Fulton, MO, USA
N923SH
Robinson R22
The pilot was conducting a solo cross-country flight in the helicopter as part of his instruction to obtain his rotorcraft rating. The intent of the flight was to fly solo cross-country to perform touch and go landings at another airport. About 36 minutes after departing on the flight, the helicopter impacted terrain. One witness reported that he saw the helicopter flying and heard the engine stop. He stated that the helicopter began to lose altitude and began to spin in a downward spiral. Another witness reported that he observed the helicopter flying toward him with its nose pointed downward about 45°. He stated that the blades were turning slowly and that the helicopter was making a circular motion along its axis, but not a spiral, as it flew toward the ground. The helicopter impacted terrain in a nose-low, left-skid-down attitude. Examination of the airframe and engine did not reveal any mechanical pre-impact anomalies. The main rotor blade damage was consistent with low rotor rpm at the time of impact, and the low rotor rpm caution light filament was stretched, consistent with the light being illuminated at the time of impact. Examination of the rotating engine components did not reveal any evidence consistent with rotation at the time of impact. The carburetor heat control in the cockpit was found unlocked and found in the "on" position. According to a carburetor icing probability chart, the atmospheric conditions about the time of the accident were conducive to moderate icing at cruise power settings and serious icing at descent power settings. Thus, it is likely that the helicopter experienced a loss of engine power due to carburetor ice accumulation. Although the carburetor heat was found in the on position, it is likely that the pilot applied the carburetor heat to address the loss of engine power but it was too late and at too low an altitude to restore engine power. Following the loss of engine power, the main rotor rpm would have rapidly begun to deteriorate unless the pilot immediately reduced collective and entered an autorotation. Given the signatures of low rotor rpm present at the site, it is likely that the pilot failed to initiate the autorotation in a timely manner, which allowed the main rotor rpm to deteriorate to the extent that the main rotor stalled and the pilot would be unable to recover.
HISTORY OF FLIGHTOn October 17, 2018, about 1421 central daylight time, a Robinson R22 Beta helicopter, N923SH, was destroyed when it was involved in an accident near Fulton, Missouri. The pilot was fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 solo instructional flight. According to the operator, the pilot had been receiving instruction toward his rotorcraft-helicopter rating. The intent of the flight was to fly solo from Moscow Mills, Missouri, to Columbia Regional Airport (COU), Columbia, Missouri, and perform touch-and-go landings at COU. The helicopter departed Moscow Mills about 1345 and impacted terrain about 36 minutes later while on a direct route to COU. One witness reported that he saw the helicopter flying and heard the engine stop. He stated that the helicopter began to lose altitude and spin in a downward spiral. Another witness reported that he observed the helicopter flying toward him with its nose pointed downward about 45°. He stated that the blades were turning slowly and that the helicopter was making a circular motion along its axis, but not a spiral, as it flew toward the ground. There were no distress calls from the pilot and no radar information was available for the flight. PERSONNEL INFORMATIONThe pilot had a rotorcraft endorsement for solo flight in the R22 helicopter. AIRCRAFT INFORMATIONAccording to information provided by the operator, the helicopter departed Moscow Mills with 28 gallons of fuel onboard. Information published by the manufacturer indicated that the helicopter consumed between 7 and 10 gallons of fuel per hour. METEOROLOGICAL INFORMATIONAccording to a carburetor icing probability chart, the atmospheric conditions were conducive to moderate icing at cruise power settings and serious icing at descent power settings. AIRPORT INFORMATIONAccording to information provided by the operator, the helicopter departed Moscow Mills with 28 gallons of fuel onboard. Information published by the manufacturer indicated that the helicopter consumed between 7 and 10 gallons of fuel per hour. WRECKAGE AND IMPACT INFORMATIONThe helicopter wreckage was found in a grassy area in a slight depression in the Potowatami recreational area near Fulton, MO. Initial examination of the wreckage at the accident site showed evidence that the helicopter impacted in a nose-low, left-skid-down attitude. Most of the helicopter wreckage was found resting on its left side, in a 6-foot-wide, 2-foot-deep crater. Portions of the wreckage were partially embedded in the dirt. All components were identified at the accident site, with small pieces of debris and plexiglass from the windscreen located within 30 ft of the main wreckage. Damage to the main rotor blades was consistent with low rotor rpm at the time of impact. The low rotor rpm annunciator light was examined with a 10x magnifier and showed stretched filaments consistent with it being illuminated at the time of impact. Examination of the airframe components, flight controls, and main and tail rotor drive systems did not reveal any mechanical pre-impact anomalies. The carburetor heat control in the cockpit was unlocked and bent flush with the console about 1 inch up (in the "on" position). The carburetor heat slider valve was about 1.4-inches open at the carburetor. The main fuel tank was severely impact-damaged and ruptured. A small amount of fuel was present in the auxiliary tank. The carburetor was disassembled, and the float bowls were wet but contained no fuel. The engine was successfully rotated from the crankshaft to the gearbox. Thumb compression was confirmed on all four cylinders. Engine continuity was confirmed from the crankshaft to the engine accessory gears to the fan. The dowel pin (locator pin) on the crankshaft gear was not sheared. There were both imprint marks and score marks on the oil cooler from the starter ring gears. The score marks were slightly angled and aft of the imprint marks. There were imprints on the cooling panels adjacent to the ring gear on the engine right (aircraft left side). There was no evidence of contact between the alternator fan and the engine, and no evidence of rotational scoring marks on the fan or fan scroll. There was a dent on the left side of the upper frames adjacent to the upper sheave, but no evidence of rotational scoring. No mechanical pre-impact anomalies were observed during the engine examination. MEDICAL AND PATHOLOGICAL INFORMATIONThe Boone/Callaway County Medical Examiner's Office, Columbia, Missouri, performed an autopsy of the pilot. The pilot's cause of death was multiple blunt force injuries. No significant natural disease was noted. The FAA's Forensic Sciences Laboratory performed toxicology testing on the pilot's tissue samples. The toxicology tests were negative for drugs and ethanol. ADDITIONAL INFORMATION Robinson Helicopter Company Safety Notice SN-24, "Low RPM Rotor Stall Can Be Fatal," stated: Rotor stall is very similar to the stall of an airplane wings at low airspeeds. As the airspeed of an airplane gets lower…the angle of attack of the wing must be higher for the wing to produce the lift required to support the weight of the airplane…The same thing happens during rotor stall with a helicopter except it occurs due to low rotor RPM instead of low airspeed. As the RPM of the rotor gets lower, the angle of attack of the rotor blades must be higher to generate the lift required to support the weight of the helicopter…Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air to the rotor provides the necessary increase in blade angle of attack…The increased drag on the blades acts like a huge rotor brake causing the rotor RPM to rapidly decrease, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle of attack on the slowly-rotating blades, making recovery virtually impossible, even with full down collective.
A total loss of engine power due to carburetor icing, the pilot's delayed application of carburetor heat, and the pilot's delay in entering an autorotation following the loss of engine power, which resulted in a main rotor stall due to low rotor rpm and an uncontrolled descent into terrain.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
