In his memo to the staff, Mr. Early noted a "rash" of controller errors since the first of the year.

Nationally, controller errors are on the decline, falling to 1,047 in 1988 from 1,200 in 1987.

The safety board report, noting the increase in controller errors, said: "This increase should serve as a warning or a precursor to a potential accident."

These included at least four collisions and, most recently, two close calls caused by controller errors.

But current equipment usually requires that trainees practice near an experienced controller, who assumes the role of a pilot and watches for controller errors.

The concerns have been generated by months of increasing reports of near misses, controller errors, and pilot mistakes.

A fourth controller error, also involving two airliners, occurred Tuesday evening.

The letter preceded the board's announcement June 8 of an investigation into a sharp rise in controller error at the Chicago airport.

The others crashed because of mechanical failures, weather or controller error.

The safety board investigated several controller errors last year and issued a string of highly critical reports.