TEMECULA, CA, USA
N90215
Bell 206B
The helicopter departed from the fuel truck and subsequently experienced a partial loss of engine power at about 10 feet above sloping terrain. The aircraft settled and impacted the ground. The pilot stated that the engine was still spooling down as he exited the aircraft. When the engine compressor section was examined, it was determined that the internal compressor case plastic coating was cracked with sections of the plastic missing. Erosion which exceeded the manufacturer's allowable limits was visible through the first four vane stages and was most evident in the lower case half. According to the engine manufacturer, when compressor erosion is unchecked it will eventually result in a fatigue fracture of one of the vanes due to a loss of stiffness. The maintenance manual states that the compressor case should be replaced if any vanes are cracked, broken, or show any evidence of tip rub on the rotor spacer. It further states that corrosion or erosion can cause damage to the compressor blades and vanes that can result in engine failure. The manufacturer specifies inspection intervals of the compressor case should be determined by the operating environment, but in any case, should not exceed 300 hours. The compressor case halves had were replaced in December 1996 and had accumulated approximately 600 hours at the time of the accident. They had not been inspected since being overhauled. The pilot reported that the helicopter had been operating in desert terrain.
HISTORY OF FLIGHT On May 14, 1998, at 1810 hours Pacific daylight time, a Bell 206B, N90215, experienced a loss of power in the takeoff initial climb and crashed in a field approximately 2 miles west of the Temecula, California, airport. The aircraft was destroyed; however, the commercial pilot, the sole occupant, was not injured. The aerial application flight, conducted under 14 CFR Part 137, originated from Fallbrook, California, at 0645 and had made multiple refueling stops in the fields being worked. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he had been spraying local avocado groves. He had loaded the aircraft with 90 gallons of spray materials and was taking off. Just after the helicopter passed through translational lift, the aircraft yawed and the pilot noticed a decrease in engine noise. The pilot reported that at that time, he was about 10 feet agl over sloping terrain. The aircraft settled and impacted the ground. The pilot stated that the engine was still spooling down as he exited through the right side door. ENGINE EXAMINATION The Safety Board conducted a teardown inspection of the airframe and powerplant on June 22, 1998. Basic control continuity was established. Visual examination of the compressor inlet showed blade and vane distress commencing at the third stage. A full report of the inspection is appended to this file. TESTS AND RESEARCH The compressor case halves, compressor rotor assembly, and compressor front support were sent to the Safety Board Materials Laboratory for further examination. A copy of the metallurgist's report is appended to this file. The compressor case is composed of two longitudinal halves with an attached inlet guide vane (IGV) assembly. The IGV's are part of the compressor front support that is mounted to the forward flange of the compressor case halves. When assembled and operational, air first enters the compressor through the inlet guide vane assembly, then passes through six stages of axial compression and a final centrifugal compressor stage. Each stage of axial compression is composed of a rotating set of compressor blades followed by a fixed set of stator vanes. None of the compressor blades were fractured; however, the blades did exhibit impact damage on the trailing edges of the third stage through the centrifugal compressor stage. The fourth and fifth stages displayed chipping, bending, and curling of the individual blades, with the bending in the direction opposite of rotation. Some leading edge tip erosion was also noted on the first stage blades. Each compressor case half has six rows (stages) of radially oriented vanes projecting inward from the case shell. In each stage, the vanes are individually attached to separate vane bands that are in turn attached to the case halves. The inside diameter of the case assembly is covered by a thick plastic erosion coating. Many of the vanes in the third and fourth stages were fractured, particularly in the lower case half. The remaining vanes in these stages exhibited scarring and bending. The vane bending was in the direction of rotation. Several of the remaining third stage vanes were bent slightly forward and into the rotational path of the third stage rotor and exhibited machining. Vanes in stages four and five also displayed damage, but none of the vanes had separated. In the third stage, 11 of the 28 total vanes were fractured, and 23 of the 32 vanes in the fourth stage were also fractured. Subsequent visual and magnified examinations found that all of the vane fracture surfaces had been smeared by post-separation mechanical damage. A spiraling path of erosion was visible in the plastic case liner material. The erosion path was visible through the first four vane stages and was most evident in the lower case half. Beginning with the first stage vanes, the erosion had completely abraded through the plastic to the vane band adjacent to five vanes. At the second stage the plastic material was penetrated adjacent to eight vanes. The plastic liner was partially missing between the vanes of the third and fourth stages. The plastic material was also abraded away from the forward and aft edges of the fifth stage bleed air slot on both case halves. The Allison Operations and Maintenance Manual for the 250-C20 series engines provides the following compressor vane erosion limits: "If erosion of the case plastic coating between and/or around the vane contour is to the degree that any of the vane band is exposed, replace the entire vane segment (both halves)." Erosion of the individual vanes was noted on some or all of the remaining vanes in the first, second, and third stages. It was adjacent to the case surface and present at both the leading and trailing edges of the vanes, but was most prominent on the second stage. The magnitude of erosion was measured using a video-based, computer-aided, measurement system at the leading and trailing edges of three vanes in the first and second stages. Two of the vanes from each stage were located in areas of erosion in which the plastic coating had been penetrated. Erosion was measured photographically at the leading edge of one blade in the third stage. In each case, the material loss was determined by projecting an undamaged portion of the respective edge over the eroded area and measuring the distance between the actual surface and the projected undamaged surface. The maintenance manual limits the total chordal loss of material from any vane in the first three vane stages to 0.030 inches. Two of the three measured blades in the second stage and the only blade measured from the third stage exceeded 0.030 inches of total erosion loss. The Allison Maintenance manual states that in addition to the 100-hour and applicable 200-hour inspection items, the frequency of inspections of the compressor case should be based on the nature of the erosive environment, and in no case should the inspection interval exceed 300 hours. At 1750, the compressor case must be inspected regardless of the environment. The manual also directs that the compressor case be replaced if any vanes are cracked or broken off or show evidence of tip rub on the rotor spacer. It states that corrosion or erosion will result in damage to the compressor blades and vanes that can ultimately result in engine failure. According to the Allison metallurgist, it has been his experience that when compressor erosion is unchecked it will eventually result in fatigue fractures of the vanes due to a loss of stiffness in the blade material. Review of the engine maintenance records disclosed that the compressor case halves were overhauled in December 1996 and had accumulated approximately 600 hours at the time of the accident. They had not been inspected since the overhaul. The pilot reported that the helicopter had been operating in desert terrain in Palmdale, California.
A loss of engine power due to a deteriorated compressor air seal which resulted in the fracture and separation of several stator vanes. Deterioration was not detected as a result of an inadequate maintenance inspection frequency.
Source: NTSB Aviation Accident Database
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