAST INTO AN EAST DEPARTING ACFT OFF THE PARALLEL RWY. RPTR STATED THEY FELT VERY LUCKY THAT THE PILOTS WERE VMC AND ABLE TO SEPARATE THEMSELVES THUS PREVENTING AN AIRBORNE INCIDENT. THE RPTR STATED THAT THE CHART FORMAT IS CONFUSING AND SHOULD BE CHANGED. AFTER CONSULTING WITH SEVERAL PILOTS AND CTLRS FAMILIAR WITH THE MEM DEPARTURE; THE RPTR ALSO SUGGESTS REMOVING UNNECESSARY NAVAIDS TO BETTER EXPOSE THE DEMARCATION LINES AND THE FREQS. ADDITIONALLY; THE 'EXCEPTION' FOR ACFT 7000 FT AND BELOWSHOULD BE PLACED NEARER TO THE PLACE ON THE CHART THAT IT APPLIES TO. THE MOST IMPORTANT CHANGE; HOWEVER; SHOULD BE REMOVING CLUTTER THUS ALLOWING INSTRUCTIONS TO STANDOUT.