WASHINGTON (AP) _ Errors by air traffic controllers were to blame for the collision last May of a skywriting plane and another small aircraft near Orlando, Fla., in which all four people aboard the two planes were killed, a federal investigation concluded Tuesday.

The National Transportation Safety Board in its report on the accident said that the collision occurred as a result of a mixup when one of the aircraft was being handed off from one controller to the other.

The safety board also cited as contributing factors in the accident the failure of air traffic control radar to continually track the two aircraft prior to the collision and the lack of traffic advisories to the pilots.

The skywriting plane, which was owned by Rosie O'Grady's of Orlando Inc., was returning after a flight over Disney World last May 1 and about to make an approach to the Orlando Executive Airport when it collided with the twin- engine Cessna at about 3,000 feet.

The Cessna, which was operated by Midwest Packaging Materials Co., of Fort Madison, Iowa, was about to land at the Orlando International Airport after a flight from Iowa.

Shortly before the accident, a controller at the Orlando airport tower attempted verbally to turn the single engine skywriting plane over to another controller, but found the second controller too busy to take the handoff, according to investigators.

So the controller directed the pilot of the skywriting plane to descend to 4,000 feet, turn right and contact the other controller. When the pilot came up on the second controller's frequency, he was told to descend to 1,500 feet and fly north directly to the executive airport

But the second controller was not aware of the fact the skywriting pilot was turning his aircraft as he had been directed to do and was suddenly on a collision course with the Cessna, investigators said.

A glitch in the radar computer kept the two planes from being continually tracked for about 46 seconds prior to the collision and added to the controller's confusion, investigators said.

The NTSB report said that as the planes converged their radar signal was blocked out causing the air traffic control computer to project a targets that were no longer current.

In its probable cause, the NTSB concluded that the collision occurred because of the failure of the first controller ''to coordinate the handoff of traffic'' and the failure of the second controller ''to maintain radar target identification.''

''Contributing to the accident were the limited capabilities of the radar system to continually track the targets in close proximity to one another and the lack of traffic avisories,'' the safety board said.

Investigators said that the controller who first handled the skywriting plane warned the pilot of a Boeing 727 jetliner in the vincinity, but gave no advisory about the Cessna which also was on his radar screen.