SPRINGDALE, AR, USA
N27AE
Bell 206L-3
Shortly after the helicopter lifted off from a hospital helipad en route to pick up a patient, the engine lost power. The helicopter descended into a parking lot, landed hard, and rolled over coming to rest on its right side. Disassembly of the engine revealed that both the N1 (gas producer turbine) and the N2 (power turbine) shafts had separated. Coke deposits and rub marks were noted on the outside diameter of the N1 shaft. Coke deposits were also present on the inside diameter of the N2 shaft. Metallurgical examination of the shafts determined that coke build up consistent with reduced oil flow led to rubbing between the shafts. The rubbing produced frictional heat that resulted in softening and subsequent failure of the shafts. An o-ring was found lodged in the main oil passage of the accessory gearbox housing. Oil flow testing and calculations showed that this o-ring reduced the oil flow to the turbine shafting from .82 to between .00 and .10 lbs/min. Maintenance records indicated that the engine had accumulated 531 hours since a 3,500 hour gear train inspection, which included disassembly, cleaning, and reassembly of all accessory gearbox components.
HISTORY OF FLIGHT On May 24, 1998, approximately 1235 central daylight time, a Bell 206L-3 helicopter, N27AE, operated by Air Evac EMS, Inc., of West Plains, Missouri, sustained substantial damage during a hard landing following a loss of engine power on takeoff from the Northwest Medical Center in Springdale, Arkansas. The commercial pilot and the two medical crewmembers received serious injuries. The positioning flight was operating under Title 14 CFR Part 91 and was en route to Bella Vista, Arkansas, to pick up a patient for transport back to Northwest Medical Center. Visual meteorological conditions prevailed and a company flight plan was filed. The pilot reported that following a "normal start and run-up," the takeoff was made into an 8-knot southwest wind. A "good rate of climb" was established, and the helicopter "cleared the highest and immediate obstacles." The pilot's next recollection was of events after the accident. One of the medical crewmembers stated that after takeoff, the pilot initiated a "normal" left turn "to circle around to the north." The helicopter was facing south when there was a "loud explosion/backfire type of noise from above" which was "followed by complete silence." The helicopter then "rotated around to the left while falling." Witnesses reported that they heard a loud noise and saw black smoke coming from the helicopter shortly after it lifted off from the hospital helipad. According to local authorities, the helicopter descended into a parking lot, one of the main rotor blades struck a light pole, and the helicopter rolled over coming to rest on its right side. AIRCRAFT INFORMATION The helicopter, a Bell 206L-3, serial number 51141, was manufactured on May 15, 1985. Review of the helicopter's maintenance records by the NTSB investigator-in-charge (IIC) revealed that it had accumulated 3,247 hours at the time of the accident. The records indicated that the engine, an Allison 250-C30P, serial number CAE-895188, was installed new when the helicopter was manufactured and had accumulated 3,151 hours and 7,312 cycles at the time of the accident. According to entries in the engine logbook, on December 14, 1995, the turbine assembly was removed at an engine total time of 2,527 hours and 5,990 cycles. On January 10, 1996, the turbine assembly was reinstalled after repair by Dallas Airmotive, Inc., of Dallas, Texas. The repair, as detailed in Dallas Airmotive's Work Order No. EM0016, entailed replacing the 3rd and 4th stage turbine wheels due to cycle limitations (6,000 cycle life limit) and performing a 1,750 hour inspection of the N1 (gas producer turbine) shafting. During the N1 shafting inspection, the N1 turbine-to-compressor coupling shaft or "pea shooter" was found to be "stepped" and a new shaft, part number 23032345, serial number AE68962, was installed. On May 23, 1996, the engine was removed "because of vibration" at an engine total time of 2,620 hours and 6,218 cycles. On August 14, 1996, the engine was reinstalled in the helicopter after repair by Dallas Airmotive. According to Dallas Airmotive's Work Order No. TR0078, the repair included overhauling the turbine assembly, performing a 3,500 hour gear train inspection on the accessory gearbox and a 3,500 hour magnetic particle inspection on the compressor impeller, and test running the engine. At the time of the accident, the engine had accumulated 531 hours and 1,094 cycles since this repair. Review of Dallas Airmotive's complete file on Work Order No. TR0078 indicated that the N1 coupling shaft, part number 23032345, serial number AE68962, was inspected, found to be serviceable, and reinstalled. At the time of the accident, this shaft had accumulated 607 total operating hours. The inner power turbine shaft, part number 23038137, serial number 64110, and the outer power turbine shaft, part number 23038136, serial number 66364, were inspected, found to be serviceable, "alseal" coated, and reinstalled. The main oil delivery tube or "piccolo tube," part number 23033876, serial number 30641, was flow tested, found to be serviceable, and reinstalled. The file for Work Order No. TR0078 contained a completed Dallas Airmotive Form No. A250-504-2, entitled "A250 Assembly C-28/C-30 Gearbox." Item number 17 of this form stated: Place an "O" ring AS3085-010 on each end of the 6896458 oil delivery tube and one "O" ring AS3085-010 on the oil screen. Install the oil screen and delivery tube in housing. Install the shouldered attaching screw and torque to 22-26 inch lbs. Secure with lock wire. This item of the form included boxes for both a mechanic's stamp and an inspector's stamp, and both boxes were stamped. The file for Work Order No. TR0078 also contained a completed Dallas Airmotive Form No. A250-505-3, entitled "C30 Final Build." Item number 2 of this form detailed the turbine to gearbox installation, and step A of this item stated, in part: "insure 'O' rings on spur adapter gearshaft are in good condition. Replace with new if necessary." The mechanic's stamp box and the inspector's stamp box for step A of item 2 were both stamped. WRECKAGE AND IMPACT INFORMATION The helicopter was examined at the accident site by an FAA operations inspector on the afternoon of May 24, 1998. It was then moved to a hangar at the Springdale Municipal Airport where it was examined on May 26 and 27, 1998, under the supervision of an FAA airworthiness inspector by representatives from the engine and airframe manufacturers. On May 27, 1998, the engine was removed from the helicopter and crated for shipment to the engine manufacturer's facility. Examination of photos and reports provided by the FAA, the Springdale Police Department, and the engine manufacturer revealed the following information. The accident site was located in the parking lot behind a restaurant at 813 West Maple Street in Springdale, Arkansas. The helicopter came to a stop heading south-southwest, lying on its right side, in the southwest corner of the parking lot. A metal utility pole located to the right of the helicopter's nose was damaged about 9 feet above ground level. Additionally, a stock trailer parked facing south to the left of the helicopter's nose sustained damage to the upper rear corner of its right side. The left skid separated from both crosstubes, and the aft crosstube separated from the fuselage. Both steps on the left side of the fuselage were broken off, while both steps on the right side remained attached but were bent upward. The fuselage was buckled and vertically crushed with the most severe damage to the bottom aft section. The tailboom was displaced downward, and the bottom of the vertical fin was crushed upward. Both main rotor blades remained attached to the main rotor head. The blue blade was nearly separated approximately 3 feet outboard of the rotor head by a tear running diagonally inward from the leading edge to within about 2 inches of the trailing edge. The white blade was torn chordwise from the trailing edge to the spar approximately 3 feet inboard of the tip. The inboard bottom surfaces of both blades were coated with black soot. Initially, the main rotor head/main transmission could not be rotated by hand. After the engine-to-transmission drive shaft was disconnected, the main rotor head/main transmission turned freely. Tail rotor drive continuity was established from the tail rotor to a torsional fracture in the tail rotor drive shaft between the aft bulkhead coupling and the #1 hanger bearing. Following removal of the engine, binding of the N2 (power turbine) was confirmed by unsuccessful attempts to hand rotate the 4th stage turbine wheel. The N1 (gas producer turbine) was also found bound in place. After several attempts to turn the N1 by hand rotating the compressor impeller, a slight rotation of about 10 degrees could be accomplished, then binding would occur. During this slight rotation of the impeller, corresponding rotation of the 4th stage turbine wheel was observed. TESTS AND RESEARCH On June 16 and 17, 1998, the engine was disassembled under the supervision of the NTSB IIC at the engine manufacturer's facility in Indianapolis, Indiana. Initially, the turbine section was unbolted from the accessory gearbox exposing the aft (turbine) end of the spur adapter gear shaft (SAGS). There was moderate to heavy coke build up on the splines and inside diameter at the aft end of the SAGS. The forward o-ring was not in its ring groove on the SAGS, but was found forward of the groove. The aft o-ring was properly positioned in its groove, and small pieces of material were missing from this o-ring. Disassembly of the turbine section revealed that both the N1 turbine-to-compressor coupling shaft and the power turbine inner shaft had separated. Multiple pieces of the N1 coupling shaft and the power turbine inner shaft remained within the 3rd and 4th stage wheel bores. Coke deposits and rub marks were noted on the outside diameter of the N1 coupling shaft. The coke build up appeared to be heavier towards the aft (turbine) end of the shaft. Coke deposits were also present on the inside diameter of the power turbine inner shaft. The accessory gearbox was opened, and the main oil delivery tube was removed. The inlet filtration screen, located in the main oil inlet passage just below the main oil delivery tube, was also removed. During removal of the screen, an o-ring came out with the screen. A flashlight was then used to look into the oil passage, and it was noted that the passage appeared to be approximately 25% of its normal size. Using a dental pick, a second o-ring was removed from the oil passage. The second o-ring appeared to be the same size as the one removed with the inlet screen. The engine manufacturer performed a metallurgical examination of the SAGS, the N1 coupling shaft, the power turbine inner shaft, and other components. The examination determined that "coke build up consistent with reduced oil flow caused rubbing" between the N1 coupling shaft and the power turbine inner shaft. The rubbing produced frictional heat that resulted in softening and subsequent failure of the shafts. Additionally, the metallurgical report concluded that the extra o-ring found in the main oil passage "probably reduced oil flow to the Oil Delivery Tube which would lead to coke build up in the turbine shafting system. The damaged O-ring on the Spur Adapter Gearshaft could have been secondary to the shaft failures and probably was not a contributing factor in the coking." On July 21, 1998, under the supervision of an FAA inspector, oil flow tests were conducted by the engine manufacturer to quantify the effect of the extra o-ring found in the main oil passage on the amount of oil flowing from hole "N" of the main oil delivery tube. (Oil flowing from hole N sprays onto the SAGS. Part of this oil passes through an opening in the SAGS and flows aft to lubricate the N1 coupling shaft.) A series of tests were conducted by reassembling the accessory gearbox housing, main oil delivery tube and inlet filtration screen from the subject engine with the two o-rings from the subject engine positioned in various configurations. For each configuration, the housing was attached to the production oil flow rig, and the amount of oil flowing from hole N was measured. During a test conducted in the normal (no extra o-ring) configuration, the total flow through hole N measured 3.33 lbs/min. Several tests were conducted with the extra o-ring positioned in the passage to approximate the reduction in passage diameter observed during the engine disassembly. During these tests, the total flow through hole N measured from 1.66 to 2.13 lbs/min. Using the flow test data, flow calculations were performed by the engine manufacturer to determine the effect of the extra o-ring on the amount of oil passing through the opening in the SAGS and flowing to the N1 coupling shaft. With the extra o-ring installed, the amount of oil entering the SAGS was calculated at .00 to .10 lbs/min, as compared to .82 lbs/min in the normal configuration. The minimum flow allowable by blueprint was calculated at .32 lbs/min. For further details of the engine disassembly, metallurgical examination, flow testing and flow calculations see the attached manufacturer's report. ADDITIONAL INFORMATION The helicopter, with the exception of the engine, was released to the owner on May 27, 1998. The engine was released to the owner on November 5, 1998.
The failure of maintenance personnel to properly assemble the engine's accessory gearbox, which resulted in a total loss of engine power due to partial blockage of the main oil passage by an o-ring. The blockage reduced oil flow to the turbine shafting, which led to the total failure of the gas producer turbine shaft and the power (free) turbine shaft. A factor was the lack of suitable terrain for the forced landing.
Source: NTSB Aviation Accident Database
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