Flight 232: A Story of Disaster and Survival (45 page)

“The most likely thing is that it has split from a fatigue crack,” Cherolis said. “The old guy in the group gets out his pictures and says, ‘See, here’s the history,’ and he had a cross section of an engine with little circles showing past failures of the same sort.” As a metallurgist in the field of failure analysis at a company that made turbine engines, Cherolis knew that he would eventually be called upon to analyze titanium that had been tainted. It was a matter of letting the wheels spin and being patient. Cherolis also knew that disk 00385 could have been damaged in handling or that the stresses induced in the disk during manufacture might not have been properly accounted for. “But by that time this engine series was well wrung out,” he said, “and that possibility was pretty slim. There were still problems in the turbine [of the CF6-6], but the fan hadn’t had any stress problems.”

Cherolis first looked at the larger piece of the disk on Thursday morning, October 11. He said, “You could see it [the crack] had come from the bore area. You can read a fracture backwards to where it starts. And it doesn’t look like a normal fracture that’s purely fatigue. There’s a depression of missing material at the origin that a layman would call a pit.” The crack had begun in that pit, the chipped-out cavity that Wizniak had called the origin. Cherolis expected to see smooth, continuous machined metal on the inside surface of the bore. He saw instead that tiny hole where a piece of material had fallen out. To Cherolis’s eye, it was evidence as damning as a bloody thumbprint at a murder scene.

Once the disk was cleaned, Cherolis and his team began taking photographs of that surface and the crack that grew from it, all the way to the rim of the disk. First he mounted a Polaroid camera on a tripod to take overall photos. Then he used a device called a macroscope to take photographs at five to ten times magnification. The team established X and Y coordinates on the fractured metal so that each photograph could be matched with others to orient all the features in space. This also allowed them to determine the direction in which the fracture was propagating and how fast it had been growing.

Wildey, leading the team, could see how the cracked surface changed as it moved out from the bore. The crack clearly began as a fatigue crack, but then it abruptly changed to what Wildey called “single load,” something that was ripped apart by brute force, or in the language of metallurgy, “with a single application of load.” The fatigue area nearer the bore looked completely different as the result of repeated cracking by small increments each time a load was put on the part. The word
load
means pull or force. The pull comes from centrifugal force, the force that results from spinning. Usually such a fatigue crack would let go on takeoff because that’s when the load is greatest. (And this is what the engineers hope for, since the pilot can usually abort the takeoff.)

Above that fatigue crack, along the edge of the single-load cracking, Wildey could see a shear lip, a flap of metal that droops off of one side at about a 45-degree angle when metal is torn apart. He said that when the piece pulled apart on the day of the crash, it was “like Thor smiting it with his hammer. This is a big deal. This is the Big Bang that everybody hears when this thing breaks. This is the clap of thunder” heard by farmers on the ground. “You’re at a stress level that’s exceedingly high and most often you’re just ripping the part apart and it’s all done. But because we don’t have that shear lip down there in that fracture region, it says that there’s some kind of brittle fracture mechanism that’s occurring down there. An experienced fractographer would be drawn to that area without a shear lip instantaneously.” And he would say, “There is a lack of deformation going on here that indicates for sure that there is a brittle fracture mechanism.” Wildey knew. Cherolis knew. Now the team had to prove it.

As the day wore on, Cherolis viewed the small pit inside the bore at higher and higher magnifications, taking photos at each step. “And you compare the surface finish away from it to the surface finish in it,” he said. On the healthy metal surrounding the pit, he could see the dimpled appearance that resulted from shot peening. That process gives a texture to the titanium that is almost like skin.
It compresses the metal near the surface
and thereby introduces stresses that help prevent cracks. But down in the pit, at higher magnifications, he could see two things of great interest. One was evidence of shot peening inside the pit. That meant that before the disk was completely finished, a bit of material had already fallen out to create the pit. Moreover, inside the pit he could see that the shot peening had fractured the metal further, which told him that the material was, indeed, brittle in there. “It looked like little pieces were missing out of that,” Cherolis said. “Well, it doesn’t do that on nice ductile material.” Healthy titanium alloy should not fracture under the force of shot peening. So the material in the pit had not behaved the way titanium should. Titanium should stretch under the load of spinning. The material in the pit, almost ceramic in its consistency, would not stretch. “The GE guys really don’t like to admit that there could be micro-cracks within the [pit],” said Cherolis.

United Airlines, for its part, denied that the cavity existed in a detectable form before the first flight. In its official report United said, “
At all times prior to the inflight event
, the cavity was filled with metal and was not visually detectable.” In fact, the major parties to the event wrote their own reports detailing what they wanted the NTSB to conclude. General Electric went so far as to print its report in the exact same format as the official NTSB report, including the same typefaces, so that the GE report could easily be mistaken for a genuine NTSB report.

In terms of whether or not the defect was detectable, Cherolis said that during the machining process, tools would cut metal from every surface to achieve the final shape of the disk, and when a tool hit the brittle material in the pit, “it was like hitting a rock in peanut butter.” His conclusion: “In my mind it was probably already micro-cracked” before it was ever installed on an engine. Tellingly, electron micrographs would clearly show cracks in the micro-structure, “and not all in the direction that the stress [of spinning] would make them.” There are “extra little cracks in random directions.” In a sense, it makes no difference who’s wrong and who’s right. If the defect hadn’t begun to crack during manufacture, it would have cracked the first time the engine was spun up to speed.

In its final report, the NTSB put it this way:

The Safety Board believes that at the time of manufacture
of the disk, the cavity at the fatigue origin point was originally filled, or nearly filled . . . making the defect more difficult to detect. . . . The cavity was most likely created during the final machining and/or shot peening process and . . . the shot peening probably created the microcracking parallel to and just below the cavity surface. Moreover, the shot peening quite likely created the mechanical deformation on portions of the cavity bottom.

After that, every time an engine bearing fan disk 00385 was started, the crack grew a bit more, elongating outward from the bore toward the rim and the dovetail slot where the number 10 blade was attached. When that fatigue crack had grown to about one inch long and half an inch deep, the disk was ready to let go on the next flight. That flight took place July 19, 1989. You can see it in the photographs: the fatigue crack is flat where the metal broke as a crack through a broken plate of glass would be. But when the disk let go, the crack propagated and tore through healthy titanium as if through living flesh, leaving ragged edges. The fracture traveled through the metal at the speed of sound.

“It’s trying to fly apart all the time,” Cherolis said, “and the whole art of making a jet engine is keeping it all together and figuring out how long your parts will last. And you actually retire them with no cracks in them and throw them away, because statistically one could start cracking some time in the next several hundred cycles. And without a defect, that all works wonderfully. But you put a defect in there, then the crack starts right away, and all your calculations are trying to avoid starting a crack. And here it’s already there.”

Wildey and Cherolis now had to answer the next question. They knew how the disk broke. It had something brittle in that little pit, something that cracked and fell out, destroying the integrity of the titanium matrix. So what was that something? By the time Cherolis finished his work, it was late at night. Someone at GE telephoned Floyd Brate at about midnight and woke him up, saying he’d better get to the lab now. He was an expert in making detailed replicas of cracks. Other technicians were called in too. It was time to find out what material lay inside the pit.

CHAPTER TWENTY-TWO

C
harles Martz, the ex-Navy fighter pilot
, sat in C-Zone next to eighty-year-old Luella Neubacher. She was telling him about her career as a senior Olympic runner. She was headed to Austria for a big race. In 1953 at the age of twenty-two, Martz had begun his career as a fighter pilot. He flew the F9F-8 Cougar off of the aircraft carrier USS
Bennington
in Navy Fighter Squadron 13. “That was the best kind of duty you can ever have,” he said. Martz was a real old-fashioned fighter jockey. He had a low, growling voice and a slow, measured way of saying things. When you’re in the cockpit of a swept-wing, jet-powered fighter plane, where unsuccessful landings on the pitching deck of a ship are usually fatal, you learn to move with deliberate precision. When he left the Navy, Martz continued to fly private planes. In fact, he should never have been on that DC-10 because he had a leased Cheyenne II, a Piper six-seat plane, in which he flew himself around the country looking at cable television systems for his company to buy. But it was the slow season for his business, and it seemed more economical on that trip to hop a commercial flight.

Martz was quite frank about saying that he was terrified. “I’d been the guy up front, and here I’m sitting in this aluminum tunnel and had no control whatsoever.” Neubacher had no idea what was going on and continued chattering away with her travel stories, while Martz stared at the dead control surfaces out the window and nodded dumbly, barely able to hear what she was saying. He listened to the changes in power, watched the tilting of the wings, and soon realized that the crew was steering the plane with the engines. Charlie Martz may have been the only person outside the cockpit who completely understood the situation. “There was no way to land this airplane without major damage.” He began saying his mental good-byes to his wife Janie and family and friends. He was fifty-eight years old and recognized that he’d had a good life. He was overcome by a sense of sorrow because he had left early that morning and had not kissed Janie good-bye. She’d been sound asleep. He hoped that she would know how much he loved her. As the plane descended through a thin layer of scattered clouds, Martz decided that he had better say something to the chatty Luella Neubacher to let her in on what she was about to experience. He turned to her and said, “I hate to tell you this, but I’ve been flying a long time, and I know this is going to be more than a hard landing. We’re going to crash.” Neubacher fell silent at last. She studied Martz for a moment and thought about what he’d said. Captain Haynes had already announced that the landing would be just as bad as could be imagined. Jan Brown was now giving her final briefing, instructing people on how to brace. Susan White and Jan Murray and the other flight attendants were cruising the aisles, giving last-minute advice. Rene Le Beau was walking backward through first class, trembling while holding up the seat-pocket card, living out the last moments of her life while bravely doing her duty. Brad Griffin and Peter Allen, Gerald and Joanne Dobson, were all watching her.

Luella Neubacher deliberately unlatched her seat belt, stood up, and took her old gray overcoat from the overhead bin. She said, “You never know when you’re going to have to walk home.” Georgeann del Castillo scurried over and told her to get back in her seat, and she did so in her own sweet time. Nobody was going to tell Luella Neubacher how to live the last minutes of her life.

“Suddenly I became very, very angry,” Martz recalled. “I had been in love with aviation since I was five years old and had always told myself that if I die in an aircraft crash, I want to go down in flames, sitting up front, fighting for my life. But here I was locked in this tin can with three hundred other people, unable to do a damn thing to help myself or anyone else.” He called it “outrage,” as he battled “the urge to scream.” A few days before the flight, he had been sitting on his patio in the sunshine, having breakfast with his daughter Gail and reflecting on why so many bad things had to happen to her mother Janie. She’d had three cesarean sections and three surgeries for cancer, and only the week before had discovered a lump in her left breast. She was scheduled for a biopsy in a few days. He told his daughter that he’d been flying airplanes for thirty-five years and never experienced anything more serious than a small hydraulic leak. As 1819 Uniform slewed back and forth, up and down, he reflected that the chickens had come home to roost “because there was no way to get this plane on the ground safely.”

When the announcement came over the loudspeaker that four minutes remained before touchdown, Charles Martz knew far too much for his own good. Then the two-minute warning came and the command to brace, and Martz said, “I was so angry by then with my frustration that I gave the seat belt another tug, held a pillow between my hands against the seat in front of me, turned my head to the right on the pillow, and watched out through the window.” If he was going to crash, he was by god going to witness it.

Working in the high-security laboratory at GE
, technicians took all the photographs at various magnifications that Wildey and his team thought they needed of fan disk 00385. Then they moved the larger of the two pieces of titanium to a power saw and cut the entire fracture from bore to rim as you might cut a piece out of a doughnut. “You essentially make a cut parallel to the entire fracture,” said Cherolis. They now had a long piece that was light enough for one person to carry. It contained the pit where the crack had begun, the progressive fatigue area, as well as the tear created when the disk let go.