CHI95IA142

The flightcrew reported that during the application of takeoff power, when the throttles were in approximately the vertical position, the numbers 1 and 3 engines were indicating approximately 70% N1. The number 2 engine was indicating about 48% N1 and not accelerating. They heard a 'crack' and aborted the takeoff. Examination of the engine revealed that the low pressure turbine (LPT) stage one disk had separated from the LPT rotor at the stage one to two flange joint. Seven intact toroid bolts and sets of associated hardware were recovered. No evidence of fatigue cracking or corrosion (pitting) was noted on any of the bolt pieces recovered from the engine. The shank of one of the toroid bolts exhibited significant damage. The lug arm, used for safetying the bolt, exhibited relatively minor damage. Examination of several of the lug arms revealed 'noticeable wear patterns' and 'crack arrest positions indicative of fatigue cracking.' Statements by UAL maintenance personnel indicate failures of the toroid bolt safeties are occasionally discovered when the engines are disassembled for major overhaul.

On May 1, 1995, about 1120 central standard time, a McDonnell Douglas DC10-10, N1811U, operated by United Airlines (UAL) as United Airlines Flight 492, sustained minor damage following the uncontained failure of the number two engine during takeoff roll.

The flight crew aborted the takeoff, returned to the gate, and deplaned the passengers without further incident. There were no injuries to the crew of 12, or the 180 passengers aboard the airplane. The scheduled domestic passenger flight, operated under 14 CFR Part 121, originated at the Chicago O'Hare International Airport with a planned destination of the Washington Dulles International Airport, Washington, D. C. An IFR flight plan was filed, and visual meteorological conditions prevailed in Chicago.

The flight crew reported that during the application of takeoff power, when the throttles were in approximately the vertical position, the number one and three engines were indicating approximately 70 percent N1. The number two engine was indicating about 48 percent N1 and not accelerating. They heard a "crack". They aborted the takeoff, pulled the number two engine fire handle and discharged the fire bottle.

The airplane was transported to the UAL maintenance hanger at O'Hare International airport where it was subsequently examined. Fragments of the turbine blades, shroud material, bolts, and the engine case were recovered from the runway, and from within the engine cowl and the engine. A gap was located in the thrust reverser latch seam which spanned approximately three inches at the trailing edge and .25 inches at the leading edge. The left cowling had a six inch by 21 inch puncture at the ten o'clock position and the right side cowling had a four inch by eighteen inch puncture at the four o'clock position.

The engine was removed from the airframe revealing a circumferential penetration of the engine case, about one inch wide, extending around about 270 degrees of the circumference from the three o'clock position to the twelve o'clock position, in the vicinity of the first low pressure turbine. One oil line on the left side of the engine was fractured and the upper surface of the left stabilizer was oil covered.

The engine was transported to the UAL engine overhaul facility, San Francisco, California, where it was examined in the presence of the Powerplants Group Chairman. In his factual report of the examination, he reported "the LPT stage one disk was separated from the LPT rotor at the stage one to two flange joint. The bore, web and rim portion of the disk were intact...Several stage one to two disk flange bolts were recovered in the debris. The fracture surfaces on the majority of the recovered bolt fragments were smoothed and rounded. Six stage one to stage two flange bolts were fractured in the shank, adjacent to the threads, in which two bolts had coarse looking fracture surfaces with varying degrees of discoloration."

The LPT components were examined by a National Resource Specialist at the NTSB laboratory, Washington, D. C. In his factual report he stated that "the internal surfaces of the engine hardware in the low pressure turbine section contained multiple small-object impact marks". Seven intact toroid bolts and sets of associated hardware were recovered. X-ray energy dispersive spectroscopy (EDS) examination of the discolored areas of the bolts revealed substantial peaks for oxygen, aluminum, sulfur, and silver. "No evidence of fatigue cracking or corrosion (pitting) was noted on any of the bolt pieces recovered from the engine." A portion of the missing bolt, identified as an unthreaded portion of the shank, designated as bolt number six, was located with the fragments recovered from within the engine. The shank of the bolt exhibited significant damage. The lug arm, used for safetying the number six bolt, exhibited relatively minor damage. Examination of several of the lug arms revealed "noticeable wear patterns" and "crack arrest positions indicative of fatigue cracking."

Statements by UAL maintenance personnel indicate failures of the toroid bolt safeties are occasionally discovered when the engines are disassembled for major overhaul.