Pilot Debrief

BOEING 777 near Ugolny Airport — 2013-07-02

Final reportENG13IA033
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Date
2013-07-02
Location
Ugolny Airport, RU
Aircraft
BOEING 777
Registration
HL8275
Category
Airplane
Fatalities
0
Phase of flight
Cruise

Probable cause

The failure of the transfer gearbox resulting from the fracture and separation of the radial gearshaft from fatigue cracking as a result of a combination of high residual tensile stresses produced by local decarburization during the manufacturing process coupled with the normal operating stresses.

NTSB narrative

On July 2, 2013, a Korean Air Lines (KAL) Boeing 777-300ER, registration number HL8275, powered by two General Electric (GE) GE90-115B turbofan engines, experienced a No. 1 engine (left) in-flight shutdown (IFSD) while crossing the Bering Sea. The pilots diverted the airplane to Ugolny Airport (DYR), in Siberia, Russia where an uneventful single engine landing was made. The Korean ARAIB requested delegation of the investigation and the NTSB accepted responsibility for investigating this incident. Post landing examination of the No. 1 engine revealed that the radial gearshaft within the transfer gearbox housing was fractured. Metallurgical examination of the radial gearshaft found an 0.049-inch axial crack with striation features indicative of fatigue propagation and a morphology consistent with initial fatigue initiation occurring under a torsional and/or biaxial stress condition. This axial crack was considered the primary crack and the fracture origin area. Microhardness tests near the fracture origin revealed a reduction in near surface hardness (lower hardness values than required). GE concluded that the near surface low hardness was attributable to decarburization due to marginal (thin or detached) copper plating existing on the gearshaft surface during the hardening process. Through computer modeling and testing of the radial gearshaft, GE concluded that the initiation of the axial crack was likely the result of a combination of high residual tensile stresses produced by local decarburization coupled with the operating stresses experienced at the outer diameter surface. To address the manufacturing process deficiency, GE issued seven service bulletins to remove, inspect, and repair the suspect radial gearshafts. To expedite the removal of the suspect radial gearshafts, the Federal Aviation Administration issued three separate Airworthiness Directives (AD). According to GE, all affected radial gearshafts addressed by the various FAA ADs have been removed from service.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Manufacturing process deficiencies could have been addressed before the gearshaft was used.

NTSB coding

  • Loss of engine power (total) · Enroute-cruise
  • Engine Failure Enroute

Evidence available

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

Docket documents4

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