Gustavus, AK, USA
N907PL
AIRBUS HELICOPTERS INC AS350
The private pilot had just purchased the helicopter and was returning it to his home airport accompanied by a commercial-certificated safety pilot and two passengers. Video from the cockpit image recorder indicated that the pilot, seated in the right seat, was manipulating the flight controls from takeoff until the accident occurred. The surviving passenger stated that, while en route, the pilot indicated that they would be landing on a beach in order to stretch their legs. The recovered data showed the helicopter flying about 500 to 700 ft over water when the pilots began conversing and pointing toward the shore. The safety pilot, whose hands were on his lap, then raised his right hand in a manner that appeared that he was guarding the cyclic control in anticipation that his assistance or intervention might be required. Shortly thereafter, the pilot twisted the collective twist grip throttle from FLIGHT to IDLE; data from the engine data recorder (EDR) indicated that, at this time, the engine fuel control was set to the idle power setting. Such action is consistent with a practice autorotation. The collective control was adjusted downward slightly, but the main rotor speed (Nr) gauge showed Nr decay from the normal "green" operating range of 375 to 405 rpm to the "yellow" cautionary range of 320 to 375 rpm about 5 seconds after the twist grip was set to IDLE. An Nr value of less than 360 rpm would have resulted in a continuous aural tone to alert the pilots of the low rotor speed. About 7 seconds after the twist grip was set to IDLE, the pilot reached toward the center console, after which the "HORN" caution light illuminated, consistent with the pilot muting the aural tone signaling the low Nr condition. After the horn was muted, Nr continued to decay to a low of 254 rpm. Several small cyclic inputs were observed, then the recorded data ended. The helicopter impacted the water and was destroyed. The four occupants were ejected from the helicopter; one of the passengers survived the accident and swam to shore. Postaccident examination of the helicopter and the recovered data did not reveal any helicopter malfunctions or failures that would have precluded normal operation. Since recorded cockpit imagery did not capture microphone audio, the reason for the pilot's decision to roll the throttle to IDLE could not be determined, and what conversations the pilots may have had before and during the maneuver could not be determined. The passengers' headphones were muted from the pilots' conversation, and the surviving passenger did not know if they planned to practice an autorotation to the beach. Cockpit video recordings from previous flights indicated that the safety pilot did not guard the flight controls when the pilot was performing takeoffs or landings. It is plausible that the pilot's intention was to practice or perform an autorotation to the beach, with the safety pilot guarding the controls. However, the beach was not the most suitable location for a practice autorotation to landing procedure and was out of the ordinary from their previous flights, as observed on the recorded cockpit imagery. When the twist grip throttle is set to IDLE position during normal flight, Nr will decay unless the pilot takes action to maintain it, such as a reduction in main rotor collective pitch or returning the twist grip throttle to the FLIGHT position. However, after the throttle was set to IDLE, the pilot failed to reduce main rotor collective pitch, which resulted in low rotor rpm. The pilot's action of muting the low rotor rpm alert horn indicated that he was aware of the annunciation to an anomalous condition, but it could not be determined if the pilot understood the nature of why the horn annunciated or the criticality of the situation.
HISTORY OF FLIGHTOn September 28, 2018, at 1057 Alaska daylight time, an Airbus Helicopters AS350-B3e helicopter, N907PL, was destroyed when it was involved in an accident in Glacier Bay National Park, about 60 miles northwest of Gustavus, Alaska. The safety pilot was fatally injured, one passenger sustained serious injuries, and the pilot and another passenger remain missing and are presumed fatal. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The purpose of the trip was to deliver the newly-purchased helicopter to Anchorage from the Airbus Helicopters factory in Texas. The left seat safety pilot was onboard for insurance coverage purposes and was acting as a safety pilot. The right seat pilot-in-command, who owned the helicopter, planned to drop off the safety pilot in Wasilla, Alaska, then proceed to Anchorage with the passengers. The trip began on September 25, 2018, and included more than 30 stops for sightseeing, fuel, and rest. In a postaccident interview, the surviving passenger, who was seated in the left rear seat, stated that the accident flight departed Juneau International Airport (JNU), Juneau, Alaska, and proceeded north toward Yakutat Airport (YAK), Yakutat, Alaska, at low altitude parallel to the shoreline. The passengers' headphones were muted, but at one point, the pilot unmuted the passengers' headphones and asked if the passengers wanted to land on a beach to stretch their legs. About 1 minute later, the safety pilot pointed his hand to the right toward a long stretch of beach. The passenger recalled that the pilot initiated a controlled right turn and the helicopter began to descend; the safety pilot did not have his hands on the flight controls. He stated that the pilot pulled up on the collective and rolled the throttle off. He never heard anything abnormal with the engine noise or noticed any anomalies with the helicopter. The pilot left the collective up and the helicopter entered a free fall from about 500 ft agl, then about 30 ft agl the pilot increased the throttle again. Before impact, he heard the pilot yell "NO" and continue to manipulate the flight controls. He felt the helicopter impact the water and noticed water splash in the cabin before he lost consciousness. He later awoke in the water and swam to shore, where he awaited rescue. He was unable to locate the other occupants. A review of the onboard cockpit image recorder, an Appareo Vision 1000, revealed that the helicopter was refueled at JNU before departing on the accident flight. The helicopter departed JNU and proceeded west over the mountains about 3,000 to 4,000 ft mean sea level (msl), then northwest along the coastline about 500 to 700 ft msl. Figure 1 depicts the helicopter's flight path from JNU to the accident site. Figure 1 – The helicopter flight track in orange from JNU to the accident site. PERSONNEL INFORMATIONThe pilot began his helicopter flight training in a Robinson R44 helicopter and had accumulated 59 hours of helicopter flight experience. On forms provided to Airbus before conducting AS350-B3e model transition training, the pilot reported no experience in the accident helicopter make and model. On June 4, 2018, the pilot completed transition training for the AS350-B3e with an Airbus Helicopters flight instructor at the Airbus factory in Grand Prairie, Texas; this training included 3 hours of flight time and 1 hour of simulator time. From June 18 to August 29, 2018, the pilot completed at least 10 flights (more than 18.3 hours) in an AS350-B2 model operated by the safety pilot's company in Alaska. in Alaska. Of the 18.3 hours, 11.4 were conducted with the accident safety pilot and 6.9 were conducted with a company flight instructor. On August 5, 2018, the pilot completed a 1.5-hour flight at the Airbus factory with the same Airbus Helicopters flight instructor in an AS350-B3e. The pilot had accumulated 4.5 flight hours in the B3e before departing Texas with the accident helicopter. The safety pilot was the owner, director of operations, and chief pilot for two different commercial helicopter operators in Alaska that operated several AS350-B2 model helicopters. The surviving passenger was not a pilot, but he did attend helicopter ground school classes with the accident pilot and had knowledge of helicopter procedures and helicopter flight theory. The passenger stated that, when taking delivery of the accident helicopter in Texas, the safety pilot did not seem recently familiar with the B3e model and the options that were installed on the accident helicopter. The safety pilot asked a lot of questions about the B3e, and the pilot appeared more familiar with the helicopter systems than the safety pilot. AIRCRAFT INFORMATIONThe Airbus Helicopters was equipped with a three-bladed main rotor system and a two-bladed tail rotor system. The flight controls are were hydraulically assisted by a dual hydraulic system. The helicopter was equipped with both pilot and safety pilot controls and a Genesys Aerosystems HeliSAS autopilot and stability augmentation system. The collective-mounted engine control twist grip throttle contains two positions: IDLE and FLIGHT. In the normal procedures section of the AS350 B3e rotorcraft flight manual (RFM), the twist grip throttle is moved from the IDLE position to the FLIGHT position during the run-up checks. The twist grip throttle remains in the FLIGHT position until the postlanding engine and rotor shutdown procedures are performed. When the twist grip throttle is outside of its FLIGHT position, a "TWT GRIP" warning light illuminates on the cockpit caution and warning panel (CWP). According to Airbus Helicopters, the airframe and engine had accumulated a total time of 13.7 hours at the time of delivery. An estimated 25-30 hours was accumulated from delivery until the accident. METEOROLOGICAL INFORMATIONThe National Weather Service (NWS) Alaska Aviation Weather Unit issued flying weather graphics, which forecast marginal visual flight rules and no low-level turbulence for the accident area. The FAA Aviation Weather Cameras for YAK and Cape Spencer, Alaska, revealed a broken to overcast cloud layer and clear visibility around the time of the accident flight. The Appareo Vision 1000 onboard the helicopter showed brief periods of light rain on the helicopter's windscreen. The rain stopped about 7 minutes before the accident. AIRPORT INFORMATIONThe Airbus Helicopters was equipped with a three-bladed main rotor system and a two-bladed tail rotor system. The flight controls are were hydraulically assisted by a dual hydraulic system. The helicopter was equipped with both pilot and safety pilot controls and a Genesys Aerosystems HeliSAS autopilot and stability augmentation system. The collective-mounted engine control twist grip throttle contains two positions: IDLE and FLIGHT. In the normal procedures section of the AS350 B3e rotorcraft flight manual (RFM), the twist grip throttle is moved from the IDLE position to the FLIGHT position during the run-up checks. The twist grip throttle remains in the FLIGHT position until the postlanding engine and rotor shutdown procedures are performed. When the twist grip throttle is outside of its FLIGHT position, a "TWT GRIP" warning light illuminates on the cockpit caution and warning panel (CWP). According to Airbus Helicopters, the airframe and engine had accumulated a total time of 13.7 hours at the time of delivery. An estimated 25-30 hours was accumulated from delivery until the accident. WRECKAGE AND IMPACT INFORMATIONThe main fuselage was found on a beach on its left side and partially embedded in the sand as shown in Figure 3. The structure forward of the aft cabin bulkhead had separated from the main fuselage. The left and right aft bench seats remained attached to the aft cabin bulkhead. The two longitudinal floor beams for the cabin floor structure were fractured several inches forward of the aft cabin bulkhead. The rear structure of the main fuselage was crushed inward throughout its circumference. The engine firewall remained attached to the main fuselage; its base was deformed in the aft direction and its upper portion was partially fractured. The engine deck was deformed downward. The engine air inlet barrier filter remained installed within its frame, but the cowling surrounding the frame had separated. Sand was found within the barrier filter. Figure 3 – Helicopter main wreckage partially embedded in the sand at low tide. The cockpit floor, with the front seats attached, was recovered. The cockpit and cabin roof structure, about 5.5 ft in length measured from the upper windshield attachment, was also recovered. The Vision 1000 remained attached to the roof structure. The three main rotor blades were found separated from the rotor hub and were broken into large pieces. The tail boom, tail rotor assembly, and most of the instrument panel were not found. The fuel tank remained installed within the center fuselage and was removed at the accident site by the investigation team in support of the wreckage recovery. About 25 gallons of fuel was removed from the fuel tank at the site; additional fuel was removed after recovery of the fuel tank. Evidence of saltwater was observed in the fuel removed from the fuel tank. All engine electrical, oil, fuel, and pneumatic connections were installed and intact. These connections were separated to facilitate removal of the engine from the airframe. One of the first stage compressor blades exhibited curling deformation in the direction opposite of normal rotation at its tip end. The pilot cyclic control was fractured from its lower attachment but remained with the wreckage via electrical wiring through its post. The grip remained attached to the pilot cyclic control. The safety pilot cyclic control and grip remained installed but exhibited deformation in multiple locations. Fractures were observed on multiple control tubes between the pilot and safety pilot cyclic controls to the mixing unit. The pilot and safety pilot collective controls remained installed. The pilot collective head was present on the control stick but was partially separated and rotated to the right. The safety pilot collective head had separated and was missing from the control stick. The engine throttle twist grip was present on both the pilot and safety pilot collective controls and both twist grips were in the FLIGHT position. An attempt to manually rotate the twist grip was unsuccessful. Fractures were observed on multiple control tubes between the pilot and co- pilot collective controls to the mixing unit. All fractures exhibited signatures consistent with overload. The pilot and safety pilot pedals remained installed. The pilot pedals moved independently of each other and the interconnect linkages were fractured. The safety pilot pedal set remained interconnected. There were several impact-related separations leading up to the tail boom separation point. The tail rotor controls aft of the tail boom separation point were not recovered. The rotor brake and the emergency fuel shutoff handles remained installed on the roof and were in the stowed position. Examination of the turboshaft engine revealed that the linking tube that housed the power transmission shaft was dented on its underside where it normally contacts the engine rear mount saddle. The reduction gearbox was removed from the engine. The index mark on the splined nut (in the reduction gearbox) was slightly offset in the tightening direction. The offset was measured to be greater than 0 millimeter (mm) but less than 1 mm. The splined nut and its mating splines did not exhibit anomalous damage. The reduction gearbox geartrain exhibited continuity, but rotation was limited with evidence of binding. Sand was observed within the visible interior surfaces of the reduction gearbox. All free turbine blades were present and did not exhibit anomalous damage. Once separated from the engine, the free turbine spun freely when manually rotated. The free turbine nozzle guide vanes were removed, revealing the high pressure turbine. All high pressure turbine blades were present and did not exhibit anomalous damage. The gas generator module could not be rotated by hand. Sand was observed within the gas generator module. The axial compressor and gas generator modules were subsequently removed, after which continuity of drive through the accessory gearbox was confirmed via manual rotation of the accessory gearbox input drive. The fuel filter was removed and the filter element was clean. Sand was observed in the fuel filter bowl along with residual fuel. The oil filter was removed and the filter element exhibited no evidence of debris. A residual liquid with the appearance of an oil and water mixture was observed within the oil filter bowl. The turbine inlet temperature thermocouples were removed and exhibited no evidence of melting. Removal of the engine's two magnetic chip detectors revealed evidence of sand. Removal of the engine's electronic chip detector revealed no evidence of debris. The bleed valve port and the auxiliary port were packed with sand. Examination of the engine and airframe did not reveal any preimpact mechanical malfunctions or failures that would have precluded normal operation. For more detailed information, reference the Airworthiness Factual Report in the public docket for this investigation. ADDITIONAL INFORMATIONAccording to the helicopter's RFM, the bleed valve is normally open when the engine is shut down, during starting, and at low power settings. The bleed valve open indicator, displayed as a solid green-colored flag with two vertical white stripes, appears above the FLI when the bleed valve is open. The bleed valve open indicator disappears when the bleed valve closes. Fuel Sample Immediately after the accident, a manager at the fixed based operator at JNU where the helicopter had refueled secured a fuel sample from their fuel supply. The sample was tested for water and visible contaminants; no water or other contamination was found. Appareo Vision 1000 A review of additional images from the unit showed that during the most recent flights preceding the accident flight, the safety pilot did not guard the cyclic or collective while the pilot was executing a takeoff or landing maneuver. Autorotation Procedures In an autorotation maneuver, after the removal of engine power, whether due to engine failure or intentional throttle reduction, the pilot is required to manipulate the flight controls in order to maintain Nr and a normal attitude during the autorotative descent. The primary means to control Nr during an autorotation is the collective control. Excessive Nr can result in a main rotor overspeed, and an increase in collective pitch is required to reduce Nr to the required range. If Nr becomes too slow, a decrease in collective pitch is required to increase Nr to the required range. Supplement 6 of the AS350 B3e RFM contained procedures for autorotation landing training. According to Supplement 6, in the event of an engine failure or a loss of engine power, "the helicopter will yaw to the right, some red warnings may come on associated with the gong audio warning, the Nr will decay, and the low Nr audio warning will sound if Nr goes below 360 rpm." A loss of engine power could be simulated by setting the twist grip to the IDLE detent, which would produce the same symptoms as that of an actual engine failure or loss of engine power. A note within the emergency procedures of Supplement 6 stated that, "if necessary, it is possible to quickly turn the twist grip back to the FLIGHT position at any time and for any Nr value." FLIGHT RECORDERSAppareo Vision 1000 The helicopter was equipped with an Appareo Vision 1000 cockpit image and data recorder at the time of manufacture. The unit was capable of recording images, audio, GPS coordinates, and pitch, roll, yaw and acceleration data. The unit was mounted in the aft center ceiling of the cockpit. The unit recorded images, ambient audio, and parametric data for the entire accident flight. The field of view included over-the-shoulder images of the forward cockpit, including both cyclic controls and the
The pilot's failure to maintain main rotor speed after setting the engine fuel control to idle, which resulted in a loss of helicopter control and impact with water.
Source: NTSB Aviation Accident Database
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