Putnam, TX, USA
N115TV
Bell 206B
The single-engine helicopter impacted the ground hard during a precautionary autorotational landing following an engine overspeed indication. According to the pilot, he observed the rotor rpm increase and attempted to decrease the rpm to no avail. He then reduced the throttle and entered an autorotation. The pilot did not remember hearing an engine or rotor rpm increase. Post-accident examination of the wreckage revealed the dual tachometer indicator had a faulty turbine rpm indication and depicted an overspeed situation when functionally tested. The rotor tach generator and indication tested satisfactorily. Review of the helicopter's maintenance records revealed the dual tachometer indicator was overhauled 39 days prior to the accident, and was installed on the accident helicopter 37 days prior to the accident. No other anomalies were noted with the engine or helicopter. Examination of the engine control unit's nonvolatile information revealed there was no rpm exceedence, and in fact there was an underspeed situation recorded, which was attributed to a high engine rpm indication. The pilot more than likely noted an exceedence in engine rpm on the dual tachometer and interpreted it as a rotor rpm exceedence. He then failed to confirm the indication and autorotated with an operating engine and rotor system.
HISTORY OF FLIGHT On January 9, 2003, at 0943 central standard time, a Bell 206B helicopter, N115TV, was substantially damaged during a precautionary autorotational landing following a reported rotor overspeed near Putnam, Texas. The helicopter was registered to and operated by US Helicopters, Inc., of Marshville, North Carolina. The commercial pilot, sole occupant, was seriously injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 CFR Part 91 positioning flight. The cross-country flight departed Midland, Texas, at 0808, and was destined for Grand Prairie, Texas. On April 14, 2003, the NTSB investigator-in-charge (IIC) interviewed the pilot over the telephone. According to the pilot, he observed the rotor rpm (Nr) increase and attempted to decrease the rpm with the collective-mounted governor increase/decrease switch. He then observed the rpm go "sky high". The pilot increased the collective to control the rpm, to no avail. He then reduced the throttle and entered an autorotation. The pilot did not remember hearing an engine or rotor rpm increase. PERSONNEL INFORMATION The pilot held a commercial helicopter certificate and a second-class medical certificate with no limitations. The medical certificate was issued on March 8, 2002. According to the FAA inspector, who responded to the accident site, the pilot accumulated a total of 2,000 hours of flight time, of which 630 hours were logged in the same make and model as the accident helicopter. The pilot accumulated a total of 240 hours of flight time within the preceding 90 days. AIRCRAFT INFORMATION The single-engine helicopter (serial number 3568) was in the process of being repositioned from Arizona to North Carolina. The helicopter was equipped with an Allison 250-C20B turboshaft engine (serial number CAE 833679). A review of maintenance records revealed the helicopter underwent its last annual inspection on July 31, 2002. On November 25, 2002, at an airframe total time of 6,878.1hours, the helicopter's dual tachometer indicator (part number 206-070-265-11) was removed and replaced with an overhauled unit (serial number 918D). On December 6, 2002, at an airframe total time of 6,896.4 hours, the dual tachometer indicator was removed and replaced with another overhauled unit (serial number 637). On December 13, 2002, at an airframe total time of 6,908.3 hours, the helicopter underwent a 100- and 300-hour inspection. At the time of the 100- and 300-hour inspection, the engine total time was recorded as 6,038.0 hours. The dual tachometer indicator depicts speeds in percentage taken from the engine's N2 tach generator and the main rotor's tach generator. There are two needles on the tachometer gauge; one long needle labeled "T" and one short needle labeled "R". The long "T" needle represents the turbine (N2) rpm, and the short "R" needle represents the rotor rpm. According to work orders provided by the maintenance facility that performed the aforementioned work/inspections, the initial dual tachometer indicator was removed because the unit was "inoperable." On December 6, 2002, a work order entry indicated that the "N2 and NR needles [were] fluctuating on dual tach[ometer]." On December 6, 2002, the overhauled unit, serial number 637, was installed. The maintenance facility that performed the aforementioned work also provided the NTSB IIC the work order and airworthiness approval tag (FAA Form 8130-3) from the company who performed the dual tachometer overhauls. Dual tachometer 637 was overhauled, calibrated and tested on December 4, 2002. The work order indicated the "T pointer above zero position. R is inop. Motors noisy. T out of cal[ibration] 100% + 4%." During the overhaul, the "motor bearings were found dirty/dry," and were cleaned and lubricated. In addition, the "R motor magnet stop plate" was found "loose" and was repaired. Dual tachometer indicator 637 was the unit installed in the helicopter at the time of the accident. The helicopter was also equipped with an Altair Avionics IntelliStart Plus Engine Monitoring System. The system's purpose was to "limit the amount of fuel delivered to the engine during the starting process and to record inflight engine and performance data for trend analysis." WRECKAGE AND IMPACT INFORMATION The helicopter wreckage was located on the north side of interstate I-20 and west of farm road 880 at 32 degrees 22.570 minutes north latitude and 99 degrees 10.206 west longitude. The fuselage was upright, facing north, and came to rest at the base of the I-20 overpass sod slope. Photographs taken at the accident site revealed the tail boom was separated from the fuselage approximately one foot aft of the fuselage area, and was fractured in two major sections. One section (located approximately 60 yards north of the fuselage) extended aft from the fuselage separation point, to the vertical stabilizer attach point. This section included the right side of the horizontal stabilizer. The left horizontal stabilizer was separated from this section of the tail boom, but was located within the debris field between the fuselage and the forward section of tailboom. The second section (located adjacent to and on the right rear side of the fuselage) included the upper and lower vertical stabilizers, along with the tail rotor gearbox and tail rotor blades. The landing skids were shattered and the aft crosstube was forced up approximately two feet through the fuselage structure just aft of the fuel filler port. The bottom half of the fuselage sustained considerable buckling on both sides of the fuselage. The main rotor blades remained attached to the mast, and the mast remained attached to the transmission. The pitch change link tubes were fractured at their approximate mid-point and were bent and pinched at the fracture areas. Photographs of the instrument panel at the accident site revealed the "R" needle of the dual tachometer indicator was reading zero, while the "T" needle was pegged at the maximum position of 120% rpm. The wreckage was transported to Air Salvage of Dallas (ASOD), Lancaster, Texas, for further examination. TESTS AND RESEARCH On January 30, 2003, the NTSB IIC, an FAA inspector, and investigators from the engine and helicopter manufacturer examined the wreckage at ASOD. Examination of the helicopter revealed no mechanical anomalies that would have led to a rotor rpm exceedence. External examination of the engine did not reveal any damage. Manual rotation of the N1 section resulted in free and continuous rotation from the compressor to the N1 drivetrain in the accessory gearbox. Manual rotation of the N2 section resulted in continuous rotation with moderate drag when compared to the N1 section. It was noted that the dual tachometer's "R" needle was indicating 0% rpm and the "T" needle was indicated 17% rpm with no power applied to the unit. The dual tachometer instrument panel mount screws were loose. The instrument's N2 tach generator connector was in place and tight against the dual tachometer. The instrument's rotor tach generator connector was noticeably looser. Continuity was confirmed from the N2 and rotor tach generators to the dual tachometer indicator. A flange on the rotor tach generator (the outside diameter of the retaining ring flange) was fractured and a wire to the generator was found pinched and separated near the generator. The area of damaged generator wire and flange coincided with a damaged area at the left forward transmission mount. The dual tachometer instrument, rotor tach generator, and N2 tach generator were removed and taken to Bell Helicopter's Receiving Inspection area for additional testing. Both the rotor and N2 tach generators satisfactorily passed functional testing. Testing of the dual tachometer indicator resulted in satisfactory test results for the rotor rpm. However, the "T" needle (N2 turbine rpm) did not move until 130 rpm was applied. The needle then peaked to the full deflection area (120% rpm). The Altair Avionics IntelliStart Plus Engine Monitoring System's electronic control unit was removed and shipped to the manufacturer for download of recorded information. According to the manufacturer, flight data was retrieved dating back to October 2001, and contained data for the accident flight. The manufacturer reported that the system was designed to capture a rotor speed exceedence above 103% rpm while under engine power and 107% without engine power. No rotor speed exceedance was captured for the accident flight. In addition, the recorded power settings showed an average torque of 85% and both turbine and rotor rpm at 95% (normal operating rotor rpm is between 97-100%). According to the IntelliStart system manufacturer, this may have been due to a high dual tachometer indication. There was no indication of an engine failure or other anomalies. ADDITIONAL INFORMATION The wreckage was released to the owner's representative on August 12, 2003.
the pilot's failure to properly interpret and confirm a faulty tachometer instrument indication and his improper flare during the ensuing precautionary autorotation, which resulted in a hard landing. Also causal was the failure of the tachometer.
Source: NTSB Aviation Accident Database
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