Incident Overview
On August 6, 2011, at approximately 1142 Eastern Daylight Time, a Delta Air Lines McDonnell Douglas DC9-51, registration N676MC, was conducting a takeoff roll at Hartsfield-Jackson International Airport (ATL) in Atlanta, Georgia. The aircraft was equipped with two Pratt & Whitney JT8D-17 turbofan engines. During the takeoff roll at about 80 knots, the flight crew heard a loud bang and felt the airplane shudder. The captain rejected the takeoff and brought the aircraft to a stop on the runway. Due to a brake lockup, the airplane could not taxi. Passengers were deplaned and transported to the terminal by bus. The flight was a scheduled passenger service under 14 Code of Federal Regulations Part 121, en route to Pittsburgh International Airport.
Injuries and Damage
No injuries were reported among passengers or crew. A visual inspection of the aircraft revealed a penetration in the No. 2 engine cowling and an impact mark on the outer pane of a window on the right side of the fuselage forward of the engine. One fan blade from the No. 2 engine was found separated below the platform, and three small holes were observed on the compressor case.
Engine Examination
The engine was removed and shipped to the Turbine Engine Center in Miami, Florida, for inspection and disassembly from September 20 to 21, 2011. Examination recovered the separated fan blade in three pieces: the blade root still in the fan hub blade slot and two airfoil sections found loose inside the engine. Damage was noted to the inlet guide vanes, remaining fan blades, fan exit guide vanes, and visible low-pressure compressor stages behind the fan. The separated blade pieces, seven additional damaged but intact fan blades, and the fuel oil cooler aft mounting bracket were sent to Pratt & Whitney's materials lab for analysis.
Metallurgical analysis determined that the separated fan blade failed due to a high cycle fatigue crack that originated approximately 0.9375 inch aft of the leading edge on the suction side of the blade. Replicas were made of six undamaged fan blades along the leading edge and superimposed over a new blade profile to measure leading edge erosion. Four of the six replicas were taken at blade chord positions that are repairable per the JT8D-17 repair manual. Measurements from these four replicas confirmed that all had material loss within allowable limits, but three of the four blades had chord loss greater than Pratt & Whitney's erosion estimate of 0.035 inches for engines that had accumulated 11,000 engine cycles. The erosion estimates were derived from field data on PW4000 series engines, which use the same fan blade material as the JT8D-17.
Maintenance Information
Fan blade fatigue fracture due to excessive leading edge erosion was addressed by Pratt & Whitney in Maintenance Advisory Notice MAN-JT8D-2-06, released on November 20, 2006. The advisory notice recommends fan blade refurbishment every 4,000 cycles to reduce fatigue fractures caused by vibratory stress from leading edge erosion. The Federal Aviation Administration subsequently issued Airworthiness Directive 2010-21-17 for all JT8D engines on November 12, 2010, mandating fan blade leading edge overhaul after 4,000 cycles since the last blade overhaul. The incident engine had been overhauled prior to the AD release, and blade refurbishment had not yet been performed in accordance with the MAN. Compliance with the MAN is at the discretion of the engine operator as it is supplemental information not covered in engine manuals.