Pilot Debrief

Agusta A109 near Penobscot, ME — 2019-11-16

Final reportENG20IA005
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Date
2019-11-16
Location
Penobscot, ME, USA
Aircraft
Agusta A109
Registration
N901EM
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Climb

Probable cause

Fatigue fracture of the main rotor blade tip cap leading edge and subsequent partial separation of the tip cap, resulting in increased vibrations in the main rotor.

NTSB narrative

The main rotor blade tip cap leading edge fractures initiated due to fatigue cracking with multiple initiation sites. At the time of the incident, the possibility of fatigue cracking of the leading edge of the P/N 709-0103-29-109 tip cap was a known issue, and Leonardo technical bulletin (bollettino tecnico, or BT) No. 109EP-85 and Federal Aviation Administration (FAA) airworthiness directive (AD) 2007-26-52 were previously issued to inspect for incipient cracks before they grew large enough to result in an inflight separation of the tip cap. These inspections primarily relied on visual inspection of the tip cap leading edge using magnification, and only required dye penetrant inspection of the leading edge if there are doubts about the presence of cracks or if damage other than a crack was found. It is possible that the fatigue crack on tip cap serial number L206 was present during its last inspection, but the visual inspection did not detect the presence of this crack. The crack on the tip cap leading edge remained undetected and propagated until the incident flight, when aerodynamic forces led to partial separation of the upper skin from the lower skin and core. While a visual inspection method can detect surface cracks, variances in the inspection environment (such as lighting), inspector experience, and operators’ maintenance practices can affect the success in detecting surface damage, such as a crack. After the initial issuance of BT No. 109EP-85 and FAA AD 2007-26-52, there continued to be a recurrence of partial and complete inflight separations of tip caps, with at least six known tip cap failures due to fatigue cracking. This indicates that reliance primarily on a visual inspection was not sufficient to eliminate the issue of detecting fatigue cracks before they resulted in a failure of the tip cap. A revision to BT No. 109EP-85 required dye penetrant inspection of the tip caps, which should provide a more reliable inspection method for identifying surface cracks.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Performing more thorough inspections of rotor blade tip caps before the flight.

NTSB coding

  • Sys/Comp malf/fail (non-power) · Enroute-cruise

Evidence available

  • 7 docket documents
View NTSB final reportView NTSB docket

Docket documents7

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