WPR11GA431

During an external load operation to drop water on a fire, the helicopter was about 100 feet above ground level at 10 knots when the engine rpm light illuminated and the low rotor rpm horn sounded. The emergency procedures section of the flight manual states that in the event of an engine failure or low rpm, a red light will illuminate and an audio signal will sound when the audio switch is in the AUDIO position. The flight manual instructs the pilot to immediately execute an autorotative descent. The pilot released the water, and made a left-pedal turn to exit the canyon and move away from the fire. He checked his engine rotor rpm gauge and saw that the needles had split: the rotor needle was at the 4-5 o'clock position, and the engine needle was at the 6-o'clock position, which he stated indicated maximum rpm. He maneuvered to establish an autorotation into a landing zone. The helicopter sustained substantial damage to the airframe and tail boom as the result of a hard landing, which collapsed the landing skid. Postaccident examination discovered that the N2 tachometer drive shaft was sheered as a result of torsional overstress. The N2 tachometer drive delivers engine rpm readings to the cockpit engine tachometer; failure of the N2 tachometer drive would send erroneous engine rpm readings to the cockpit. Accordingly, the pilot's instruments indicated that there was an engine overspeed, but the warning lights and audio were indicating a low power condition. The pilot elected to perform an autorotative landing in accordance with the flight manual instructions for a low rotor rpm.

On September 4, 2011, about 1500 Pacific daylight time, a Bell 205 A1, N205WW, landed hard during an off field emergency autorotation near Tehachapi, California. The Kern County Fire Department was operating the helicopter as a public-use fire suppression flight under the provisions of 14 Code of Federal Regulations (CFR) Part 133. The commercial rated pilot sustained minor injuries; the helicopter sustained substantial damage to the tail boom from impact forces. A company visual flight rules (VFR) flight plan had been filed for the local flight.The helicopter had a water bucket as an external load, and the pilot was preparing to drop a load on the leading edge of the fire. The helicopter was about 100 feet above ground level (agl) at 10 knots, when the engine revolutions per minute (rpm) light illuminated, and the low rotor rpm horn sounded. The pilot released the water, and made a left pedal turn to exit the canyon and move away from the fire. He checked his engine rotor rpm gauge and saw that the needles had split. The rotor rpm needle was at the 4-5 o'clock position, and the engine rpm needle was at the 6-o'clock position, which he stated indicated maximum rpm. The helicopter was now about 400 feet agl, and the pilot spotted a landing zone for a landing. He perceived that the helicopter had a high side fuel control failure. He rolled off the throttle, and lowered the collective. After he placed the governor switch into manual mode, he rolled the throttle on while raising the collective. He felt the rotor rpm decrease as he pulled the collective, and saw that the gauge was at 90 percent. He was 250-300 feet agl, and felt that he was running out of altitude and rpm, so he lowered the collective to regain rpm, and established an autorotation into the landing zone. After landing hard, which collapsed the landing skids, the pilot closed the throttle, turned off the fuel switches, and checked for fire/warning lights. He exited the helicopter to stop approaching civilians. Once he felt the area was secure, he checked for fire and assessed the damage. The helicopter sustained substantial damage to the airframe and tail boom. He reentered the cockpit, and reset the switches to post flight mode out of habit, and assessed the interior damage. The helicopter was recovered to the operator's facility in Renton, Washington. An investigator from the engine manufacturer examined the engine under the supervision of the FAA. He confirmed both N1 and N2 control continuity from the pilot's seat to the engine, and observed that both controls traveled from stop to stop. The only anomaly discovered was a sheared N2 tachometer drive shaft. A specialist from the National Transportation Safety Board's Materials Laboratory examined the N2 shaft. He determined that the shaft failed as a result of torsional overstress. The N2 tachometer drive delivers engine rpm readings to the cockpit engine tachometer; failure of the N2 tachometer drive would send erroneous engine rpmreadings to the cockpit. The pilot's instruments indicated that there was an engine overspeed, but the warning lights and audio were indicating a low power condition. The emergency procedures section of the flight manual states that in the event of an engine failure or low rotor rpm, a red light will illuminate and an audio signal will sound when the audio switch is in the AUDIO position. The flight manual instructs the pilot to immediately execute an autorotative descent.