Yuma, AZ, USA
N82857
BELL OH 58C
The pilot of the helicopter was performing an aerial application flight in dark night conditions when he felt a vibration. The pilot pulled back on the cyclic control to stop the helicopter's forward movement and maintained this position for the remainder of the flight. Seconds later, the helicopter had descended to 40 ft above ground level (agl), and the low rotor rpm light indication illuminated, accompanied by multiple other warning lights. The helicopter continued to descend and impacted terrain upright. The main rotor blades severed the tail boom, the left skid dug into the ground, and the helicopter rolled over and came to rest on its right side. Examination of the helicopter revealed no anomalies with the flight control system. The engine inspection revealed thermal damage to the combustion liner and 1st stage gas turbine producer vanes, consistent with an irregular spray pattern from the fuel nozzle due to carbon buildup. Although the helicopter's weight at the time of the accident exceeded its published maximum gross weight by 100 lbs, whether this contributed to the helicopter's inability to maintain altitude following the loss of engine power was not determined. The engine logbook indicated that the fuel nozzles were inspected in accordance with the manufacturer's 100-hour inspection checklist about two years before the accident. Although a 100-hour inspection was completed about 4 months before the accident, the maintenance log entry did not specify the work completed, and whether the fuel nozzles were inspected at this time could not be determined; however, the carbon buildup on the fuel nozzles suggested long-term accumulation, and it is likely that maintenance personnel either omitted this inspection item or did not adequately inspect the fuel nozzles. Given the carbon accumulation of the fuel nozzle and signatures of irregular fuel spray, it is likely that the helicopter experienced a partial loss of engine power, which resulted in the subsequent descent into terrain.
HISTORY OF FLIGHTOn May 11, 2018, about 2030 mountain standard time, an OH-58C helicopter, N82857, was substantially damaged when it was involved in an accident near Yuma, Arizona. The commercial pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot departed in dark night conditions to apply chemical insecticide to three separate fields. After completing the first pass over the first field, he climbed the helicopter and started a left turn at 20 mph and an altitude of about 60 ft above ground level (agl), but immediately felt a vibration that he perceived was coming from the main rotor system. Seconds later, at an altitude of about 40 ft agl, the low rotor rpm light indication illuminated, accompanied by multiple other warning lights. The pilot added that, after the low rotor rpm light illuminated, the vibration was present for the remainder of the flight. The vibration was noticeable but did not obstruct the pilot's visibility of the instrument panel. The pilot turned to the southeast while the helicopter began to descend, and as it approached the ground, the pilot flared the helicopter to reduce its vertical speed. The helicopter landed flat in a bed of sudangrass, the main rotor blades severed the tailboom, the left skid dug into the ground, and the helicopter rolled over on its nose before it came to rest on its right side. The pilot reported that he did not attempt an autorotative landing at any point following the partial loss of power. During a follow-up interview, the pilot reported that he had never experienced a similar vibration. At the time of the vibration, he pulled back on the cyclic control to stop the helicopter's forward movement and held it in the aft position for the remainder of the flight. He further stated that the engine governor trim was at maximum, which is customary for him, and that he normally flew the helicopter at or near its maximum gross weight. AIRCRAFT INFORMATIONThe helicopter was powered by an Allison M250-C20C (T-63-A720), s/n AE-406024, turboshaft, gas-coupled free-power turbine, 420 shaft hp engine. Review of the aircraft logbooks revealed that the helicopter's most recent airframe annual inspection and 300-hour inspection was completed on March 23, 2018, at an accumulated flight time of 7,300.2 total hours. The airframe had accrued a total of 7,314 hours, and the engine a total of 8,682 flight hours at the time of the accident, with 14 hours in service following its most recent inspection. The engine's most recent 100-hour inspection was completed on January 4, 2018, but the records did not specify the work that was completed. According to an entry dated February 16, 2016, the engine received a 100-hour inspection in accordance with the "Rolls Royce 250-C20 Series Inspection Checklist (Table 602)." Item 21G of this checklist required the removal, inspection, and cleaning of the fuel nozzle. The turbine assembly was inspected on July 9, 2015, at a total time of 6,963.4 flight hours, due to an engine overtemperature during startup, which included a replacement of the 1st stage axial wheel. The pilot reported that he departed the operator's base airport with about 32 gallons of fuel onboard for his 20-minute flight to the field he intended to spray. The helicopter consumed about 28 gallons per hour, and he had about 15 gallons of fuel on board at the time of the accident. Weight and Balance At the time of the accident, the helicopter's total weight was 3,342 lbs, about 142 lbs above its maximum gross weight. AIRPORT INFORMATIONThe helicopter was powered by an Allison M250-C20C (T-63-A720), s/n AE-406024, turboshaft, gas-coupled free-power turbine, 420 shaft hp engine. Review of the aircraft logbooks revealed that the helicopter's most recent airframe annual inspection and 300-hour inspection was completed on March 23, 2018, at an accumulated flight time of 7,300.2 total hours. The airframe had accrued a total of 7,314 hours, and the engine a total of 8,682 flight hours at the time of the accident, with 14 hours in service following its most recent inspection. The engine's most recent 100-hour inspection was completed on January 4, 2018, but the records did not specify the work that was completed. According to an entry dated February 16, 2016, the engine received a 100-hour inspection in accordance with the "Rolls Royce 250-C20 Series Inspection Checklist (Table 602)." Item 21G of this checklist required the removal, inspection, and cleaning of the fuel nozzle. The turbine assembly was inspected on July 9, 2015, at a total time of 6,963.4 flight hours, due to an engine overtemperature during startup, which included a replacement of the 1st stage axial wheel. The pilot reported that he departed the operator's base airport with about 32 gallons of fuel onboard for his 20-minute flight to the field he intended to spray. The helicopter consumed about 28 gallons per hour, and he had about 15 gallons of fuel on board at the time of the accident. Weight and Balance At the time of the accident, the helicopter's total weight was 3,342 lbs, about 142 lbs above its maximum gross weight. WRECKAGE AND IMPACT INFORMATIONThe helicopter came to rest in a field about 8 nautical miles north of the departure airport, adjacent to a dirt road. The main rotor blades separated from the blade mast and were located several feet away from the helicopter. The nose exhibited vertical crush damage and the lower windshield was broken. Both landing skids remained attached to the helicopter; however, the left side step was broken at the forward mount. The right-side aft fuselage was compressed along the upper deck and compression and buckling were observed along the lower aft fuselage. The upper deck was collapsed, and the transmission was bent aft. The tail boom was collapsed, and the empennage was separated from the tail boom. The tail rotor drive system was continuous from the tail rotor to the tail rotor driveshaft segment at a separation in the tail boom. The second driveshaft segment displayed a torsional fracture. The third driveshaft segment was continuous to the oil cooler blower shaft, which was disconnected during the impact sequence. The anti-torque control tube was continuous from a fracture separation at the horizontal stabilizer to the tail rotor blades. Both tail rotor blades showed smooth and unrestricted movement when turned by hand and the pitch angle movement was unremarkable. The main rotor system was continuous from the engine to the pitch change links at the main rotor blades. Movement was confirmed from the KAflex main driveshaft coupling through the transmission to a fracture in the mast. Both main rotor blades displayed fractures and bending at the end of the doublers. The collective and cyclic controls were continuous from the cockpit to the main rotor system. Throttle control continuity was confirmed from the cockpit to the fuel controller, which could be actuated from the flight idle position to the full-throttle position. Anti-torque control was confirmed from the pedals to the aft fuselage bell crank, where the pushrod had separated. The aft pushrod was continuous from the bell crank to the horizontal stabilizer, where it had fractured at the threaded end. About 20 gallons of Jet-A fuel was drained from the fuel tank. The fuel pump functioned normally, and the airframe fuel filter was free of obstructions. Engine Examination Control continuity was not confirmed from the collective to the power turbine governor (PTG) due to damage sustained by the linkage system; however, the PTG arm moved from stop to stop by hand with no resistance. The PTG trim actuator was in the full retracted position, which corresponds to the full increase position on the PTG. The PTG connection rod that adjusted the trim movement at the PTG tested normally when power was applied to the airframe. The helicopter was equipped with an inlet barrier. Numerous pieces of vegetation debris were observed along the circumference of the inlet. The 2nd stage compressor rotor blades were bent opposite the direction of rotation. Both the N1 and N2 rotors were locked and could not be rotated by hand at either the compressor inlet or the 2nd stage power turbine wheel. The oil system did not display any preimpact anomalies. The engine was examined at a third-party facility in Mesa, Arizona by a representative of the engine manufacturer with oversight from the NTSB investigator-in-charge. The 4th stage power turbine blades were bound; however, examination of the trailing edge of the blades did not show any signs of foreign object debris ingestion. The PTG drive shaft was partially engaged but rotated freely within the governor by hand without resistance. Disassembly of the engine-driven fuel pump revealed a considerable amount of clean, clear liquid that resembled JET-A fuel. The unit, filter, and driveshaft were unremarkable. The outer combustion case was removed to inspect the inside combustion liner, which displayed two visually noticeable burn signatures, about 2 inches in diameter, on the outside wall adjacent to the large dilution at the 3 o'clock position. Removal of the combustion liner revealed the presence of 4 damaged vanes about the 2 o'clock position of the 1st stage turbine nozzle, which was consistent with one of the burn markings. (see Figure 1.) Figure 1. Burn Markings on Both Sides of Large Dilution Hole The fuel nozzle and nozzle nut were marked before the nozzle was removed. During subsequent testing at various pressures, the nozzle showed streaking at each pressure setting. Carbon debris was observed at the fuel nozzle. The streaking position was consistent with the position of the thermal signatures on the combustion liner and damage to the 1st stage gas producer turbine nozzle, which showed 5 vanes with varying amounts of distress. Postaccident examination of the engine did not present any anomalies that could have precluded normal operation. Functional testing of the PTG could not be performed due to extensive postaccident damage; however, inspection of the PTG and a functional test of the fuel control unit revealed no anomalies that could have precluded their normal operation.
A partial loss of engine power during an aerial application flight due to thermal damage to the 1st stage gas producer turbine nozzle and an irregular fuel spray pattern due to carbon buildup of the fuel nozzle. Contributing to the accident was maintenance personnel's inadequate inspection of the fuel nozzle.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
