Uvalde, TX, USA
N484AB
Robinson R44
The student pilot and flight instructor were conducting a local, instructional flight. The instructor reported that, as the student was transitioning the helicopter through effective translational lift about 40 knots airspeed, he raised the collective to increase the available power to the takeoff power setting. Shortly thereafter, he heard a loud "pop/bang sound," which was followed by an uncommanded right yaw and severe vibrations. Due to the low altitude and airspeed, the instructor immediately took the flight controls and initiated an autorotation. Upon touchdown, the helicopter tilted forward, and the instructor applied aft cyclic to correct; however, the main rotor blades contacted the tailboom, which resulted in the tailboom partially separating from the helicopter. During postaccident examination of the helicopter, the clutch assembly lubricant was drained, and metallic debris was found in the strainer. Disassembly of the clutch assembly revealed galling of the sprags and the corresponding sprag contact surfaces on the clutch shaft and hub. It is likely that the excessive galling resulted in the sprag clutch slipping, which produced the uncommanded right yaw and led to the student's and instructor's inability to maintain helicopter control.
On January 8, 2019, about 1000 central standard time, a Robinson R44 II helicopter, N484AB, experienced a hard landing during an autorotation shortly after takeoff from Garner Field Airport, Uvalde, Texas. The flight instructor sustained minor injuries, the student pilot was not injured, and the helicopter sustained substantial damage. The helicopter was registered to and operated by a private individual as a Title 14 Code of Federal Regulations Part 91 instructional flight. Visual meteorological conditions prevailed at the time of the accident, and a flight plan was not filed. The local flight was originating at the time of the accident. According to the flight instructor, all pre-takeoff checks were normal with no anomalies noted. Once the helicopter was through effective translational lift and about 40 knots airspeed, the collective was raised to increase the available power to takeoff power setting. Shortly thereafter, a loud pop/bang was heard followed by an uncommanded right yaw and severe vibrations. Due to the low altitude and airspeed, the flight instructor immediately initiated an autorotation. Upon touchdown, the helicopter tilted forward with the main rotor blades about 6 inches from contacting the ground. The flight instructor applied aft cyclic to correct, and the main rotor blades contacted the tailboom. Postaccident examination of the helicopter revealed the tailboom was partially separated, and the tail rotor driveshaft system was damaged. During the examination, the clutch assembly lubricant was drained and an unusual amount of metallic debris was noted in the strainer. The clutch assembly was removed and sent to Robinson Helicopter Company for further examination. Examination and disassembly of the clutch assembly revealed galling of the sprags and the corresponding sprag contact surfaces on the shaft and hub. According to the Robinson R44 maintenance manual, the clutch assemblies are to be inspected for metallic flakes every 500 hours. The most recent inspection on the clutch assembly was completed on April 18, 2018, about 245 hours prior to the accident.
The slipping of the sprag clutch due to galling of the sprags and corresponding sprag contact surfaces within the clutch assembly, which rendered the helicopter uncontrollable.
Source: NTSB Aviation Accident Database
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