Pilot Debrief

HUGHES 369 near Canton, MI — 2024-09-11

Final reportCEN24LA358
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Date
2024-09-11
Location
Canton, MI, USA
Aircraft
HUGHES 369
Registration
N8969F
Category
Helicopter
Highest injury
None
Fatalities
0

Probable cause

Fatigue cracking of the main rotor blade that occurred at an undetermined time.

NTSB narrative

The pilot of the helicopter was performing external lift operations when he noticed an “intermittent and unusual vibration.” The pilot performed a precautionary landing, and a postflight inspection of the main rotor system revealed a crack in one main rotor blade, which constituted substantial damage to the helicopter. Postaccident examination of the main rotor blade revealed that a large crack extended from the leading edge spar on the lower surface to the middle of the main rotor blade along the doubler leading edge on the upper surface of the blade. There was an area of chipped paint adjacent to the crack at the outboard tip of the root fitting on the lower side of the blade. After sectioning, the large crack was mechanically exposed and examined stereoscopically. Feathery features consistent with fatigue were observed on the lower side fracture surfaces of the doubler, skin, and spar. Scanning electron microscope examination revealed numerous ratchet marks at the doubler-skin and skin-spar interfaces emanating at the outboard sides of both layers, consistent with multiple initiation fatigue starting from the outboard side. Dark-colored deposits were observed on the spar fracture surface at the tip and on the adjacent skin fracture surface due to the rubbing of mating fracture surfaces. The large size of the crack and the rubbing of the mating fracture surfaces was consistent with fatigue crack growth over an extended period of time. The accident helicopter main rotor blades were subject to an FAA Airworthiness Directive (AD) for regular inspections due to the helicopter’s use for external lift operations. The cracked main rotor blade was installed onto the accident helicopter the month before the accident and had accrued about 67 hours of operation since installation. The operator reported that an inspection in accordance with the AD (AD 2005-21-02) was most recently performed 6 days (12.4 flight hours) before the accident. Maintenance personnel noted no defects during recent inspections. According to the operator, the inboard portion of the main rotor blade was painted gloss white by the operator’s director of maintenance sometime during the month of the accident; however, there were no maintenance records reflecting when or by whom the painting was performed or inspected. It is likely that the fatigue crack occurred sometime between when the main rotor blade was installed onto the helicopter and before the accident flight; however, when the fatigue crack occurred could not be determined. Manufacturer guidance stated that the identification of cracks in main rotor blades requires a careful examination, including the use of supplemental lighting and optical magnification. Whether maintenance personnel performed the inspections with adequate lighting and magnification was not determined. Additionally, it could not be determined whether the fatigue cracking may have been concealed by the recent painting of the main rotor blade.

NTSB coding

Evidence available

  • Video
  • Photos
  • 15 docket documents
View NTSB final reportView NTSB docket

Docket documents15

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