Pilot Debrief

BOEING MD-88 near Atlanta, GA — 2012-11-07

Final reportENG13IA005
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Date
2012-11-07
Location
Atlanta, GA, USA
Aircraft
BOEING MD-88
Registration
N908DE
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Initial climb

Probable cause

The 4th stage turbine blade separation that resulted in an uncontained release of low pressure turbine blade material was Delta Air Lines' repair process for the turbine blades' tip shrouds. It was not possible to identify the part of the repair process that led to the premature wear of the blade's tip shoud and subsequent separation.

NTSB narrative

A Delta Air Lines Boeing (previously McDonnell Douglas) MD-88 airplane, N908DE, experienced an uncontained 4th stage turbine blade separation and loss of power from the No. 1 (left) engine, a Pratt & Whitney JT8D-219, as the airplane was climbing through FL 240 after departing from the Hartsfield-Jackson Atlanta International Airport, Atlanta (ATL), Georgia. The pilots reported hearing a loud bang and the indications on the No. 1 engine's rpm and engine pressure ratio gages dropped while the indication on the exhaust gas temperature gage increased. The pilots retarded the No. 1 engine's power lever to idle and noted that the engine continued to run. The pilots declared an emergency and requested to return to ATL. While enroute back to ATL, the pilots shut down the No. 1 engine after the oil quantity had decreased to near zero and the oil pressure was less than 10 pounds per square inch. The airplane returned to ATL for a single-engine landing without further incident. The examination of the airplane revealed a large hole in the bottom of the No. 1 engine's cowl coincident with where the engine's fan duct was ruptured. The disassembly and examination of the No. 1 engine revealed all of the 4th stage turbine blades were fractured transversely across the airfoil adjacent to the blade root platform. There was one 4th stage turbine blade that had a flat, blue-colored, elliptical-shaped area on the fracture surface on the convex (suction) side of the airfoil that was identified by the NTSB's Materials Laboratory as a high-cycle fatigue (HCF) fracture. HCF fractures at that location on JT8D-219 4th stage turbine blades are consistent with wear on the blade tip shroud notches that cause an increase in the stresses in the airfoil. There is an airworthiness directive that requires an inspection of JT8D-219 4th stage turbine blades for shroud notch wear, however the incident occurred prior to the blades having accumulated enough time in service to have required the blades to be inspected. All of the 4th stage turbine blades had the tip shroud notches repaired at Delta's turbine blade shop prior to installation in the engine. Delta reported that they had seen an increase in the number of JT8D-219 4th stage turbine blade fracture incidents, almost all that involved blades that had been repaired in their blade repair shop. As a result of the number of JT8D-219 4th stage turbine blade fracture incidents that had occurred, Delta had discontinued repairing those turbine blades in their shop and were using either new blades or blades that were repaired at a turbine blade repair facility. Although a special dimensional inspection to measure the turbine blade's shroud notches was developed, there were no Delta-repaired 4th stage turbine blades available to measure.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Inspection of 4th stage turbine blades before the incident could have prevented failure.

NTSB coding

  • Uncontained engine failure · Initial climb

Evidence available

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

Docket documents21

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