Cold Springs, NV, USA
N174EH
Bell 412
While flying along a mountain ridgeline to make a water drop on a wild fire, the helicopter lost power in one engine and collided with terrain as the pilot turned downslope toward a landing area. Ground crews watching the helicopter make its drop run observed smoke emanating from the right engine, then the helicopter made a left descending turn and impacted the downsloping mountainous terrain. A trailing pilot saw the helicopter about 150 feet above the ridgeline, then it made a sudden left descending turn. He did not see the pilot jettison either the water or the bucket. A teardown inspection and metallurgical examination of the No. 1 and No. 2 power sections was conducted. The examination of the No. 1 power section CT disc revealed that the firtree serrations adjacent to the No.s 24 and 25 blade positions were fractured above the blade retaining rivet hole, and that the No.s 27-29 firtree serrations were fractured at the blade roots. During the metallurgical examination, the failure of the CT disc was attributed to cyclic stress rupture due to extended and repeated operation of the engine at, near, or above its temperature/power limits. Dimensional measurements of the blades showed growth and deformation to the disk in the areas of the fractures. There were no material, manufacture, or design deficiencies identified during the metallurgical examination of the CT disc. The examination of the No. 2 power section revealed that the intermediate drive shaft fractured in a counterclockwise direction due to sudden stoppage of the left engine while it was at a high power level. Due to the degree of destruction and lack of dispatch records, the investigation was not able to accurately determine the operating weight of the helicopter at the time of the accident; however, for the 9,500-foot density altitude, it is believed that the helicopter's weight with the water load was at a point that resulted in marginal single engine capability at best. The accident helicopter had been modified with the installation of a water bucket and long line system. The long line and water bucket circuit breakers, and the emergency electrical release, were connected to the nonessential bus. This system was installed on a Form 337 field approval. According to the helicopter manufacturer, the electrical system is designed so that if one generator and/or engine failed both of the nonessential buses would automatically drop offline. Thus the emergency electrical release of the water bucket and long line would have been rendered inoperable in the event of a generator and/or engine failure. An override switch on the electrical panel can restore power to the nonessential buses; however, based on the event timeline reported by the witnesses, it is unlikely that the pilot could have restored power to the nonessential busses in time to prevent a collision with the ground.
1.1 HISTORY OF FLIGHT On August 13, 2000, at 1645 Pacific daylight time, a Bell 412 twin-engine helicopter, N174EH, collided with mountainous terrain while conducting a long line water drop along a ridgeline during a wildfire suppression operation near Cold Springs, Nevada. The wildfire was named the Twin Peaks fire. The helicopter was certified under 14 CFR Part 133 for external load operations and was being operated by the Bureau of Land Management (BLM) as a public-use firefighting aircraft. The helicopter, owned by Era Aviation, was destroyed. The airline transport pilot, the sole occupant, sustained fatal injuries. Visual meteorological conditions prevailed, and a company visual flight rules (VFR) flight plan had been filed. The helicopter departed the Twin Peaks helibase, located at Cold Springs, at 1605. The primary wreckage was at 39.34.83 north latitude and 117.48.56 west longitude. The accident pilot flew several missions the day of the accident. On the accident flight the pilot flew to the dip site, 3 miles south of the accident location, and then flew to the accident area to make a water drop. According to another pilot working the same dip site and fire, the accident helicopter was to make a water drop along the ridgeline to support the ground fire crews. The trailing pilot was going to make his water drop behind the accident helicopter. He was about 1 mile behind the accident helicopter, and observed the accident helicopter flying along the ridgeline. The accident helicopter made a sudden 90-degree left descending turn and impacted the downsloping mountainous terrain. There were no radio communications with the accident pilot immediately prior to the turn. The trailing pilot estimated the accident helicopter's altitude to be about 150 feet above the ridgeline, and that the accident helicopter was flying into the wind, which was 10 to 15 knots along the ridgeline. The trailing pilot indicated that the helicopter was "very low until impact" about 2/3 of the way down the hill. He did not know if the accident pilot had gotten rid of the bucket. He indicated that he did not see the accident pilot jettison the water. The fire crew from the Texas Initial Attack (IA) #1 were witnesses to the accident. A compilation of the witness statements indicated that they were about 1 ¼ miles away from the accident site. The witnesses stated that they observed the helicopter make a sudden left descending turn and impact the ground. They did not see the bucket release from the helicopter. One witness stated that he saw the helicopter start a climb along the ridgeline and saw a "puff of gray smoke come from his engine exhaust." Another witness stated that he saw the helicopter start up the ridgeline when he saw "white [and] blue smoke" coming from the right side of the engine. The crew boss assigned to the Texas IA #1 indicated that he was standing with the crew boss trainee just south of the crew bus. He stated that the crew boss trainee had just finished communicating via radio with the accident pilot; however, it was unclear if the pilot responded. The crew boss stated that the helicopter's forward motion "halted," and he saw about three or four "puffs of white smoke" coming from the exhaust area. The helicopter began to "roll and yaw," dropping to the side of the slope that he was traveling beside. Fueling records were not obtained at the accident site. However, in the operator's written statement (Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2), they estimated that the pilot departed the helibase with 1,000 pounds of Jet A fuel on board. 1.1.1 Helicopter Activities Prior to the Accident A couple of days before the accident, two ground crewmen from the Nevada Army National Guard watched for about an hour as the accident pilot filled up the Bambi bucket at a pond used as a helicopter dip site. On the east side of the pond there was a 5-foot berm. On the northwest corner was a small dry creek bed. According to the witnesses, the pilot would try and maneuver through the dry creek bed so that he didn't have to climb over the berm. One witness indicated that on some of his flares to fill the bucket, the pilot had "flared so hard the rotor wash would catch the Bambi and flip it up towards the rotor blades and come right out in front." The witness indicated that there was an abrupt manner when the pilot was filling the bucket. He observed the nose pitch down "quite a bit" to where it was about 6 feet off the water and then the pilot would try and get the bucket out of the water. There were a couple of times that the pilot would get the bucket "flying about 6 or 8 inches off the water," and if "he was not pulling enough power" the bucket would "go slamming back into the water." He stated, "It was a pretty abrupt move again to try and stop the aircraft and the bucket from going into the berm." A couple of times he had to put the bucket back into the water because the helicopter couldn't make it over the berm. The witness indicated that a couple of times, from his vantage point, it appeared that the bucket slid across the top of the berm. He further stated that at times during the pilot's maneuvers, he could see the tail boom "actually kind of almost wrinkle up," with some coning of the main rotor blades. At one point the pilot had to abort the dip because it appeared that he wasn't going to clear the berm. When the bucket went in "it pulled the back of the aircraft up. The back of the aircraft settled in and the second witness made a comment about how he [the pilot] had almost struck the tail rotor in the water." At that point both ground crew moved farther up the hill for safety concerns. The second witness indicated that the accident helicopter seemed to be in more of a hurry to fill the bucket than the other aircraft in the area. He also noted that the pilot's approach to "dipping" was very different from the other helicopter pilots. The pilot made a high speed pass, "kind of a high angle of bank turn then basically let the collective out of the [helicopter] and autorotate into the dip site and as the bucket hit the water he started to dragging it to fill it and then just pulled in all the power that he had." The accident pilot flew the bucket down the drainage area due to "bleeding rotor as he was going through translational lift." The second witness also observed, on a couple of passes, where the bucket swung out in front of the helicopter. The witness further indicated that the accident pilot appeared impatient, and that at one point tried to "sneak in front of the other helicopters." 1.5 PERSONNEL INFORMATION The National Transportation Safety Board investigator-in-charge (IIC) reviewed the pilot's Federal Aviation Administration (FAA) certified Airman and Medical information, as well as flight training records from Era Aviation, and Era's daily flight duty logs for the Twin Peaks fire. 1.5.1 Federal Aviation Administration Records Review of the pilot's medical information revealed that the most recent first-class medical certificate was issued on March 29, 2000. The medical contained limitations for vision, which indicated that the holder shall wear lenses that correct for distance vision and possess glasses that correct for near vision. On his medical application he reported having accumulated 14,200 hours of total pilot flight time, with 100 hours in the last 6 months. Review of the pilot's airman certification records disclosed that the pilot held an airline transport pilot certificate, as well as a commercial pilot certificate, both with ratings for rotorcraft-helicopter, along with an instrument rating for the BV-234. The pilot was type rated for the BH-212, BH-206 (VFR only), and BV-234. 1.5.2 Operator Records 1.5.2.1 Operator's Pilot Training Records The accident pilot's training records from Era Aviation indicated that he took a 14 CFR 135.293/ .297, and 14 CFR 135.299 check ride for airman competency/proficiency on March 29, 2000. His training records further indicated that he received an Office of Aircraft Services (OAS) Interagency Helicopter Pilot Qualifications and Approval (OAS Card) on April 6, 2000. His most recent sling load recurrency check was done at the time of the OAS check. 1.5.2.2 Operator's Daily Flight Logs Review of the daily flight logs for July and August 2000 revealed that the accident pilot reported for duty on July 24 - 29th, and flew 36.8 hours. He was off duty July 30-31, and returned to duty August 1, 2000. He flew from the 1st to the 8th and accrued 32.4 hours of flight time. He went off duty August 9-10, and reported back for duty on August 11th. A relief pilot flew the accident helicopter during the time when the accident pilot was off duty. Between August 11th and 12th the pilot accrued 12 hours of flight time. His total flight time for the month of August was 44.4 hours. Not all of the daily flight logs were completely filled out. According to the operator, the responsibility to fill out the form was with the pilot. Two days prior to the accident, and the day before the accident, the pilot wrote in the ON/OFF block: 905.4, 906.4, 908.4, 910.0, 912.3, and 913.0. In the FLT. TIME block the pilot recorded the corresponding flight times as 1.0, 2,0, 1.7, 2,2, and 0.7. There was not a daily flight log for the day of the accident. 1.5.3 Bureau of Land Management (BLM) Duty Limitations On August 6, 2000, a Phase 2 Duty Limitations (24.13.C.1 of the National Interagency Mobilization Guide) were issued for helicopter firefighting operations. The Phase 2 Duty Limitations was enacted due to the sustained high volume of fire suppression activities throughout the United States. The Phase 2 Duty Limitations are more restrictive than the standard Interagency Fire requirements for crew rest. The Duty Limitation required specific rest periods for pilots, with exemptions and provisions set aside for duty days. During each duty day cycle, flight crewmembers were required to have a minimum of 12 consecutive hours of uninterrupted rest (off duty). The standard day was no longer than 12 hours, except with a crew duty extension not to exceed a cumulative 14-hour duty day. (Document contained in the Docket file for this accident.) According to the daily flight duty logs, the pilot met the requirements for the duty limitation rest periods. 1.6 AIRCRAFT INFORMATION 1.6.1 Airframe The accident helicopter was a Bell 412, serial number 33085. A review of the helicopter's maintenance records revealed that it accumulated a total airframe time of 7,683.1 hours prior to the accident flight. The helicopter was inspected under the company's approved Continuous Airworthiness program. The last inspection was completed on August 8, 2000. The aircraft total time at the last inspection was 7,651.4 hours, with 31.7 hours flown since the last inspection. According to Era's maintenance records, the last visual inspection of the temperature strips (heat sensor strips) was on August 10, 2000. The aircraft total time was 7,671.1 hours, with the next inspection of the temperature strips due at an aircraft total time of 7,696.1. The helicopter was equipped with two emergency cargo release mechanisms. One was electrical, and activated by depressing a button on the cyclic. The other was mechanical, and activated by depressing a foot pedal located in between the antitorque pedals for either the pilot or copilot positions. On March 6, 1998, the helicopter had been modified in accordance with FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), with the installation of a water bucket and long line system. The installation called for the long line circuit breaker and the water bucket circuit breaker to be installed on a fabricated bracket located in the "cabin overhead at station 82." According to FAA Form 337, the installation required that the circuit breakers be connected to the nonessential bus. According to the manufacturer's approved flight manual, section 2 (Systems Description), pg. 2-11 under electrical systems: "In the event that one generator or engine should fail, both nonessential buses are automatically dropped, and all essential dc loads are supplied by the remaining generator. An override switch is available so that the pilot, at his discretion, can manually restore power to the nonessential buses." Era's recurring maintenance records indicated that the cargo hook inspection is a daily requirement. The last check was on August 12, 2000, at an aircraft total time of 7,681.3. There were no entries for the accident date. According to the manufacturer, the belly cargo hook release mechanism is powered by the emergency bus and would operate in any scenario where one or both generators failed, and one or both engines failed. The emergency bus is a continuous electrical system. 1.6.2 Power Plants The helicopter utilized Pratt and Whitney's PT6T-3B twin-pack system. The system utilizes two PT6 engines (labeled #1 Power Section (left engine) and #2 Power Section (right engine)), and a combining gearbox. 1.6.2.1 Number 1 Power Section (left engine) Review of aircraft records indicated that the engine had been installed on N415EH. UNC Airwork Corporation, in Miami, Florida, overhauled the No. 1 engine on July 30, 1996 (work order F07705), at a total time of 10,043.8 hours since new. They installed a new compressor turbine disc P/N 3024211, S/N 23B827, on the No. 1 Power Section during the 1996 overhaul. On December 8, 1997, UNC sent the compressor turbine to Coltec Industries in Peabody, Massachusetts, for repair following an overtemp condition. December 11, 1997 Coltec rejected the CT disc, and shipped it back to UNC Airwork Corporation. According to UNC Airwork Corporation paperwork, their engineering department examined the CT disc and the following work was completed: January 7-9, 1998 UNC Airwork Corporation paperwork indicated that the repair to the CT disc was to "Polish rivet head area per PI 002-384-01" followed by nondestructive testing, and a final inspection. January 22, 1998 UNC Airwork Corporation rejected the CT disc with no remarks listed in the "Reason for Replacement and Remarks" section. February 3, 1998 FAA form 337 MAJOR REPAIR AND ALTERATION, the power section (work order F09113) was "inspected and repaired for over-temperature in accordance with the manufacturer's specifications and requirements." FAA form 337 also indicated that "Airwork authorized repairs in accordance with SFAR 36: A01-510-01." Once the repairs were completed the part was installed on the No. 1 power section and returned to service. The No. 1 power section was installed on the accident helicopter on March 19, 1998, at an engine total time of 10,807.2 hours. The No. 1 power section, serial number 62224, had a total time of 12,784.7 hours, with 2,740.9 hours since overhaul (aircraft total time 7,683.1 hours). 1.6.2.2 Number 2 Power Section (right engine) The No. 2 power section was serial number 62059. The No. 2 power section had a total time of 11,060.9 hours, with 2,560.0 since overhaul. The No. 2 power section was installed on the accident helicopter on January 14, 2000, at an engine total time of 10,534.0 hours. Pratt and Whitney Canada Service Centre, Quebec, Canada, overhauled the No. 2 power section on August 21, 1996, with an engine total time of 8,500.0 hours (work order No. 13819). 1.6.2.3 Combining gearbox The combining gearbox, serial number 1936, had a total time of 1,977.5 hours. 1.6.3 Bambi Bucket The helicopter was outfitted with an SEI Industries International Bambi Bucket model 3542. It held 420 (U.S.) gallons of water; the empty weight was 167 pounds, and the gross weight was 3,667 pounds. On-scene inspection of the bambi bucket revealed that a knot in the Frusto-Conical Arrest System (FCAS) cinch strap was adjacent to the 80-percent setting, as well as an additional ring inside the 80-percent setting. At the 80-percent setting the bucket was capable of holding 336 U.S. gallons (2,789 pounds). According to the FCAS manual, the cinch strap allows the pilot to reduce
failure of the compressor turbine disc due to cyclic fatigue brought about by repeated operation near or above the engines' temperature/power limits by company personnel over an extended period of time. Factors in the accident were: 1) the high density altitude, mountainous terrain, and the helicopter's resulting marginal single engine performance capability; 2) the design, fabrication, and installation of the emergency external load release system, which had the power supply wired to the nonessential bus that would automatically drop offline during an engine or generator failure; and 3) the pilot's resulting inability to electrically release the water load, bucket, or line while dealing with the engine failure.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
