League City, TX, USA
N355CV
Eurocopter AS355N
"THIS CASE WAS MODIFIED MAY 30, 2006." The 13,800-hour airline transport pilot and five passengers departed on a short pleasure flight after satisfactorily completing a maintenance check with the engines running on the ground. The pilot flew to a local airport and made a normal straight-in approach, executed an autorotation with a power recovery, then proceeded back to the home airport. While en route, after the helicopter crossed over a large row of power lines, the pilot reported the helicopter lost all engine power. However, none of the anticipated aural warnings, annuciator lights, abnormal vibrations or sounds associated with a power failure was observed. The pilot entered an autorotation and the helicopter contacted the ground hard and rolled over. Examination of the helicopter revealed continuity to all flight controls and the main gearbox. The #1 engine was placed on a test cell, started immediately, and was run at various power settings without interruption. Due to impact damage the #2 engine could not be test-run. Examination of the #1 and #2 engines' main gearbox coupling shafts by an NTSB Materials Specialist revealed that the shafts were both fractured at the reduced section adjacent to the main gear box input splines. The fracture plane, presence of dimpling, and direction of fracture were consistent with ductile torsional overstress with the engines running. In addition, data retrieved from the digital engine control units and damage to the main gear drive shafts indicated that both engines were operating at the time of impact. No mechanical deficiencies were noted that would have attributed to a loss of power on either engine. Several attempts were made to obtain a copy of NTSB Form 6120.1/2, Pilot/Operator Accident Report; however, the pilot failed to submit a completed report.
HISTORY OF FLIGHT On May 28, 2004, at 0935 central daylight time, a twin-engine Eurocopter AS355N helicopter, N355CV, was substantially damaged during a hard landing following an autorotation to a field near League City, Texas. The airline transport pilot and three passengers sustained minor injuries. Two passengers were not injured. The helicopter was registered to Summit Seafood Supply Incorporated, of Wilmington, Delaware, and was being operated by Landry's Seafood, of Houston, Texas. No flight plan was filed for the local flight that originated from the William P. Hobby Airport, near Houston, Texas, at 0920. Visual meteorological conditions prevailed for the personal flight conducted under 14 Code of Federal Regulations Part 91. The operator needed to comply with a mandatory Federal Aviation Administration (FAA) Airworthiness Directive (AD) that required a vibration check of the number two engine starter generator. The AD predicated the check be conducted during a ground run; however, the operator elected to test vibration levels in flight as well. As an appreciative gesture, the pilot invited four employees (in addition to an airframe and power plant mechanic) to be onboard the helicopter for the flight. During an interview with an FAA inspector, the pilot stated that after a vibration check was completed on the ground, he departed Hobby Airport and flew to Clover Field, near Pearland, Texas. He made a normal straight-in approach to Runway 14, and then executed an autorotation with a power recovery. The pilot then departed Clover Field to the south and climbed to an altitude of 700 feet above ground level. Shortly after the helicopter crossed over a large row of power lines, it "lost all power" and the pilot noticed a "zero" reading on one of the engine gauges, but could not recall which gauge. In addition, the helicopter would not respond to collective power changes. The pilot said there were no aural warnings, annuciator lights, abnormal vibrations or sounds associated with the power failure. He entered an autorotation and "used the tail boom to absorb the impact." The helicopter rolled and came to rest on its right side. In a written statement, the pilot described the power failure as "a sort of anomaly, power interruption, or power surge" that caused him concern. He recalled telling the mechanic that he had no power available and "vaguely" remembered looking at "a gauge" and seeing it read "zero or zeroes or not having any readings." In a written statement, the mechanic said that after the ground-run vibration check was completed, the pilot departed Hobby Airport and flew toward Clover Field. While en route, the in-flight vibration check was satisfactorily completed. The pilot then made a normal approach into Clover Field, hovered for a few moments, then departed to the south. Shortly after departure, the helicopter crossed over some high-tension power lines and the pilot made a right turn back toward Hobby Airport. The mechanic said, "Moments after beginning the turn it felt like the bottom was falling out beneath us and the helicopter was pivoting on its vertical axis to the right. I could see and feel that we were just falling to the ground at what felt like a pretty good pace. I heard [the pilot] say something to the effect of, 'Oh [expletive]...Hang on, were going to hit.' " He added that the helicopter impacted the ground "pretty violently." PERSONNEL INFORMATION The pilot held an airline transport pilot certificate with ratings for airplane multi-engine land and rotorcraft-helicopter. He also held a commercial pilot certificate for airplane single-engine land and airplane single-engine sea. In addition, he was type rated in several aircraft. His most recent FAA first class medical was issued on January 5, 2004. At that time, he reported a total of 13,800 flight hours. The pilot was also an FAA designated pilot examiner and a former FAA safety inspector. At the time of the accident, the pilot was a contract employee for the owner/operator. METEOROLOGY INFORMATION Weather reported at the Hobby Airport at 0915 was reported as wind from 210 degrees at 9 knots, visibility 9 statute miles, broken clouds at 16,000 feet, temperature 81 degrees Fahrenheit, dewpoint 75 Fahrenheit, and a barometric pressure setting of 29.89 inches of Mercury. WRECKAGE AND IMPACT INFORMATION Two FAA inspectors performed an on-scene examination of the helicopter on the day of the accident. All major components were accounted for at the site. The helicopter came to rest on its right side facing a westerly direction. The right skid and tail rotor assembly were separated, and the tail boom was broken in half. The main rotor blades remained attached to the hub and were bent and damaged. About 30 feet forward of the main wreckage was a dense tree line that ran northeast/southwest. In addition, there were three rows of approximately 100-foot-tall high-tension power lines that ran east/west about 100 feet north of the main wreckage. Flight control continuity was established for the cyclic and collective from the cockpit to the main rotor head. The main rotor pitch links were still intact and connected to each blade horn. Continuity was also established for the tail rotor system. However, the tail rotor pitch change control rod was broken near the tail boom-to-fuselage junction, and again at the point where the tail boom had separated. Continuity for the main gearbox was established by manually rotating the rotor head. The main rotor mast was rotated and both freewheeling clutches moved freely. Gear continuity was intact through the tail rotor drive shaft and the air conditioner drive located on the front of the combining gearbox. However, continuity was not established for either engine to the main gearbox drive shafts. Both main gearbox-coupling shafts were sheared at the transmission input. The flared couplings of both shafts exhibited circumferential scoring. The ends of the flex couplings were flattened, exhibited material smearing, and partial delamination. The #2 engine gearbox input flange was broken at the attachment to the gear, and the left gearbox input was intact. External examination of the engines revealed that the #2 engine was not damaged, but the #1 engine power turbine had separated and the case sustained damage. A 6x2-inch-wide puncture hole was found on the composite cowling as well as on the firewall between both engines. In addition, pieces of power turbine blades were imbedded on the interior wall of the engine cowling. The power turbine disk was found imbedded in the ground near the main wreckage. A kickback of dirt was near the imbedded wheel; indicative that it was rotating at the time it contacted the ground. The #1 engine was placed on a test cell. It started immediately and was run at various power settings without interruption. No pre-mishap anomalies were noted. The #2 engine sustained damage and it could not be test-run. Examination of the engine revealed that the power turbine attachment point exhibited rotational/fracture damage at the four o'clock position. The power turbine blades made contact with the abradeable material of the power turbine labyrinth. Debris was found inside the chamber between the swirl plate and diffuser in a pool of oil. Internal scoring was noted on the power turbine-bearing race. In addition, there were corresponding scoring marks on the outer surface of the impeller blades. There were impact marks imbedded approximately 180 degrees around the compressor cap from the impeller blades. The blades also exhibited some damage to the trailing edges. The chip detector was absent of debris. The reduction gears were seized. Disassembly revealed damage to the main drive shaft, due to contact with the oil pressure sensor. No mechanical deficiencies were noted. The #1 and #2 engines' main gearbox coupling shafts and flared couplings, along with the #1engine's power turbine flange, power turbine nozzle guide vane, drive shaft, blades, and disk were examined by a Materials Specialist at the Safety Board's Materials Laboratory, Washington, DC. Visual examination of both engines' main gearbox coupling shafts revealed they were both fractured at the reduced section adjacent to the MGB input splines. The fractures were perpendicular to the surface and very flat. Optical and scanning electron microscope (SEM) examination of the mating fracture surfaces on the forward side showed that both had elongated ductile dimples oriented in the clockwise direction. The fracture plane, presence of dimpling, and direction of fracture were consistent with ductile torsional overstress with the engines running (arrest of the gearbox side of the shaft). Extensive fracture surface smearing was observed between the areas of dimpling from the fracture surfaces rubbing together. The flared couplings did not contain significant damage, except for a small piece of the #1 engine flared coupling forward region. Fractures on the #1 engine flared coupling, the power turbine shield, input flange and the nozzle were all typical of overstress. Examination of the power turbine revealed that it contained 23 out of the 29 blades, with six turbine blades completely missing. The 23 blades were all fractured at approximately the same height at the forward side of the disc posts. The fracture surface of the blades exhibited rough features with uniform oxidation, which was consistent with overstress. No evidence of fatigue was found on any of the blades. The disk itself sustained extensive circumferential smearing on the aft face and all five-bolt holes were elongated with preferential damage on the aft side of the hole. On the forward side of the disk, a section of the inner rabbet that fits into the power turbine drive shaft was sheared off. The aft end of the power turbine drive shaft attached to the forward face of the power turbine disk. The aft flange of the drive shaft contained remnants of the bolts in the bolt holes, and heavy smearing damage was found on the face and inner rabbet that mated with a section of the power turbine disk that fractured. The drive shaft was blackened adjacent to the aft-most ball bearing, and the shaft was bent. Examination of the five bolt remnants revealed fractures consistent with overstress. Examination of the aft bearing cage and split inner races revealed that cage exhibited regions of oil coking. The cage pockets were not significantly rubbed whereas the inner and out diameter surfaced adjacent to the retaining tabs showed circumferential smearing marks. The inner race mating surfaces were fractured 360-degrees around with missing material. The race shoulders exhibited evidence of circumferential scoring. According to the manufacturer, the engines' over speed protection system would automatically shut down an engine when an overspeed is detected and also disarm the overspeed protection on the operating engine. This would allow a pilot to have additional power on the remaining engine to make a safe landing. The helicopter was also designed to climb to 7,000 feet mean sea level on one engine at a gross weight of 2,250 kg. Both digital engine control units (DECU) were examined at Turbomeca's facility in France. Examination of the right engine DECU revealed that there were no failure messages during the accident flight. However, the over speed protection was triggered and the engine was shut down automatically, as designed. Examination of the left engine DECU revealed that there were no failure messages during the accident flight. The over speed protection was not available (as designed) and there was also an incorrect T4.5 (thermocouple) measurement, which would not attribute to a power loss. No premishap anomalies were noted with the engine. ADDITIONAL INFORMATION According to the Eurocopter AS 355N Flight Manual, Chapter 3 section 3.1, Autorotative Landing, stated, "The following procedures are to be applied where necessary (unlikely): On land or on water with emergency flotation gear. - Reduce collective pitch to obtain an NR near the nominal speed - Fly at an IAS of 65 knots ( 120Km/h - 75 MPH) into wind - Set OFF-FLT (ARRET-VOL) selector switches to OFF (ARRET) - Close fuel shut-off cocks - Switch OFF booster pumps - Engage EMERGENCY CUT-OUT (COUPE TOUT) switch (on final approach) - At a height between 50 and 100 feet (15 and 30m), depending on the weight and external conditions (wind, terrain), start to flare out to a nose-up attitude of 15 to 20 degrees to reduce the forward speed and the rate of descent. Monitor NR." - At a height of between 10 and 15 feet reduce the nose-up attitude to approximately 5 degrees and start to apply collective pitch. - Immediately prior to touch-down apply collective pitch progressively up to the high pitch stop if necessary whilst holding back slightly on the cyclic pitch control. - After touch-down, reduce the collective pitch slowly to prevent an abrupt stop if any forward speed is present. - Carry out the safety procedures if necessary (emergency cut-out switch, fire extinguisher) - Apply the rotor brake - Evacuate the aircraft by jettisoning the doors if necessary Several attempts were made to obtain a copy of NTSB Form 6120.1/2, Pilot/Operator Accident Report; however, the pilot failed to submit a completed report. The wreckage was released to a representative of the owner's insurance company.
"THIS CASE WAS MODIFIED MAY 30, 2006." The pilot's misjudged flare during an autorotation, which resulted in a hard landing and subsequent rollover.
Source: NTSB Aviation Accident Database
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