Narrative:

My student was flying the aircraft; approaching the airport for pattern work. The student directed my attention to the rotor RPM; which was beginning to decay. I immediately took the controls; lowering the collective to the full down position as the rotor RPM dropped below the green arc. At the same time I banked the aircraft to the left in an attempt to increase rotor RPM and head towards a suitable landing area. I called out my location to tower at the same time and told them I was making an emergency landing. A police helicopter 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 approach with power. At this time we were approximately 300 ft AGL and I was unable to judge exactly where the rotor RPM was without a working tach. The engine RPM showed the engine was producing power and was above the red line. Without a governor equipped on the aircraft it was unknown if the rotor RPM was still correlated with the engine. I reduced engine 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 aircraft in an unoccupied livestock field using a run-on landing at approximately 15 KTS. There was no damage to the aircraft; 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 problem. The only corrective action possible was to lower the collective and then identify if it was an instrument error or not. With the engine producing power; a drive shaft failure was still a possibility. The student helped me by pointing out engine instruments that were exceeding limitations. His attention to the instrumentation was excellent.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.