Pilot Debrief

Robinson R44 near Uvalde, TX — 2019-01-08

Final reportCEN19LA065
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Date
2019-01-08
Location
Uvalde, TX, USA
Airport
UVA
Aircraft
Robinson R44
Registration
N484AB
Category
Helicopter
Highest injury
Minor
Fatalities
0
Phase of flight
Takeoff roll

Probable cause

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.

NTSB narrative

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Inspect clutch assembly for metallic flakes as per maintenance manual.

NTSB coding

Evidence available

  • 7 docket documents
View NTSB final reportView NTSB docket

Docket documents7

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