Citing its accident investigation board, the Army said the lead scout helicopter in October's accident over southwest Baghdad slowed and made an unexpected 360-degree right turn during the period when crews aboard both OH-58D Kiowa helicopters could not see each other.

The trailing helicopter, still flying roughly twice as fast, crashed 26 seconds later when its main rotor struck the other's tail, the Army said. Both wrecked helicopters fell into a farm field.

Investigators said the only radio signal between the helicopter crews was a single command, "Buffalo," minutes before the crash. That was the unit's code word to slow and begin flying the reconnaissance assignment. The trailing helicopter's crew replied, "Roger, Buffalo."

But in the ensuing minutes, neither crew radioed the other.

"When a trail aircraft loses sight of lead, an immediate radio call must be made," said a preliminary report published yesterday by the U.S. Army Safety Center in Fort Rucker, Ala.

"Likewise, when the lead aircraft makes turns that are not standard during a mission, the turn direction should be called back to trail," the report said.

The Army said its accident board recommended that military flight schools teach new procedures to pilots about warning other aircraft when they lose sight of each other during missions.

The crash on Oct. 16 killed two pilots based in Hawai'i — Chief Warrant Officer William I. Brennan, 36, of Bethlehem, Conn., and Capt. Christopher B. Johnson, 29, of Excelsior Springs, Mo. Both were aboard the trailing helicopter, assigned to the 1st Battalion, 25th Aviation Regiment at Wheeler Army Air Field.