R22 PILOT ON APPROACH LANDS ON CITY STREET AFTER ROTOR RPM DECAY SEEMS TO INDICATE AN ENGINE FAILURE.

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ON THURSDAY; I RECEIVED NOTIFICATION THAT ONE OF OUR HELI PLTS HAD MADE A TOUCHDOWN AUTOROTATION ON A CITY STREET CLOSE TO OUR ARPT OF OP DUE TO A SUSPECTED ENG FAILURE. I ARRIVED A FEW MOMENTS LATER AND FOUND THAT DRIVE LINE SYS AND ENG WERE IN GOOD OPERATING CONDITION BUT THE TACHOMETER APPEARED TO WORK ERRATICALLY. SUSPECTING EITHER A GAUGE OR AN ELECTRICAL PROB TO THAT GAUGE; I ELECTED TO PREFLT THE MACHINE; SECURE THE ROADS IN THE IMMEDIATE AREA WITH A FLAGGING CREW; AND FLY THE HELI TO THE ARPT (A FEW HUNDRED YARDS AWAY). DUE TO THE FACT THAT CARS WERE DRIVING BY THE HELI SO CLOSELY WHILE IT WAS ON THE GND; THAT THE TACHOMETER PROB SEEMED TO BE FIXED BY RESETTING A BREAKER AND THE ARPT WAS SO CLOSE; I FELT THAT IT MADE SENSE TO FLY IT HOME DESPITE THAT AN FAA INSPECTOR MAY FEEL THE MACHINE SHOULD NOT HAVE BEEN FLOWN. I BELIEVE THAT CARRYING A CELL PHONE AND CALLING THE FAA FOR A FERRY PERMIT OR FINDING A WAY TO MOVE THE HELI BY GND MAY HAVE BEEN A WISER DECISION EVEN IF IT MEANT A BIT MORE WORK TO KEEP THE HELI OUT OF HARM'S WAY ON THE GND. IN THE FUTURE; I WOULD TAKE A BIT MORE TIME IN TRYING TO CONSIDER ALTERNATE SOLUTIONS. SUPPLEMENTAL INFO FROM ACN 780596: ON AN INSTRUCTIONAL FLT; TURNING XWIND; I HEARD THE LOW RPM HORN COME ON. I THEN SAW AN OVERSPEEDING ENG TACHOMETER; AND A DROOPING ROTOR TACHOMETER. AT THE LOW LEVEL OF ALT I WAS; AND AT THE RATE OF RPM DECAY; I ELECTED TO ENTER FULL AUTOROTATION AND LANDED ON A CITY STREET. IN THE FUTURE; I WOULD OPT TO TROUBLESHOOT AND CONFIRM ENG FAILURE WITH OTHER GAUGES. ALSO; TRYING TO HAVE BETTER THROTTLE MGMNT WOULD HELP.