Northrop Corp. said yesterday that the crash of one of its three F20 fighter planes last October was due to pilot error, not any fault in the plane.

The jet crashed during a demonstration flight in South Korea on Oct. 10, killing its pilot and only occupant, Darrell E. Cornell, Northrop's chief test pilot. The crash was viewed by industry officials as a setback to Northrop's efforts, unsuccessful so far, to find a customer for the F20.

To demonstrate its faith in the plane, Northrop began construction of a fourth F20 soon after the crash. A statement released by the company yesterday said a three-month investigation showed that "no technical or design changes to the aircraft are required."

Northrop said the Air Force participated in the investigation and agreed that it "reflects logically derived conclusions." An Air Force spokesman said he could not find officials who could confirm that yesterday, a federal holiday here.

The company did not explain why it believes that the pilot erred but said that all aircraft systems "functioned properly" and did not contribute to the plane's fatal stall.

"At the end of a demonstration flight and in preparation for landing, the pilot began a climbing roll and stopped the roll while inverted at low speed with the landing gear and flaps extended," the statement said. "The aircraft was at an altitude of 1,200 to 1,800 feet above the ground, insufficient to recover from the stall."

Cornell, 52, was "widely considered one of the world's finest test pilots," according to a Northrop statement released in November. "He was the best of the best," Thomas V. Jones, Northrop chairman and chief executive officer, said then.

The F20 was designed for export to developing countries, although Northrop recently tried unsuccessfully to sell the plane to the Navy for use as an "aggressor" aircraft in training exercises. It has been praised as a capable and maintainable plane but is thought to suffer in the export market because the U.S. military does not use it.