Incident Overview
On September 4, 2011, at approximately 1500 Pacific daylight time, a Bell 205 A1 helicopter (registration N205WW) made a hard landing during an off-field emergency autorotation near Tehachapi, California. The helicopter was operated by the Kern County Fire Department as a public-use fire suppression flight under 14 Code of Federal Regulations Part 133. The commercial pilot sustained minor injuries, while 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.
Sequence of Events
The helicopter was carrying a water bucket as an external load, and the pilot was preparing to drop water on the leading edge of a fire. At about 100 feet above ground level (agl) and 10 knots, the engine revolutions per minute (rpm) light illuminated and the low rotor rpm horn sounded. The pilot released the water and executed a left pedal turn to exit the canyon and move away from the fire. He then checked the engine rotor rpm gauge and observed 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 interpreted as indicating maximum rpm.
At that point, the helicopter was about 400 feet agl, and the pilot identified a landing zone. Believing that a high-side fuel control failure had occurred, he rolled off the throttle and lowered the collective. He placed the governor switch into manual mode, then rolled the throttle on while raising the collective. He felt the rotor rpm decrease and saw the gauge at 90 percent. With altitude at 250-300 feet agl and sensing a loss of altitude and rpm, he lowered the collective to regain rotor rpm and established an autorotation into the landing zone.
Landing and Post-Accident Actions
The helicopter landed hard, collapsing the landing skids. The pilot closed the throttle, turned off the fuel switches, and checked for fire or warning lights. He exited the helicopter to prevent civilians from approaching and, once the area was secure, checked for fire and assessed the damage. The helicopter sustained substantial damage to the airframe and tail boom. He reentered the cockpit, reset the switches to post-flight mode out of habit, and assessed interior damage.
Examination Findings
The helicopter was recovered to the operator's facility in Renton, Washington. An investigator from the engine manufacturer, under FAA supervision, examined the engine. The investigator 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 and determined that it failed due to 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. The pilot's instruments indicated an engine overspeed, while the warning lights and audio indicated a low-power condition. According to the flight manual's emergency procedures, in the event of an engine failure or low rotor rpm, a red light would illuminate and an audio signal would sound when the audio switch was in the AUDIO position. The manual instructs the pilot to immediately execute an autorotative descent.