BALLSTON, OR, USA
N921RB
Bell 206B3
Witnesses observed the helicopter begin a left turn from north to west, then begin rotating to the left, after lifting an external load of Christmas trees. The helicopter rotated about 1 to 1 1/2 turns, then impacted the ground on its left side. Local winds at the time were from the southeast at 5 to 15 knots, according to the witnesses. On-site investigators found no evidence of airframe or engine malfunction, and noted dirt and packed mud in the engine compressor. The pilot, who was seriously injured, had no recollection of the accident sequence. In a test run of the engine at the manufacturer's facilities, the engine would not produce more than normal cruise power, but the manufacturer's test report stated that the only observable cause for this was extensive foreign object damage to the inlet guide vanes and first stage compressor blades. The engine fuel control and governor were also functionally tested and found to be slightly out of limits at certain test points, but the manufacturer stated that the exceedances observed would not significantly affect engine power output.
On November 18, 1996, approximately 1608 Pacific standard time, a Bell 206B-3 helicopter, N921RB, registered to Rainbow Helicopters Inc. of Redmond, Oregon, impacted terrain while performing a 14 CFR 133 rotorcraft external-load operation near Ballston, Oregon. The commercial pilot, who was the helicopter's sole occupant, was seriously injured and the helicopter was substantially damaged. Salem, approximately 16 miles southeast of Ballston, reported a 2,500-foot overcast ceiling with 4 miles visibility in rain at 1556. No flight plan had been filed for the local flight, which originated from a site adjacent to the work area. The helicopter was lifting Christmas trees with a 25-foot line at the time of the accident. Witnesses interviewed by on-scene investigators from the FAA and Bell Helicopter Textron reported that shortly after picking up the load, the helicopter started a left turn from a northerly to a westerly heading, then began a left rotation. The witnesses reported that the helicopter rotated about 1 to 1 1/2 turns prior to impacting the ground on its left side. According to on-site investigators, the point of impact was in a level muddy field, approximately 200 to 300 feet from the spot where the load had been picked up. The external load was found to be still attached to the helicopter. The load was weighed after the accident and the weight of the load was found to be about 1,050 pounds, which is within the load limitations for the helicopter. On-scene investigators reported that they found approximately 38 gallons of fuel aboard the accident aircraft, and that this fuel appeared clean. The Bell investigator reported that the witnesses estimated local wind conditions in the accident area as being approximately southeasterly at 5 to 15 knots. Winds at Salem were reported as being northeasterly at 5 to 6 knots during the time frame of the accident. The Bell investigator stated that the local winds reported by the witnesses, in combination with the helicopter's reported heading during the accident sequence, represented conditions conducive to weathercock stability. According to FAA Advisory Circular (AC) 90-95, "Unanticipated Right Yaw in Helicopters", weathercock stability is an unanticipated yaw which can occur as a result of the tendency of the helicopter's nose to yaw into the relative wind. Weathercock stability can result in an increase in yaw rate if a yaw rate to either the left or right has been established. The increase in yaw rate, if not arrested promptly by the pilot with opposing antitorque pedal input, can result in loss of control of the helicopter. According to FAA AC 90-95, weathercock stability potential exists when winds are from 120 to 240 degrees relative to the helicopter's nose at speeds of approximately 5 to 15 knots. FAA AC 90-95 states that single-main-rotor helicopters whose main rotor rotates counterclockwise (as viewed from above the helicopter), as is the case with the Bell 206B-3, are most susceptible to weathercock stability situations in a right turn. The pilot, who sustained head injuries and was comatose for a period of time following the accident, eventually reported to investigators that the accident occurred during the last flight of the day after approximately 8 flight hours of work that day, but stated he did not have any recollection of the actual accident sequence. Investigators from the FAA and Bell Helicopter Textron examined the helicopter wreckage at the accident site and found no evidence of airframe or flight control malfunctions. Investigators from the FAA, Bell Helicopter Textron and Allison Engine Company subsequently conducted a follow-up examination of the helicopter's 250-C20B engine at the facilities of HLM Air Services, Independence, Oregon, on November 26, 1996. During this examination, it was noted that the engine compressor bleed valves closed about 10 pounds per square inch (PSI) later than their usual closing range, although the investigators noted that the regulator used for the check was of unknown accuracy. The investigators also noted foreign object damage (FOD) on most of the first stage compressor blades and several blades of all other stages, a significant amount of mud packed in the first few stages of stator vanes, and mud residue throughout all stages of the engine compressor. A photograph of the helicopter's instrument panel, provided by the Bell investigator to the NTSB, depicted the helicopter's torque gauge indicating 110%. The engine was subsequently shipped to the facilities of the Allison Engine Company in Indianapolis, Indiana, for a test cell run under FAA supervision. The engine was run in as-received condition except for cleaning the mud from the engine compressor prior to the test run. It was noted prior to beginning the run that there was a kinked Py air line, and that this line did not appear to be in a position consistent with the kink being due to impact damage. The engine did not attain its power specification at any point during the test run, and was unable to produce more than approximately normal cruise power. The kinked Py line was replaced with a serviceable line and the engine was test run again, with negligible change in the engine's power output. Allison also conducted a metallurgical analysis on the kink in the Py line. This analysis revealed a deposit of tool steel on the line at the kink, which Allison stated was indicative of the line being struck by a foreign object. The fuel control and governor from the engine were bench-tested under FAA supervision at AlliedSignal Inc., South Bend, Indiana. These units were found to be out of limits at certain test points, but AlliedSignal and Allison reported that none of the out-of-limits conditions observed would significantly affect engine performance. Additionally, the compressor bleed valve was bench-tested at Allison and found to be within test limits on closing, but slightly late in opening up as Pc pressure was reduced. Allison stated that this condition would not affect engine performance and would have no effect at all at high power settings. Allison concluded in its report on the engine examination that the only observable cause for the engine's failure to produce rated power during the test run was the FOD to the compressor inlet guide vanes and first stage compressor blades. According to available maintenance records and data furnished with Allison's engine investigation report, the last annual inspection of the helicopter was performed on April 2, 1996, at 15,714.7 airframe hours, 17,015.8 engine hours and 234.6 Hobbs hours. The Hobbs meter on the aircraft read 803.4 hours after the accident. At the time of the accident, the engine had 209.8 hours since its last hot section overhaul. The FAA inspector who responded to the accident site reported that the engine fuel control was changed on November 1, 1996, approximately 2 1/2 weeks before the accident. The FAA inspector stated that Bell recommends an engine performance check after changing a fuel control, but that none was required or accomplished following this change.
a loss of control for undetermined reasons.
Source: NTSB Aviation Accident Database
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