A plane crash last year in which Washington real estate developer Theodore R. Hagans II died was caused chiefly by Hagans' errors in piloting the twin-engine aircraft, the National Transportation Safety Board said yesterday.

The federal safety agency, concluding a one-year investigation, found that the plane's engines stopped after Hagans failed to activate fuel pumps required under federally approved operating procedures. The error was cited among "probable causes" of the accident.

Hagans, who was 58, was killed April 28, 1984, when the Piper Aerostar he was piloting crashed in a grassy field near Cockeysville, Md., shortly after taking off from Lancaster, Pa. His son, Theodore R. Hagans III, 32, a passenger in the plane, also died.

Hagans was one of the Washington area's most prominent businessmen and a political ally of District Mayor Marion Barry. His development projects included the Fort Lincoln "new town" in Northeast Washington.

His former wife, Delores Hagans, recently sued Hagans' estate, seeking $10 million for the "wrongful death" of their son. In the lawsuit, she alleged that Hagans "did not have the competence to fly" the plane and was "negligent in failing to familiarize himself with emergency procedures."

In its report, the safety board said that Hagans lacked sufficient experience in flying the recently purchased Piper. About a month before the accident, a Piper instructor at a training center in Florida had declined to issue a certificate to Hagans and recommended more flight practice, it said.

According to the safety panel, a report on Hagans' performance by the Piper instructor "listed deficiencies," including "inability to remember procedures, lack of understanding of the procedures, poor performance of flight maneuvers, and slow reaction time."

The panel found that the engines cut out because of lack of fuel as the plane was climbing at an altitude of nearly 17,000 feet. Pumps that boost fuel pressure had not been turned on, it added. According to a federally approved flight manual, the pumps must be activated during ascents above 10,000 feet.

In addition, the fuel mixture was found to have been set "on rich," the report said. "The engines cannot be restarted if the mixture is rich," it added. After the engines stopped, investigators found, the plane went into an uncontrolled dive.

The panel found that rivets were missing from an outer section of a wing, and it urged the Federal Aviation Administration to require inspections of similar planes to ensure that rivets are attached. The missing rivets were not cited as a cause of the crash. The FAA will review the issue, a spokesman said.