Pilot Debrief

ROBINSON R44 near Kailua, HI — 2019-04-29

Final reportWPR19FA123
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Date
2019-04-29
Location
Kailua, HI, USA
Aircraft
ROBINSON R44
Registration
N808NV
Category
Helicopter
Highest injury
Fatal
Fatalities
3
Phase of flight
Descent

Probable cause

The helicopter’s encounter with a strong downdraft or outflow boundary while operating at a higher than recommended airspeed in turbulence which resulted in a low-G condition, excessive main rotor flapping, and an in-flight breakup when the main rotor contacted the cabin area.

NTSB narrative

The pilot and two passengers departed on a helicopter tour flight around the island. Air traffic control radar and communications information indicated that the flight encountered deteriorating visibility conditions enroute. About this time, the helicopter slowed from about 108 knots (kts) to about 92 kts and initiated a turn. Shortly thereafter, radar data then indicated a rapid descent from an altitude about 1,700 ft above ground level. Witnesses near the accident site reported hearing the helicopter in the clouds and reported that it sounded as if it was very low. They then described the sound of metal hitting metal and saw pieces of the helicopter falling from the sky. Postaccident examination of the airframe revealed that the forward left roof and floor/chin exhibited damage consistent with contact from a main rotor blade. One main rotor blade exhibited a spanwise dent which displayed equally spaced score marks consistent with the row of screws in the windshield bow. The outboard portion of one of the main rotor blades, along with left fuselage and left side cabin components, were found early in the debris path. The main rotor drive shaft was bent about 20° above the swashplate. Arc-shaped scoring was observed on both sides of the main rotor hub adjacent to the pitch horns. Both teeter stops were crushed, and the driveshaft was dented. The droop stop bolt at the nut was sheard but remained in place. Examination of the engine revealed no anomalies and the evidence indicated that the engine was operating at the time of the accident. The accident helicopter was equipped with a two-bladed, semirigid rotor system. This design is susceptible to excessive rotor blade flapping, during periods of low-G’s or the pilot’s improper application of control inputs. Excessive rotor blade flapping can result in main rotor contact with the fuselage. A manufacturer safety notice stated, in part, that, when flying in turbulence, the helicopter should be slowed below a normal cruise speed to around 60 to 70 kts. Review of weather information for the area of the accident site around the time of the accident revealed a warm, moist environment that supported heavy rain showers that could produce unexpected strong downdrafts, reduced visibilities, wet microburst, and outflow wind conditions. At the time of the accident, weather surveillance radar reflectivity values indicated a descending core of 20 to 30 dBZ values descending towards the surface from above the accident site. The accident flight likely encountered this downdraft or outflow boundary, reduced visibilities, and rapidly changing and unexpected wind conditions. Which likely caused a low-G condition, excessive main rotor flapping, and an in-flight breakup when the main rotor contacted the cabin area.

Analysis

Primary failure mode
Loss of control
First missed decision gate
Pilot could have chosen to avoid the area with deteriorating weather conditions.

NTSB coding

Evidence available

  • ATC audio
  • Video
  • ADS-B / radar
  • 23 docket documents
View NTSB final reportView NTSB docket

Docket documents23

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