A slack cable triggered a chain of events that ended with a fatal helicopter crash during rescue exercises off Hawaii last year, the Coast Guard said Friday.
All four crew members on board were killed.
The accident occurred while the helicopter practiced lifting a rescue basket from a Coast Guard lifeboat about five miles south of Oahu.
The report said the hoist cable went slack due to the motion of the hovering helicopter and the small boat in the waves.
The extra slack formed a loop that became snagged on a protruding pipe on the boat just as its stern fell in the trough of a wave.
The cable snapped, causing the helicopter to abruptly lurch left, damaging the aircraft's rotor system.
The report said the helicopter began issuing distress calls as it climbed to an altitude of about 500 feet and vibrated abnormally. It dropped out of the sky and sank in water about 1,300 feet deep about four minutes after the cable went slack.
Coast Guard Commandant Adm. Thad W. Allen said the accident was a reminder that Guardsmen operate in extremely hazardous environments.
"Coast Guard men and women go into harm's way to train and conduct operations each day, and we must diligently direct our energies to performing operations safely and effectively," Allen wrote in a memo on the accident investigation.
His memo noted the pipe on the life boat _ a dewatering standpipe _ can be a snag hazard during helicopter hoist operations.
He said the Coast Guard has already adopted steps to reduce the risk of hoist operations, including revising checklists and procedures, updating flight manuals and using shrouds to limit life boat snag hazards.
Allen further ordered his chief of staff to alter the engineering and design of the boats to the extent practical and to train personnel to limit snag hazards.
The crew members killed were co-pilots Cmdr. Thomas Nelson and Lt. Cmdr. Andrew Wischmeier, rescue swimmer Petty Officer 1st Class David Skimin and flight mechanic Petty Officer 2nd Class Joshua Nichols.
Nelson was posthumously promoted to captain while Nichols was posthumously promoted to Petty Officer 1st Class.
The crash was the first Coast Guard helicopter accident in Hawaii since January 1982, when an HH-52 Sea Guard crashed into a cliff during a rescue mission in bad weather. Three people were killed.