N5748M
BELL 214
Witness accounts from the flight crew of the helicopter flying behind the accident helicopter indicated that, after both helicopters had climbed to an altitude between about 800 to 1,000 feet, the accident helicopter rolled sharply to the right and entered a steep, nose-down pitch, at which time the aft tailboom separated from the helicopter. The helicopter's nose-down pitch attitude continued until ground impact. There was no evidence of abnormal engine operation at the time of the accident or a failure of the tail rotor system or a major airframe structural component before the in-flight breakup. The damage observed on the right collective lever arm, idler link, and collective sleeve bearing support assemblies were consistent with an overtravel of the collective sleeve, which resulted in a severe and sudden reduction in main rotor collective pitch. Also, the damage to the left and right sides of the idler link showed that the left and right collective lever arms were in their installed position at the time of the collective sleeve overtravel. Bell Helicopter Textron's math model simulation of the Bell 214 helicopter's response to a sudden and severe reduction of main rotor collective pitch (resulting from a failure of the collective flight control system) showed that the helicopter would respond with a severe right roll and a simultaneous nose-down pitch attitude. The math model simulation also showed that a sudden and severe reduction of main rotor collective pitch would cause the main rotor hub to exceed its flapping angle and the main rotor blade tips to deflect downward and possibly contact the tailboom due to the reduced amount of clearance. The results of the math model simulation were closely aligned with the witness accounts from the flight crew of the helicopter behind the accident helicopter and the damage observed on the helicopter wreckage. For example, the nose-down pitch and right roll observed in the math model simulation was similar to the reported pitch and roll of the accident helicopter when the accident sequence began. Also, the main rotor mast showed damage consistent with mast bumping that resulted when the main rotor hub significantly exceeded its flapping angle. Thus, the evidence showed that, when the accident helicopter's main rotor hub exceeded its flapping angle, the resulting loss of clearance between the main rotor system blades and the tailboom caused the main rotor blades to contact the tailboom. This contact was severe enough to cause the aft tailboom section to completely separate from the rest of the tailboom, leading to a loss of pitch control.
HISTORY OF FLIGHTOn January 16, 2012, about 1045 Afghanistan time (0615 UTC), a Bell 214ST helicopter, N5748M, crashed 7 miles south of Camp Bastion in the Helmand province of Afghanistan. The helicopter was operated by AAR Airlift Group under contract to the Department of Defense (DOD) Air Mobility Command (the air component of the US Transportation Command). The flight was conducted under the provisions of 14 Code of Federal Regulations (CFR) Part 135. Visual meteorological conditions prevailed at the time of the accident. The three crewmembers, who were US citizens, died as a result of their injuries from the accident. The helicopter was destroyed by impact forces and postcrash fire. There were no reports of any hostilities in the area at the time of the accident. The accident helicopter was operating between several forward operating bases in Afghanistan along with another Bell 214 helicopter (N391AL) to transport military personnel between bases. The accident helicopter used the call sign "Slingshot 72"; the other helicopter used the call sign "Slingshot 71." Both helicopters departed Shindand Air Base about 0901 and flew to Camp Farah. Slingshot 72 was the lead helicopter for that flight. The Slingshot 71 flight crewmembers stated that they could see Slingshot 72 during the flight, even though the helicopters were separated "by a couple of miles." At Camp Farah, the engines on both helicopters were shut down, and the helicopters were refueled. Two passengers then boarded each helicopter for the next flights, which departed Camp Farah about 0934 for Camp Bastion. The helicopters arrived at Camp Bastion about 1031, and the passengers disembarked the helicopters. The engines continued to run while the helicopters were at Camp Bastion. Four passengers then boarded Slingshot 71; no passengers boarded Slingshot 72. The pilot-in-command (PIC) of Slingshot 71 stated that the crew chief of each helicopter (one of the three crewmembers aboard each helicopter) performed a walk-around inspection while the helicopters were at Camp Bastion. About 1040, both helicopters departed Camp Bastion for Shindand Air Base on a heading of 190º and climbed to a cruise altitude between about 800 and 1,000 feet. Slingshot 72 was the lead helicopter, and Slingshot 71 was trailing the helicopter by about ¼ to ½ mile. The Slingshot 71 PIC and second-in-command (SIC) stated that Slingshot 72 was flying slightly above and to the right of their helicopter. According to the SIC of Slingshot 71, "it was a normal climbout…" and that "they were climbing at a rate of about 300 feet per minute (FPM) at an airspeed of 120 knots." The SIC of Slingshot 71, who was the pilot flying, saw Slingshot 72 enter a "sharp" bank to the right; he estimated that the bank was about 70º to 80º. The SIC then saw Slingshot 72 begin to "come apart." The SIC stated that he flew Slingshot 71 to the left to avoid the large amount of debris coming from Slingshot 72, which included large pieces of structure. The SIC reported that the tailboom of Slingshot 72 began to "separate and fold" and estimated that about two-thirds of the tailboom came off the helicopter. The SIC then saw Slingshot 72 pitch down about 75º to 80º, impact the ground, and burst into flames. The PIC of Slingshot 71, who was the pilot monitoring, stated that, after making a radio frequency change, he looked up and saw Slingshot 72 in a "steep pitch down." The PIC stated that "nothing seemed wrong" with Slingshot 72 when he had looked down to make the radio frequency change. The PIC also stated that he saw structure starting to come off Slingshot 72 and that the debris looked similar to "confetti." The PIC noticed that the tailboom of Slingshot 72 appeared to be folded under the helicopter and stated that he could see the "zinc' color of the inside of the tailboom. Afterward, the PIC observed Slingshot 72 descend "straight down," impact the ground, and burst into flames. The PIC reported that the flight crew of Slingshot 72 made no radio transmissions indicating any problems. The flight crew of Slingshot 71 contacted controllers at the Camp Bastion tower, who coordinated crash rescue efforts. Also, Slingshot 71 circled the wreckage several times so that the crewmembers could look for any survivors aboard Slingshot 72. After US forces reached the area and secured the accident site, Slingshot 71 departed the area and landed uneventfully at Camp Bastion. The Slingshot 72 wreckage was then recovered and moved to a secure location at Camp Bastion. On January 18, 2012, the National Transportation Safety Board (NTSB) accepted full delegation of this accident investigation from the Ministry of Transport and Civil Aviation of the Islamic Republic of Afghanistan in accordance with the provisions of Annex 13 to the Convention on International Civil Aviation. Afterward, the helicopter wreckage was sent from Afghanistan to the United States for examination and analysis. PERSONNEL INFORMATIONThe three crewmembers aboard the accident helicopter were the PIC, the SIC, and the crew chief. The PIC, age 48, held an airline transport pilot certificate issued on March 2, 2011, for the BH-214ST and BK 117 helicopters with airplane single-engine land and instrument airplane ratings. The PIC held a second-class medical certificate issued on May 9, 2011, with the limitation that he must wear corrective lenses while exercising the privileges of the certificate. A review of FAA records found no prior accident, incident, or enforcement actions. The PIC was hired by AAR Airlift Group in January 2011. His most recent recurrent ground training occurred on March 1, 2011, and his most recent proficiency check in the Bell 214 helicopter and PIC line check occurred on June 16, 2011. According to employment records from AAR Airlift Group, the PIC had accumulated about 7,250 hours total pilot flying time, including about 480 hours in the Bell 214 helicopter. He had flown about 115 and 12 hours in the last 90 and 30 days, respectively. The SIC, age 51, held an airline transport pilot certificate issued on April 9, 2011, for the BH-214ST helicopter with an airplane single-engine land rating. The SIC held a second-class medical certificate issued on August 3, 2011, with no limitations. A review of FAA records found no prior accident, incident, or enforcement actions. The SIC was hired by AAR Airlift Group in November 2011. His most recent recurrent ground training occurred on December 19, 2011, his most recent proficiency check in the Bell 214 helicopter occurred on November 28, 2011, and his most recent SIC line check occurred on December 30, 2011. According to employment records from AAR Airlift Group, the SIC had accumulated about 4,170 hours total pilot flying time, including about 120 hours as an SIC in the Bell 214 helicopter. He had flown about 250 and 14 hours in the last 90 and 30 days, respectively. On the day of the accident, the PIC and SIC were scheduled for 14 flights. The accident occurred during the sixth flight of the day. The PIC and SIC had been on duty for about 4 hours when the accident occurred. AIRCRAFT INFORMATIONThe accident helicopter, serial number 28102, was built in 1984 and was approved for operations conducted under 14 CFR Parts 91 and 135. Records showed that, as of January 15, 2012 (the day before the accident), the helicopter had accumulated a total time of 12,346 flight hours. The Bell 214ST helicopter has a two-bladed, semi-rigid main rotor system, which provides helicopter lift and thrust, and a two-bladed, semi-rigid tail rotor system, which provides main rotor anti-torque and directional control. The helicopter is equipped with two General Electric CT7-2A turboshaft engines that provide power to the main and tail rotor systems via a geared transmission system and drive shafts. The helicopter's flight controls for cyclic, collective, and anti-torque/directional control are hydraulically assisted. The automatic flight controls consist of a stability control and augmentation system, attitude and altitude retention system, and a fly-by-wire elevator system. The accident helicopter was equipped with a portable Garmin GPS unit, which could store route of flight and flight time information for up to 50 individual flights. The GPS recorded date, time, latitude, longitude, and altitude parameters and derived groundspeed and course from the recorded parameters. The data extracted from the GPS aboard the accident helicopter included the flight from Camp Farah to Camp Bastion and the accident flight. These data were recorded between 0928:41 and 1044:37. The data showed that the accident helicopter was traveling at a groundspeed of about 145 knots when the last parameters were recorded. METEOROLOGICAL INFORMATIONThe sky condition was reported to be clear with unrestricted visibility at the time of the helicopters' departure from Camp Bastion. The wind was reported to be variable at 6 knots. AIRPORT INFORMATIONThe accident helicopter, serial number 28102, was built in 1984 and was approved for operations conducted under 14 CFR Parts 91 and 135. Records showed that, as of January 15, 2012 (the day before the accident), the helicopter had accumulated a total time of 12,346 flight hours. The Bell 214ST helicopter has a two-bladed, semi-rigid main rotor system, which provides helicopter lift and thrust, and a two-bladed, semi-rigid tail rotor system, which provides main rotor anti-torque and directional control. The helicopter is equipped with two General Electric CT7-2A turboshaft engines that provide power to the main and tail rotor systems via a geared transmission system and drive shafts. The helicopter's flight controls for cyclic, collective, and anti-torque/directional control are hydraulically assisted. The automatic flight controls consist of a stability control and augmentation system, attitude and altitude retention system, and a fly-by-wire elevator system. The accident helicopter was equipped with a portable Garmin GPS unit, which could store route of flight and flight time information for up to 50 individual flights. The GPS recorded date, time, latitude, longitude, and altitude parameters and derived groundspeed and course from the recorded parameters. The data extracted from the GPS aboard the accident helicopter included the flight from Camp Farah to Camp Bastion and the accident flight. These data were recorded between 0928:41 and 1044:37. The data showed that the accident helicopter was traveling at a groundspeed of about 145 knots when the last parameters were recorded. WRECKAGE AND IMPACT INFORMATIONDetailed information about the accident site and photographs of the site were provided to the NTSB by the US military and AAR Airlift Group's director of ground support operations in Afghanistan. The accident site consisted of a primary impact area and a scattered debris field, which measured about 1,575 feet north-south by 656 feet east-west. The primary impact area consisted of the main fuselage, which had been fully consumed by the postcrash fire. The forward tailboom section was found separated from the main fuselage about 49 feet north of the primary impact area, and the aft tailboom section (containing the vertical stabilizer and tail rotor) was found about 279 feet north of the primary impact area. The main debris field was located south of the primary impact area (distance unknown) and included an engine cowling, a door, and one of the two main rotor blades. The other main rotor blade was found about 426 feet southwest of the primary impact area. Examination of the wreckage found that only one of the two collective lever arms (subsequently identified as the right collective lever arm) was recovered from the accident site. Also, the areas on the main transmission's main rotor mast where the missing (left) collective lever arm was installed did not exhibit damage similar to the significant damage found in the areas where the right collective lever arm had been attached before the helicopter impacted the ground. Examination of the two holes for the bolts that secured the two collector lever arms together found no indications of hole enlargement or longitudinal wear marks on the inside of bolt holes. The right collective lever arm was not attached to the idler link or the collective sleeve. The idler link had two impact marks in similar locations on the left and right sides of its outboard edges. The impact mark on the left side of the idler link was smaller than the mark on the right side. According to Bell Helicopter Textron, the impact marks on the idler link were consistent with an overtravel of the collective lever arm in the direction of an upward movement of the collective sleeve. Impact marks were observed on the upper edge of both sides of the swashplate support assembly window. These impact marks matched the damage to the upper two of the four bolts used to secure the bearing assemblies on both sides of the collective sleeve. On the left bearing assembly, the upper left bolt head was completely sheared off; on the right bearing assembly, the upper left bolt head exhibited a downward bend. Bell Helicopter Textron indicated that these impact marks were also consistent with an overtravel of the collective sleeve in the upward direction. The left bearing assembly was relatively intact, and the right bearing assembly showed significant deformation and damage. The visible portions of the main rotor mast had two areas with impact damage that were located 180º opposite of each other and were aligned with the flapping direction of the main rotor hub. The impact damage on the main rotor mast showed evidence consistent with mast bumping, which occurs on a two-bladed semi-rigid rotor system when the system exceeds its teetering limit and strikes the mast of the helicopter with sufficient force to cause mast deformation and potentially mast failure. Continuity of tail rotor drive and tail rotor control were confirmed within the separated aft tailboom section but could not be confirmed forward of the aft tailboom section because of breaks in the tail drivetrain and control system and the loss of components due to the postcrash fire. No evidence indicated a loss of tail rotor drive or tail rotor control before the in-flight separation of the aft tailboom section. Further examination of the tailboom wreckage found overload fractures on the four tailboom attachment fittings. Both engines showed damage consistent with ground impact forces and exposure to the postcrash fire. Examination of both engines showed evidence that was consistent with the engines producing power at the time of impact. The left cyclic, right cyclic, and collective hydraulic actuators from the accident helicopter's flight control system were examined in detail at Bell Helicopter Textron facilities in Fort Worth, Texas. The examination found no evidence indicating abnormal operation. A review of maintenance records for the collective flight control system showed that the collective torque shaft support bracket had been replaced the day before the accident. The support bracket installation was found in the wreckage, and no evidence indicated that the installation of the replacement support bracket was faulty. The maintenance records did not include any discrepancy related to the collective flight control system that led to the replacement of the support bracket or any troubleshooting performed to determine that the support bracket needed replacement. The maintenance records showed that a functional check flight was completed after the support bracket replacement with no noted defects. ADDITIONAL INFORMATIONThe DOD reported that it conducted in-theater oversight of AAR Airlift Group. This oversight was conducted by an evaluator in Afghanistan who observed company operations, procedures, aircraft, and facilities. The evaluator was generally an operations specialist but was occasionally a maintenance specialist. DOD stated that it strived to conduct in-theater oversight twice a year (in 6-month intervals). The operations evaluators assessed in-flight operations. The evaluators used a cockpit evaluation form, which was similar to a pilot checklist, during their assessment and shared completed cockpit evaluation
the overtravel of the collective sleeve, which led to a severe and sudden reduction in main rotor blade collective pitch, resulting in a loss of control of the helicopter. The reason for the collective sleeve overtravel could not be determined since much of the collective control system was consumed in the post crash fire.
Source: NTSB Aviation Accident Database
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