FLAGSTAFF, AZ, USA
N2773R
BELL 206B-III
FOLLOWING AN ANNUAL INSPECTION, THE HELICOPTER HAD EXPERIENCED AN INTERMITTENT ILLUMINATION OF THE 'ENGINE OUT' ANNUNCIATOR LIGHT. THE PILOT CONTINUED FLYING AND MONITORED THE ENGINE INSTRUMENTS, PARTICULARLY THE N2 POWER TURBINE GAUGE. DURING THE ACCIDENT FLIGHT, THE ANNUNCIATOR AGAIN FLICKERED AND THEN STAYED ON. IN ADDITION, THE 'ENGINE OUT' HORN ALSO SOUNDED; HOWEVER, THE ENGINE PARAMETERS REMAINED NORMAL. THE ANNUNCIATOR LIGHT AND HORN WILL ACTIVATE IF THE N1 GAS PRODUCER FALLS BELOW 55 PERCENT. THE PILOT REPORTED THAT DURING A TURN, THE N2 BEGAN DECREASING AND HE ENTERED AN AUTOROTATION. THE HELICOPTER ROLLED OVER FOLLOWING A HARD LANDING. POSTACCIDENT TESTS REVEALED THAT THE ENGINE OPERATED WITHIN NORMAL SPECIFICATIONS. THE N2 TACH GENERATOR OPERATED NORMAL. THE N1 TACH GENERATOR FAILED.
On December 10, 1993, at 1700 hours mountain standard time, a Bell 206B-III helicopter, N2773R, crashed during a forced landing about 14 miles southeast of Flagstaff, Arizona. The helicopter was being operated as a visual flight rules (VFR) local area personal flight when the accident occurred. The helicopter, operated by the pilot, received substantial damage. The certificated private pilot and a passenger were not injured. Visual meteorological conditions prevailed. The flight departed a private helipad at the operator's residence at 1645 hours. The pilot reported that the helicopter had undergone a recent annual inspection. Since the annual, the helicopter had experienced an intermittent "engine out" warning annunciator light. The "engine out" annunciator light and warning horn are normally activated when the N1 gauge decreases to less than 55 percent (plus or minus 3 percent). The pilot verified that the engine was still running by cross-checking other engine instruments, particularly the N2 gauge. During the accident flight, the warning light again was intermittent until about 3 minutes before the accident when the "engine out" annunciator light and horn both activated. The engine parameters remained steady. During a telephone interview with the pilot, he indicated that the N2 gauge was holding steady at 92 percent until he executed a turn. The pilot then noticed the N2 gauge decreasing and he entered an autorotation into unsuitable terrain. The helicopter landed hard and rolled over on the left side. The helicopter manufacturer reported that the engine out annunciator light and horn is activated when a RPM sensor receives an insufficient RPM signal from the N1 tach generator on the engine. There is one tach generator for the engine N1 speed and the N2 speed. The normal operating range for the N1 (gas producer) is 60 to 105 percent. The normal operating range for the N2 (power turbine) is 97 to 100 percent. At normal cruise power settings (about 80 percent torque), the N2 gauge is maintained at 100 percent and the N1 gauge normally reads about 92 percent. The pilot holds a private pilot certificate with a rotorcraft helicopter rating. The most recent third-class medical certificate was issued to the pilot on August 20, 1993, and contained no limitations. According to the pilot/operator report submitted by the pilot, his total aeronautical experience consists of about 1,100 hours, of which 800 were accrued in the accident aircraft make and model. In the preceding 90 and 30 days prior to the accident, the report lists a total of 12 and 6 hours respectively flown. The helicopter and engine had accumulated a total time in service of 1,005 flight hours. Examination of the maintenance records revealed that the most recent annual inspection was accomplished on July 8,1993, 16 flight hours before the accident. The helicopter was examined by a Federal Aviation Administration (FAA) airworthiness inspector, Scottsdale Flight Standards District Office. The examination revealed that during the impact sequence, the main drive shaft separated from the forward main rotor transmission outer coupling. The forward inner coupling was missing. The aft engine inner coupling was broken within the engine outer coupling. The top of the transmission mount assembly under the flanges of the transmission outer coupling was gouged and cut. The separated portions of the main drive shaft assembly were examined by the manufacturer under the supervision of a National Transportation Safety Board investigator. The manufacturer reported that the drive shaft components had been subjected to extreme misalignment which produced deformation and fracturing of the spine teeth in the forward and aft outer couplings. The aft inner coupling was fractured as a result of the misalignment. The end plates of both outer couplings had indentations produced by the inner coupling gear teeth. Indentations and gouges indicated that the drive shaft inner coupling gears (that are bolted to the drive shaft) disengaged from the aft outer coupling (engine) in a forward direction and from the front outer coupling (transmission) in an aft direction. The manufacturer also reported that although the forward inner gear coupling was missing, the bolt holes through which it is attached to the forward end of the drive shaft were deformed. The deformation of the holes and damage to the forward outer coupling teeth indicated that the inner forward coupling was intact when the drive shaft was subjected to severe misalignment. The drive shaft had circumferential scoring marks. The flanges of the forward outer coupling had gouged and cut into the top of the transmission pylon isolation mount while rotating. The drive shaft components were released to the operator on April 13, 1994. The engine was examined by the manufacturer under the supervision of an FAA inspector. The engine manufacturer reported that the examination consisted of a visual inspection and determination that the engine was suitable for running. A subsequent engine run totalling 53 minutes revealed that the engine performed within specifications. A copy of the engine run report is available from the manufacturer. The engine was released to the operator on April 1, 1994. The tach generators were examined by the manufacturer under the supervision of an FAA inspector. The examination revealed that the N2 tach generator, serial No. 10194, performed within specifications. The N1 tach generator, serial No. 10157, failed the manufacturers test standards. Of the two tach generators, the N1 generator had been overhauled by an FAA repair station. The tach generators were released to the operator on June 27, 1994.
The pilot's improper initiation of an emergency autorotation into unsuitable terrain and subsequent hard landing and rollover. A false annunciator indication, a failure of the N1 tach generator, the pilot's continued flight with a known aircraft defect, and the pilot's improper identification of the failed N1 gauge were factors in this accident.
Source: NTSB Aviation Accident Database
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