On April 22, 2021, a catastrophic engine failure on a 460-foot-long passenger and car ferry near Bainbridge Island in Puget Sound started a fire that caused more than $3.8 million in damages, according to the National Transportation Safety Board.

The NTSB recently concluded its investigation, which tied the fire to an improperly tightened nut.

During a post-maintenance sea trial, the connecting rod assembly in the No. 3 main engine failed and ejected components that breached the crankcase and ignited hot pressurized gasses.

The fire occurred during a sea trial in Puget Sound after the fastener failure led to components from the engine being ejected and causing a fire in the engine room. None of the 13 crew members were injured.

The NTSB found that the probable cause of the mechanical failure was the connecting rod assembly that came loose due to insufficient torque on a lower basket bolt during the previous engine overhaul. The NTSB stressed that maintenance technicians should use a calibrated torque wrench in compliance with the manufacturer’s recommendations and verify that all required torque requirements had been completed.

This is not the first time that improperly torqued fasteners have caused problems. A fire aboard the Carnival Liberty cruise ship in 2015, a fire on the Nenita bulk carrier in 2016 and an engine failure on the Red Dawn offshore supply vessel in 2017 were all traced back to bad fastening, according to the NTSB.