Pilot Debrief

Robinson R44 near Logan, IA — 2020-08-08

Final reportCEN20LA330
Sign in to save
Date
2020-08-08
Location
Logan, IA, USA
Aircraft
Robinson R44
Registration
N89LA
Category
Helicopter
Highest injury
Serious
Fatalities
0
Phase of flight
Maneuvering

Probable cause

The failure of the lower actuator bearing support brackets, which resulted in the misalignment of the clutch actuator, a loss of belt tension on the main rotor drive system, and a subsequent forced landing.

NTSB narrative

The commercial pilot had just completed several spray runs in the helicopter and was turning to line up for another run, when he heard the engine emit a "pop" noise. The helicopter yawed “violently,” the engine rpm increased, and the rotor rpm decreased, which caused both the low rotor rpm horn and light to activate. The pilot initiated an autorotation to a cornfield, where the helicopter impacted an area of rising terrain and came to rest on its right side. The helicopter sustained substantial damage to the fuselage. A postaccident examination of the airframe revealed that all four V-belts that drive the rotor system were displaced from their grooves. Small areas of damage were visible in one belt. The lower bearing brackets (also known as A185 brackets) were separated from the scroll and the belt tension actuator had rotated out of position. The mating surface of the brackets and the scroll were polished smooth, and the edges of the brackets had dug into the scroll. The attaching rivets were sheared flush with the mating surface of the brackets. The rivet holes in the scroll were elongated in the direction of engine rotation. The bracket backing plates were not present. The belt tension actuator had fractured just below the upper gear housing. It is likely that, over time, the lower brackets supporting the lower actuator bearing became loose during flight operations and sheared the rivets, which allowed the actuator bearing to shift and led to a subsequent misalignment of the clutch actuator. The misalignment of the clutch actuator resulted in the loss of belt tension between the engine and the main rotor drive system. The pilot’s report of an unusual noise, the nose yaw, and an increase of engine rpm and a decrease in rotor rpm was consistent with a drive system failure. A review of the airframe maintenance logbooks showed no evidence of compliance with the helicopter manufacturer’s service bulletin RHC SB-94 and service letter RHC SL-61A. RHC SB-94 directs the inspection of the A185 brackets for evidence of fretting or looseness. RHC SL-61A directs actions to be performed whenever looseness of an A185 bracket is detected and whenever the upper or lower scroll was removed. The FAA does not require compliance with RHC SB-94 and RHC SL-61A for 14 CFR Part 91 and 14 CFR Part 137 operations.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Failure to inspect A185 brackets as per RHC SB-94 and SL-61A.

NTSB coding

Evidence available

  • Photos
  • 6 docket documents
View NTSB final reportView NTSB docket

Docket documents6

Related mishaps