An Iowa farmer harvesting corn has found key jet engine pieces that are expected to solve the mystery of why United Airlines Flight 232 crashed in Sioux City in July, investigators said yesterday. "This is the most significant development to date for us in trying to piece together what happened," said Karen Purdy, a spokeswoman for General Electric, which made the DC-10 engine that exploded. GE offered $279,100 in rewards to spur the search for the pieces. "I knew immediately that's what they were looking for," said Janice Sorenson, who found the bent and broken pieces partially buried in the ground and shadowed by 10-foot cornstalks as she was driving across her field Tuesday afternoon. "I felt a resistance against the combine. I put it in reverse and I could see some of the aluminum blades protruding from the ground," said Sorenson, who lives on a 460-acre farm about 11 miles northwest of Alta, Iowa. "I was just shocked to find it." The crash occurred after the plane's tail-mounted engine exploded 37,000 feet above northwest Iowa, spewing shrapnel that severed the jumbo jet's control lines. The pilots struggled successfully to guide the crippled plane on a dramatic 42-minute spiral to the Sioux City airport, where it cartwheeled and then burst into flames, killing 112 people; 184 survived. Missing in the wreckage were many parts of the tail engine, including the critical pieces that should indicate why it exploded. Investigators believe the engine blew apart because of a rupture in a 300-pound titanium fan disk that spins 38 blades. But without that fan assembly, they have been unable to determine why the disk broke. In August, GE offered rewards to induce farmers to look for the pieces, including a top prize of $50,000 for the disk. But farmers said then that the pieces probably would not appear until the harvest. Sorenson found about two-thirds of the disk, attached to about a half-dozen fan blades. GE offered $1,000 each for 38 missing fan blade stubs, the ends of the blades attached to the disk, and $1,000 each for certain central fragments of the 38 blades. Sorenson, in a telephone interview yesterday, said she did not know how much money she will collect. "We're just very, very happy for GE . . . . I'm certain this will help solve the puzzle for them so there will be no more tragedies like this one. "It's exciting for us, but then you think of all those families that lost loved ones, and it's saddening," she said. The pieces were shipped yesterday to GE's engine headquarters in Evendale, Ohio, where they were to be examined by company and federal investigators. A pie-shaped piece of the disk was not found, but examination of the recovered part should reveal what caused the break, stress, fatigue or a flaw in the metal, investigators said. "Now that we have a substantial part of that disk, we hope that metallurgical analysis will tell us what caused that failure," National Transportation Safety Board spokesman Ted Lopatkiewicz said. When the disk broke, it apparently shot the fan blades like knives through the control lines, which drained the jet of the hydraulic fluid needed to manipulate the rudder, flaps, slats and other tail and wing surfaces. The pilots managed to steer by altering the throttle speeds on the jet's two wing-mounted engines. By accelerating both engines, the pilots were able to keep the nose up. By accelerating one engine and slowing the other, they were able to make crude turns, but to the right. The safety board this week released a radar track that traces the last 48 minutes of the plane's harrowing path. The track starts six minutes before the engine exploded, roughly over the Sorenson's farm northwest of Alta. The path shows how the jet circled to the right as it descended, and managed one left turn before the track stops just short of the airport.