Bradenton, FL, USA
N528SF
Eurocopter Deutschland GMBH MBB BK-117 A4
The pilot said that while in cruise flight at an altitude of 1,000 feet, a speed of about 130 knots, and about 60 percent torque, without warning he heard a loud explosion from the right side of the helicopter, with simultaneous No. 2 engine fire and failure warning lights and indications, along with severe vibrations. He said that the helicopter yawed several times from left to right and he heard a "whopping" sound. He also said that at the same time the No. 1 engine torque meter needle moving rapidly from about the 9 to 3 or 4 o'clock positions on the gauge. The investigation revealed two of the power turbine blades and four rear bearing support housing (RBSH) failed in overload. The hub had been displaced from the engine centerline, and a deflection had occurred at the engine case axial midpoint, along with a misalignment of the inlet housing and the rear bearing support housing (RBSH). The resultant damage to the gas producer (GP) system, was consistent with an assembly error and mechanical failure having occurred by the loss of critical internal operating clearances and radial support for the rotors. Continued engine operation as the failure progressed led to compressor surge and the emergence of combustion gases from the No. 2 engine inlet, which ignited inlet cowling material. The No. 1 engine ingested smoke and combustion by-products from the engine cowling fire which caused a temporary stall condition.
HISTORY OF FLIGHT On August 26, 2002, about 2035, eastern daylight time, Eurocopter Deutschland GMBH MBB BK-117 A4, N528SF, registered to and operated by a Rocky Mountain Helicopters LLC, as a Title 14 CFR Part 135 aeromedical flight, had an engine fire while in cruise flight, and the pilot executed a forced landing in Bradenton, Florida. Visual meteorological conditions prevailed, and a company visual flight rules flight plan was filed. The commercial-rated pilot, three crewmembers, and one patient were not injured, and the helicopter incurred minor damage. The flight originated from Naples, Florida, the same day, about 1910. The pilot stated that he was transporting a patient from Naples, Florida, to St. Petersburg, Florida, and while flying the helicopter at a cruise altitude of 1,000 feet, a speed of about 130 knots, and about 60 percent torque, without warning, he heard a loud explosion from the right side of the helicopter, with simultaneous No. 2 engine fire and failure warning lights and indications, along with severe vibration indicative of an engine failure. He said that the helicopter yawed several times from left to right and he heard a "whopping" sound. Looking at the instruments, he said he noted the No. 1 engine torque meter needle moving rapidly from about the 9 to 3 or 4 o'clock positions on the gauge. He made a 'mayday' radio communications call to his company and performed a 180-degree turn, seeking an emergency landing site. He then completed the emergency fire procedures, and entered autorotation in anticipation of a dual engine failure, noting that the vibrations ceased just after he entered autorotation, and was able to affect a forced landing to a highway. After landing, the pilot said he instructed the crew to evacuate the helicopter, pulled both throttles aft, and activated the second fire extinguisher. Upon exiting to the right side of the aircraft, the pilot said he observed considerable smoke and fire in the intake and tailpipe areas of the No. 2 engine, and added that the entire top and aft end of the No. 2 engine cowling was "in flames." He said he grabbed the cockpit fire extinguisher and began fighting the fire, and at that time he noticed that the patient was still in the helicopter. He stated that the crew had proceeded to a point about 150 feet in front of the helicopter, according to the evacuation procedure, and further stated that he could not get to the patient from the right side of the helicopter because of the fire. He said he went to the left side of the helicopter, and was able to open the rear doors and pull the stretcher out, with the patient. He was then met by his crew who then relieved him of the stretcher with the patient, and he returned to fighting the fire. DAMAGE TO AIRCRAFT An FAA inspector stated that when he responded to the incident he found the helicopter sitting on a trailer with its main rotor blades removed. During his examination of the aircraft, the inspector noted that the No. 1 extinguisher switches were in the "Armed" position, the left and right fuel valve "Emergency Close" position guards were up and the safety wires had been broken. There was no apparent damage to the airframe structure or the skids. A main rotor blade had come to rest over the fuselage after the landing and power down, and it had been damaged by heat. According to the FAA inspector, the No. 1 engine had incurred minimal damage. The No. 2 (right) engine had ceased operating while in cruise flight, and it exhibited evidence of an in-flight engine compartment fire having occurred. The No. 2 engine had two fractured power turbine (PT) blades, a dented exhaust extractor, and when the engine power turbine rotor was turned there was binding, and it ceased its travel. The cowling had been damaged by the fire in the area of the tailpipe, as well as in areas on top of the No. 2 engine, and at the inboard inlet area of both the left and right engines. The deice harness had burned off the connector on top of the No. 2 engine, and the No. 2 engine inlet screen perimeter seal had melted and had run into the inlet, There was an oily substance on the No. 2 engine bleed valve linkage, and on the aft lower engine support link threads, and the fuel nozzle manifold was found intact. After the remaining cowling was removed, the No. 2 PT rotor was examined and could be rotated counter clockwise, but when rotated clockwise it would rotate to a point and then spring back. The aircraft structure supporting the engine fire bottles had been burnt off at the top, and the bottles were covered in soot. The forward fire bottle pressure gauge read 200 PSI, and the aft fire bottle pressure gauge read 0. The area on top of the tailboom support structure between the engine tailpipes and near the tail rotor drive shaft had been discolored due to heat and there were heat related blisters in the skin. There was also heat damage in the area of the doubler under the tail rotor drive shaft. The No. 2 engine was removed and shipped to the Honeywell Corporation, Phoenix, Arizona, for further investigation by the NTSB. On October 2, 2002, the NTSB examined the No. 2 engine at Honeywell, Corporation, Phoenix, Arizona. Neither the gas producer (GP) nor the PT systems could be rotated. There were no signs of external fuel or oil leakage, and no evidence of any thermal damage to the gearbox or inlet housing. A plastic-like material had melted, and had deposited in streaks along the inlet air path. The deposits exhibited characteristics and color traits consistent with that of the inlet screen perimeter seal. The thermocouple junction box, located on top of the engine, had also been damaged by heat. The blade tips at the inlet and compressor had been worn, and a number of the tips had curled in the direction opposite the direction of rotation. There were circumferential rub marks on the compressor impeller shroud coating, and there was also thermal discoloration and material smearing along the inlet diameter. There was a 1/8-inch gap between the compressor assembly and the impeller housing, and the centrifugal impeller retaining nut was loose and there was a 0.065-inch gap between the lock cup and the impeller. There were rub marks on the GP spacer forward impeller surface, and in addition, the stationary member of the GP labyrinth seal exhibited circumferentially scour marks. The GP shaft lock cup was intact, and was devoid of its anti-rotation features, and the nut had lost its torque. The No. 2 bearing was dry, and its aft side had scour marks. In addition, the aft bearing sump and scavenge tube were also dry. The GP blades had been missing tip material, and remaining stubs had severe damage consisting of tip fractures, thermal distortion and extensive coating loss/flaking. Combustion and power turbine sections showed that the GP shroud (GP wheel assembly blade path) had been coated with re-solidified molten metal material, which had been evenly distributed. The RBSH had distorted into an egg-shape, and the four outer wall-to-inner hub support struts had separated. Three struts had cracked through at the inner support hub, and the fourth separated outboard of the flow path ring. Additional cracks were noted in the flow path ring and at the forward inner hub flange adjacent to each of the struts. There were indications of metal transfer from the PT blade tips, with metal deposits and deep 360-degree scouring along the shroud's inner diameter (blade path). All of the PT blade tips were severely rubbed, and the outboard sections of two adjacent blades were missing. A 180-degree arc of scraping was found on the leading edge side of blade platforms, and they were radially aligned with the blade fractures. The PT shaft outer diameter had rubbed heavily in a circumferential arc of about 180 degrees, just forward of the No. 3 bearing and seal, and was also radially aligned with fractured PT blades. In addition, the PT retention probe showed a light contact mark. The examination revealed no evidence of an internal oil fire, fuel leaks, or unusual burn patterns. Damage was consistent with the power turbine (PT) blades having contacted stationary PT structures during engine operation. All 12 bolts that secured the compressor inlet housing to the impeller housing were found to be loose, with several had backed off the flange, and there was bolt-hole thread damage that was consistent with the bolts having had varying degrees of looseness during engine operation. Metallurgical examinations of PT blade as well as the four rear bearing support housing (RBSH) strut fractures showed that they had failed in overload, and the hub had been displaced from the engine centerline, and a deflection had occurred at the engine case axial midpoint, along with a misalignment of the inlet housing and the rear bearing support housing (RBSH) with resultant damage to the gas producer (GP) system. The NTSB Powerplant Group Chairman's Factual Report has been included as an attachment to this report. PERSONNEL INFORMATION The crew on board N528SF consisted of the pilot, employed by Rocky Mt. Helicopters, a flight nurse employed by Bayflight, and a respiratory therapist and a neonatal nurse both employed by All Children's Hospital. The pilot held commercial pilot and flight instructor certificates with helicopter and instrument helicopter ratings. At the time of the incident he had accumulated about 6,800 hours of total flight experience with about 1,200 flight hours in the same make and model as the incident helicopter. He had also accumulate about 400 hours of night flying experience. AIRCRAFT INFORMATION According to the FAA, a review of the engine and airframe logs did not reveal the presence of any open or deferred discrepancies. The records revealed that the last engine overhaul had been performed at the operator's facility in Provo, Utah, on October 12, 2001, about 568 operating hours, and about 10 months prior, and a PT wheel assembly was installed in the engine at that time. In addition the wheel assembly had received a periodic inspection for axial PT blade displacement, in accordance with Honeywell Service Bulletin No. LT101-72-50-0153, at 21,063 PT cycles, on May 16, 2001, and the next compliance with the service bulletin was due at 21,392 PT cycles. SURVIVAL ASPECTS N528SF had been transporting an infant when the accident occurred, and after affecting a forced landing on a highway, the pilot said as he proceeded to fight the fire he noted that the infant/patient was still inside the burning helicopter. On March 25 through 28, 2003, an NTSB Survival Factors Specialist traveled to Tampa and St. Petersburg, Florida, to interview the pilot, and medical crewmembers about the accident flight, relative to survival factors issues. The pilot confirmed the specifics he had stated earlier, and added that he had last completed emergency procedures training in April or May of 2002. Two of the three medical crewmembers were also interviewed, and both crewmembers also confirmed the specifics of the flight. The nurse who occupied the left cockpit seat, stated that her last emergency procedures training was done in February 2002, and it involved various evacuation scenarios. The respiratory therapist, seated in the forward bench seat on the right side of the cabin, said that his most recent formal training was at Careflight in 1995, and added that Bayflight requires hospital staff to receive initial emergency procedures training before flying on a Bayflight helicopter. He said he had not received formal training on evacuations, but the Bayflight safety officer had shown him how to jettison the doors of the BK-117, and he had received some other hands on training in the helicopter. ADDITIONAL INFORMATION On September 29, 2002, the NTSB released the helicopter with the exception of the No. 2 engine, to Mr. Eric Herbst, Chief Inspector, Rocky Mountain Helicopters Inc. On February 13, 2004, the NTSB released the No. 2 engine to Air Methods Inc. Air methods Inc., purchased Rocky Mountain Helicopters Inc, and the wreckage release was signed by Mr. Leroy Jackson, Risk Manager, Air Methods Inc. on October 25, 2004.
Improper maintenance/installation of the No. 2 engine power turbine (PT) wheel assembly by company maintenance personnel, which resulted in deflection at the engine case axial midpoint and misalignment between the inlet housing and the rear bearing support housing (RBSH) which resulted in damage to the engine and an in-flight fire.
Source: NTSB Aviation Accident Database
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