NTSB Factual Report

History of Flight

On November 6, 2012, about 1900 eastern standard time, a Boeing (previously McDonnell Douglas) MD-88, N908DE, operated by Delta Air Lines as flight 241 experienced an uncontained 4th stage turbine blade separation and loss of power from the left engine as the airplane was climbing through FL 240 after takeoff from the Hartsfield-Jackson Atlanta International Airport (ATL), Atlanta, Georgia. The pilots reported hearing a loud bang and noted that the left engine's engine pressure ratio (EPR) indication had dropped. The pilots retarded the left engine's power lever to idle and noted that the engine continued to run. The pilots also noted that the left engine's generator had dropped off line. The pilots declared an emergency and informed air traffic control that they needed to return to ATL. When the left engine's oil quantity had decreased to near zero and the oil pressure was down to less than 10 pounds per square inch, the pilots shut down the engine. The airplane returned to ATL for a single-engine landing without further incident. After landing, the airplane stopped on an adjacent taxiway and airport fire department personnel examined the left engine to ensure that it was safe. The airport fire department personnel reported that there was a hole in the engine cowling and that there was hydraulic fluid leaking from the engine. The flight crew pulled the left engine's fire handle, and the airplane then taxied back to the gate under its own power where the passengers and crew deplaned normally. The airplane was operating as a scheduled domestic passenger flight on an instrument flight rules flight plan under the provisions of 14 Code of Federal Regulations Part 121 from ATL to Miami International Airport, Miami, Florida.

Injuries to Persons

There were no injuries to the 2 pilots, 3 flight attendants, and 142 passengers on board.

Damage to Airplane

The airplane and right engine did not have any damage. The left engine's aft lower cowling had a 63-inch long (circumferential) by 9-inch wide (axial) hole in the bottom that was 14 inches aft of the forward edge that spanned almost the full width of the cowling.

Other Damage

There was no other reported damage.

Personnel Information

The captain, age 52, holds an airline transport license and reported having 8,145 hours of flying time with 2,562 hours being in the MD-80 series airplane. The captain reported flying 177 hours in the previous 90 days and 6 hours in the previous 24 hours. The captain is type rated in Boeing MD-80 and 737 airplanes. The captain's last line check was on April 4, 2012.

The first officer (FO), age 43, holds an airline transport license and reported having about 5,300 hours of flying time with 322 being in the MD-80 series airplane. The FO reported flying 59 hours in the previous 90 days and 6 hours in the previous 24 hours. The FO is type rated in Boeing MD-80 and Gates Learjet airplanes.

Airplane and Engine Information

The airplane, N908DE, is a Boeing MD-88, serial number (SN) 53417, with a maximum takeoff gross weight of 150,500 pounds. The airplane was equipped with 156 seats: 2 pilots, 5 flight attendants, and 149 passengers. According to Delta Air Lines' records, at the time of the incident, N908DE had accumulated 49,868 hours.

The engines installed on N908DE were Pratt & Whitney JT8D-219 turbofans. The JT8D-219 is a dual-spool, medium-bypass, fully-ducted, axial-flow turbofan engine with a single-stage fan, six-stage low pressure compressor, seven-stage high pressure compressor, nine-chamber can-annular combustor, single-stage high pressure turbine, three-stage low pressure turbine, and a mixer. The JT8D-219 engine has a normal takeoff rating of 21,000 pounds and a maximum takeoff rating of 21,700 pounds, both of which are flat-rated to 84°F.

The left engine was SN 725545. According to Delta Air Lines' maintenance records, the left engine had accumulated 55,673 hours and 41,849 cycles since new and 4,470 hours and 3,068 cycles since the last shop visit on January 25, 2011. Following that last shop visit, which was to replace life limited parts, the engine was installed on the airplane at John F. Kennedy International Airport, Jamaica, New York on January 26, 2011.

Communications

There were no reported communications issues between the pilots and air traffic control or with the fire department personnel.

Flight Recorders

The airplane was equipped with a cockpit voice recorder (CVR) and flight data recorder (FDR). Both the CVR and FDR were removed from the airplane and sent to the NTSB Flight Recorder Laboratory for playback and readout, respectively.

The CVR was an L3 Fairchild recorder model number FA-2100-1020 SN 814 that recorded 2 hours of cockpit audio. The playback of the CVR provided a recording that was of good quality that captured the entire flight from taxiing out to the runway, takeoff, flight including the event, return to ATL, landing, and taxi back to the gate. The playback of the CVR revealed the pilots responded correctly and appropriately to the inflight loss of power and there were no issues with the pilots' performance or actions. A CVR group was not formed.

The FDR was a Lockheed Aeronautical Systems Model 209 that recorded 25 hours of data. The FDR recorded the entire incident flight. The FDR data shows that from takeoff to about 11 minutes into the flight, the left and right engines performance parameters: EPR, low pressure rotor speed (N1), high pressure rotor speed (N2), fuel flow, and exhaust gas temperature (EGT); were evenly matched. At about 11 minutes after takeoff at about FL 240, the left engine's EPR dropped from about 2.0 to 0.8, the N1 dropped from about 91% to about 32%, the N2 dropped from about 94% to 61%, and the EGT increased from about 500°C to about 560°C before decreasing to about 260°C. The left engine's EPR, N1, N2, and EGT indications remained at those levels until about 34 minutes after takeoff when the N1 dropped to about 16%, N2 dropped to about 24%, EGT slowly decreased to about, and the EPR increased to about 0.9. The FDR data shows that when the airplane landed, the N1 and N2 indications decreased about 10% and 15%, respectively, the EGT had decreased to 18°C and the EPR had increased to 1.0.

Fire

There was no fire.

Tests and Research

The engine was disassembled at Delta's Technical Operations Center, Atlanta, Georgia in the presence of investigators. The examination of the engine revealed that the lower combustor fan duct was ruptured at the bottom coincident with the hole in the aft lower cowling. The disassembly of the engine revealed that the turbine case rear flange and turbine exhaust case front flange were separated at the bottom adjacent to where the lower combustor fan duct was ruptured. There were no holes through either the turbine case or the turbine exhaust case. However, the flanges at the bottom of both cases were distorted and the bolts were missing from the bolt holes. A number of broken flange bolts were recovered from inside the fan duct as well as from inside the engine cowling.

The disassembly of the engine revealed that all of the 4th stage turbine blades were fractured transversely across the airfoil adjacent to the blade root platform. The examination of the fracture surfaces on the 4th stage turbine blades revealed that there was one blade that had an elliptical-shaped, blue-colored, smooth-textured area on the suction (convex) side at the maximum thickness part of the airfoil. The remainder of the fracture surface on that blade as well as the fracture surfaces on all of the other 4th stage turbine blades were coarse and grainy.

All of the 4th stage turbine blades were examined at the NTSB's Materials Laboratory. The examination revealed that the blade with the elliptical-shaped, blue-colored, smooth textured area on the fracture surface had separated due to a high cycle fatigue (HCF) fracture. The examination of all of the other blades revealed that they had separated due to overstress.

The JT8D-219 4th stage turbine blades are the subject of an airworthiness directive (AD)-mandated recurring inspection for shroud notch wear. As the blades' shroud notches wear from normal engine operation, the vibratory stresses in the blades increase. Eventually, the stresses will increase to the point that a HCF crack will initiate and propagate to critical length resulting in a blade separation subsequently causing turbine damage and a loss of engine power. This incident occurred before the initial inspection of those 4th stage turbine blades was required by the AD.

The 4th turbine blade that had the HCF fracture as well as all of the other 4th stage turbine blades in the engine had the shroud notches repaired in Delta's engine maintenance facility at Atlanta and installed in the engine at the time of the last overhaul. Delta reported that they had recently experienced a number of JT8D-219 4th stage turbine blade fractures, most that occurred prior to the AD-required inspection, and mostly on blades that they had repaired in their shop. A special dimensional inspection was developed to measure JT8D-219 4th stage turbine shroud notches. However, Delta reported that they had discontinued repairing the blades in their shop in favor of using blades that were repaired at a turbine blade repair facility. As a result, they no longer had any of 4th stage turbine blades that they had repaired.

The disassembly of the engine revealed the flange joint between the turbine case and the turbine exhaust case at the bottom of the engine had separated and that the flange bolts were found broken. Because of previous JT8D-219 uncontained turbine blade fractures that occurred due to separation of the flanges between the turbine and turbine exhaust cases, P&W had tried various schemes to keep the flanges together in the event of a turbine blade fracture. The latest configuration that was released in Alert Service Bulletin (ASB) A6494 introduced longer high strength steel bolts along with crushable energy absorbing spacers to prevent the flanges from separating. The engine had been overhauled prior to the release of ASB A6494 and did not incorporate the longer steel bolts and crushable spacers.

NTSB 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.