Boulder City, NV, USA
N835GC
AIRBUS HELICOPTERS EC 130 T2
The helicopter was returning to the operating base following an air tour flight with six passengers. The helicopter entered a hover-taxi and the pilot initiated a slow left turn with a right crosswind of 11 knots, gusting to 20 kts. After the tail of the helicopter passed through the wind, the tail continued to swing to the right and the helicopter entered a climbing left spin. The pilot told investigators that when the helicopter started turning left, he applied right anti-torque pedal to stop the turn, and when the helicopter continued to spin left, he lowered the cyclic. The helicopter completed about 3 full left turns before descending rapidly and impacting the ground. The helicopter fuselage was substantially damaged, and 5 passengers and the pilot were seriously injured. The helicopter was equipped with an Engine Data Recorder (EDR) that stored pedal potentiometer (position) values and airport security video captured the accident sequence. The data and video evidence are consistent with the pilot initiating a left pedal turn by applying about 1/3 left pedal input (from neutral). After about 90° of heading change, as the tail passed through the wind line, the pilot applied about 3/4 right pedal input followed by reversal to 2/3 left pedal input (in the direction of rotation). The pilot then applied 1/2 - 3/4 right pedal input as he rapidly lowered the collective. The accident helicopter was equipped with an anti-torque Fenestron. Airbus Helicopters published an Information Bulletin that outlines the differing performance characteristics of a Fenestron-equipped helicopter compared to a helicopter with a conventional tail rotor (CTR). As outlined in the bulletin, on a CTR-equipped helicopter the thrust curve is more linear when compared to a Fenestron-equipped helicopter. The effect of a control input with a CTR is almost constant throughout the whole pedal range, while it significantly varies for the Fenestron. The thrust curve slope is larger, and thus the perceived efficiency of the Fenestron is greater, when coming close to the full left pedal stop. An Airbus Helicopters Safety Information Notice regarding unanticipated left yaw states “… use of the rudder pedal … may not cause the yaw to immediately subside, thus causing the pilot to make inadequate use of the pedal to correct the situation because he suspects that it is ineffective when, in fact, thrust capability of the tail rotor available to him remains undiminished.” “The key feature of an unanticipated left yaw recovery is large amplitude right pedal input. Recovery may not be immediate but will occur if the pilot persists in maintaining right pedal. In some instances, the pilot re-centered the pedal before entering again a right pedal input. This cannot help and only delays recovery from the yaw. If the yaw deceleration is not enough, more right pedal must be added, reaching the pedal end-stop if necessary.” The evidence indicates that when the pilot initiated the left hover-taxi turn he failed to apply right anti-torque pedal in a sufficient and timely manner to arrest the left turn. The helicopter subsequently entered an uncontrolled spin before impacting the ground. Six of the seven occupants received serious injuries attributable to the hard landing. The seats installed in the helicopter were equipped with energy-absorbing devices designed to reduce occupant injuries in the event of a hard landing and had been certified to standards established by the European Aviation Safety Agency (EASA) and the United States Code of Federal Regulations (CFR). Examination of the seats showed inconsistencies in the performance of the energy-absorbing devices. Measurements were taken, and it was noted that the seats did not stroke as expected when compared with the occupant’s overall stature. Energy-attenuating seats are designed to stroke to absorb an occupant’s energy, and the amount of stroke would vary based upon the weight of each occupant. In this accident there were occupants close to the size of a 50th percentile male used in certification; however, the seats did not stroke as expected. Additionally, there were larger occupants whose seats did not stroke at all, which likely contributed to the severity of the injuries of some of the occupants. A previous hard landing accident involving the same model helicopter and seats resulted in similar seat performance and occupant injuries. Those injuries were attributed to improper tightening and positioning of the seat restraints, which allowed the occupant’s positions in the seat to vary and affect the performance of the seats. Investigators could not determine if the occupant’s seat restraints were properly tightened and positioned in this accident. Performance studies of the accident determined the helicopter and the seats were subject to high lateral forces at impact as the helicopter rotated and the seat manufacturer stated the inconsistent performance of the seats was likely due to the lateral forces encountered; however neither the EASA or CFR certification standards specified lateral force testing. No anomalies were noted with the seats or energy-absorption devices during postaccident examination that would have contributed to their inconsistent performance. Therefore, the seat’s energy-absorbing devices likely performed inconsistently due to the lateral rotational forces encountered during the accident which were not required to be accounted for during the certification process.
HISTORY OF FLIGHTOn December 27, 2022, at 1635 Pacific daylight time, an Airbus Helicopters EC-130-T2, N835GC, was substantially damaged when it was involved in an accident near Boulder City, Nevada. The pilot and five passengers were seriously injured, and one passenger received minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations (CFR) Part 135 air tour flight. The accident helicopter was the second of four company helicopters returning from Kingman, Arizona, to Boulder City Municipal Airport (BVU). According to the operator, the helicopters were not flying as a formation. Security video captured the first helicopter enter the camera’s view of the landing area from the west and enter a hover-taxi over a closed runway south of the camera’s location. The first helicopter then made a left turn over a paved taxiway and proceeded north toward the ramp area. The accident helicopter was the second helicopter to return and was visible in the camera’s view entering the landing area from the southeast as the first helicopter entered the hover-taxi. The accident helicopter exited the camera’s view for a short period of time before reentering from the west, flying a similar approach to hover-taxi as the first helicopter. The video showed the accident helicopter traveling eastbound in a hover-taxi as the first helicopter entered the ramp area. The helicopter began a slow turn to the left as it approached the taxiway to the ramp area. The accident helicopter continued the left turn, entered a left climbing spin for about three revolutions, and then descended to the ground, impacting terrain 10-12 seconds after starting the left turn to align with the taxiway. The helicopter skids collapsed and the tail boom contacted the ground, resulting in substantial damage to the helicopter. The pilot told investigators that when the helicopter started turning left, he applied right anti-torque pedal to stop the turn; when the helicopter continued to spin left, he lowered the cyclic. The helicopter was equipped with an Engine Data Recorder (EDR) that records and stores specific engine parameters and some flight control parameters, including collective and pedal potentiometer values once every second (1Hz). Recovered EDR data for the accident flight is shown in Figure 1. The data showed that 10-12 seconds before ground impact the left pedal was deflected about 1/4 - 1/3 for about 4 seconds, followed by the right pedal being deflected about 3/4 towards full deflection, followed about 4 seconds later by a rapid reversal of pedal deflection to about 1/2 full left deflection, and then back to about ¼ - 3/4 full right pedal deflection until just before impact. (Note: The times and potentiometer readings are approximate due to the coarseness of the 1 Hz capture rate). Figure 1 – Collective and Pedal Potentiometer Data PERSONNEL INFORMATIONDuring a postaccident interview the pilot told investigators he had never been terminated from employment and that he had no previous experience flying EC-130 helicopters. Investigators discovered the pilot participated in EC-130 ground and flight training with Maverick Helicopters from January 3-19, 2022. Maverick Helicopters conducts air tours and operates from seven locations in Nevada, Arizona, Hawaii, and California. The pilot completed 7.6 hours of flight training before training was terminated. According to Maverick Helicopters personnel they “felt he was not experienced enough to handle our aircraft or our flight environment in a normal amount of training.” The pilot was not employed by Maverick Helicopters during the training program. The pilot was hired by Papillon Grand Canyon Helicopters (PGCH) November 30, 2022. On his pilot resume dated November 29, 2022, he stated he had 0.7 total flight hours in EC-130 helicopters, and he had accumulated that flight time in the previous 30 days. PGCH personnel stated they were unaware of the pilot’s training history with Maverick Helicopters before the accident and that they would not have extended an offer of employment to the pilot had they known of that experience. The PGCH Director of Operations further stated that there were no records of the pilot’s failure of the Maverick Helicopter’s training program in the Federal Aviation Administration’s Pilot Records Database. The pilot accumulated 7.4 flight hours during 8 training flights with PGCH. The pilot’s PGCH training records indicated the pilot achieved a satisfactory level of proficiency in all trained areas, as defined by Title 14 Code of Federal Regulations Part 135 standards. The pilot successfully completed a Part 135 flight evaluation on December 20, 2022. PGCH reported the pilot had accumulated 23 total flight hours in make and model helicopter at the time of the accident, which did not include the unreported flight time accumulated during training at Maverick Helicopters. The pilot was issued his private pilot certification in December 2019, and his commercial pilot certification in July 2020. His experience included flying air tour flights using Robinson helicopters, and his most recent experience was as second-in-command flying Boeing-Vertol 234 helicopters. ADDITIONAL INFORMATIONUnanticipated Yaw Airbus Helicopters Safety Information Notice (SIN) No. 3297-S-00 (Revision 0, 2019-07-03) regarding unanticipated left yaw, commonly referred to as Loss of Tail-rotor Effectiveness (LTE), states: “Unanticipated yaw is a flight characteristic to which all types of single rotor helicopter (regardless of anti-torque design) can be susceptible at low speed, dependent usually on the direction and strength of the wind relative to the helicopter.” “Where this type of unanticipated yaw situation is encountered, it may be rapid and most often will be in the opposite direction of the rotation of the main rotor blades (i.e. left yaw where the blades rotate clockwise). Swift corrective action is needed in response otherwise loss of control and possible accident may result.” “… use of the rudder pedal … may not cause the yaw to immediately subside, thus causing the pilot to make inadequate use of the pedal to correct the situation because he suspects that it is ineffective when, in fact, thrust capability of the tail rotor available to him remains undiminished.” “Stabilizing surfaces are installed downstream of the center of gravity. The tail rotor and the fin have this role and are well located for forward flight conditions. In a tailwind, however, their position on the helicopter is not ideal. As a result, they cause yaw instability. This can be managed as long as the pilot is aware of the wind direction relative to the helicopter.” “Most instances of unanticipated yaw which lead to accidents are to the left when the main rotor rotates clockwise. This shows that the main problem is not a tailwind or wind in the vicinity of the critical azimuth, where the pedal coming close to the 100% stop gives a clear warning. The main problem area for unanticipated left yaw is on the other side of the stability range, when the pedal position is much more benign.” “The key feature of an unanticipated left yaw recovery is large amplitude right pedal input. Recovery may not be immediate, but will occur if the pilot persists in maintaining right pedal. [Recentering the pedal and then re-entering right pedal input] cannot help and only delays recovery from the yaw. If the yaw deceleration is not enough, more right pedal must be added, reaching the pedal end-stop if necessary.” “The most probable reason for accidents following unanticipated yaw events is a late and too limited pedal input.” Airbus Helicopters Information Notice (IN) No. 3540-I-00 addresses some flight characteristics, including unanticipated yaw, that differ between Fenestron-equipped helicopters and conventional tail rotor (CTR) equipped helicopters whose main rotor rotates counterclockwise when seen from above. The notice outlines, “…some specific characteristics of the Fenestron that must be remembered, especially when transitioning from a helicopter equipped with a conventional tail rotor (CTR).” “The effect of a control input is almost constant in the whole pedal range [of a CTR equipped helicopter], while it significantly varies for the Fenestron.” “Transitioning from cruise to hover flight on a helicopter equipped with a Fenestron requires adding more left pedal than with a conventional tail rotor.” “The CTR [tail rotor thrust] curve is more linear. The effect of a control input is almost constant in the whole pedal range, while it significantly varies for the Fenestron. The slope, and thus the perceived efficiency of the control, is much larger when coming close to full left pedal stop.” The pilot told investigators that he was familiar with unanticipated yaw and that it was in “the manual” that he was responsible for reading. He stated he was not familiar with the IN that discusses the differences between Fenestron- and CTR-equipped helicopters. Papillon Grand Canyon Helicopters (PGCH) Training Program The PGCH training program consisted of both ground and flight training elements using Eurocopter manuals and training presentations. Pilots entering training were provided copies of the Eurocopter Training Manual and the Eurocopter Flight Manual, both of which provided information on the operating characteristics of the fenestron tail rotor system. At the time of the accident, PGCH also provided pilots in training with copies of SIB No. 3297-S-00 and IN No. 3540-I-00 and provided training regarding the subject matter in the notices, but they did not have focused training that covered those documents specifically. PGCH updated their training program following the accident to teach those two documents formally within their training program. INJURIES TO PERSONSFour of the seven occupants in the accident were diagnosed with vertebral fractures. One occupant had fractures at multiple vertebral levels and a fractured pelvis. Two occupants had lumbar fractures, one had a thoracic fracture, and another occupant had a right tibia and fibula fracture, and splenic laceration. SURVIVAL ASPECTSThe helicopter was equipped with eight single-occupant seats manufactured by Zodiac Seats France (Zodiac). The seats were designed and certified to the standards contained in European Aviation Safety Agency (EASA) European Technical Standard Order (ETSO) C127a. All seats consisted of a composite bucket affixed to a structural frame composing both the seat legs and seatback supports. The seatback supports contained energy-absorbing features designed to meet the requirements in 14 CFR § 27.785, 27.561, and 27.562. Corrugated absorption devices and fuses were built into either side of the seatback supports (a total of two in each seat) to absorb energy in the event of high vertical loading (figures 2 and 3). The fuses were designed to break and allow the corrugated absorption devices to function when exposed to six or more “g’s” of downward force. The composite seat bucket was affixed to the seat frame on a set of tracks via two “bucket fixings” and plastic bushings (rollers). When subjected to high vertical loads, these features allow the bucket to move downwards while the absorption devices deform (i.e. stretch) and absorb vertical energy. Figure 2 - Structural elements of the seat legs and seatback supports. Figure 3 - Close-up views of the corrugated vertical energy absorber and fuse. The seats were certified in dynamic conditions to conform to ETSO C127a. During certification, seat performance in the forward and downward directions were observed in dynamic conditions through three main tests: a 30G downward dynamic test, an 18.4G forward dynamic test, and a 20.0 G downward static test using a 170-lb (50-percentile male) test dummy. The certification standards described in ETSO C127a and 14 CFR § 27.785 and 27.561 do not require lateral (Y-axis) dynamic testing. All seats were equipped with a four-point restraint system with a rotary buckle, adjustable lap belt segments, and an inverted Y-yoke shoulder restraint that routed over the seat back and downward into an inertial reel that was secured to the seat bucket. All occupants were using the restraint system at the time of the accident. The positioning and tightness of the restraint systems at the time of the accident could not be verified for any of the occupants. The seats were documented in situ and then removed from the helicopter for additional examination. The following measurements and observations of downward stroke, forward deflection, and fuse performance were recorded. Note: Passenger seat 1 was unoccupied. No anomalies were noted with the seats or energy-absorption devices during postaccident examination that would have affected the performance of the seat’s energy-absorption capabilities. Similar seat performance and occupant injuries were documented in a previous accident investigation involving an EC-130 helicopter involved in a hard landing and equipped with the same models of seats (WPR16FA055). Five occupants received serious injuries and two received minor injuries during that accident. That investigation recovered photographic documentation of the occupants’ restraint system positioning before impact, which showed some of the occupants’ seat restraints were improperly positioned. The investigation concluded the seat displacement was directly related to the amount of vertical energy absorbed by the seat and to the severity of the occupants’ injuries. The previous investigation determined the loose and out-of-position seatbelts most likely allowed the passengers' bodies to shift out of position on the seat before and during the hard landing and did not restrain the occupants in the proper position for the seat to absorb the vertical landing loads for which it was designed. TESTS AND RESEARCHNTSB Research and Engineering staff conducted a performance study using the available security camera footage of the accident. The yaw rate and ground impact speed of the helicopter were estimated based on the video. The study estimated that the helicopter exhibited a fast counterclockwise yaw rotation as large as 139 degrees/second and that it impacted the ground with an estimated vertical speed of 43 feet/second. The average vertical deceleration of the passenger seats was estimated to be between 8.7 g and 12.4 g. If a triangular deceleration pulse is assumed, the peak deceleration was estimated to be between 17.4 g and 24.8 g. The peak deceleration was estimated by Airbus Helicopters at aircraft center of gravity to be 35.3 g downward, 10.5 g frontward and 10.5 g leftward (yaw). According to Safran Seats, the deceleration and vertical speed values determined by the NTSB are similar to their own estimations. Safran Seats further explained that the high lateral deformation observed in the pilot and passenger no.5 seats “indicated a significant Y (lateral) acceleration, which does happen in 30 g down certification.”
The pilot’s failure to apply anti-torque pedal input in a sufficient, timely, and sustained manner while attempting to arrest a turn during a hover-taxi, which resulted in a loss of directional control. Contributing to the severity of the occupants’ injuries was the inconsistent performance of the seats’ energy-absorption devices, which was likely due to the rotational forces encountered during the accident which were not required to be accounted for during the seat certification process.
Source: NTSB Aviation Accident Database
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