Metal fatigue probably caused a left rear landing skid strut to collapse on a helicopter operated by K&S Helicopters Inc. at the Turtle Bay Resort Heliport in 2009, according to a National Transportation Safety Board investigation.

None of the four passengers and pilot was injured in the accident on Nov. 20, 2009, but the McDonnell Douglas 369E helicopter sustained substantial damage to its tail skid and stabilizer when the strut broke as the helicopter prepared for ascent, tilting the helicopter to its left rear.

The NTSB said the struts had been checked 13 days before the accident as part of an 100-hour inspection.

During the investigation, the board found the left strut fractured at the bolt hole where it was attached to a fuselage drag brace.

The bolt holes on the broken strut and right strut were larger than the maximum allowed in the maintenance manual, the NTSB said in a report Thursday.

The NTSB said the manufacturer’s manual required removal of any strut exhibiting looseness.