Investigators say inspections of a fan blade that broke off during a United Airlines flight in 2018, triggering an engine failure and emergency landing, had failed to spot signs that the blade was weak.

The National Transportation Safety Board says engine maker Pratt & Whitney did not provide a formal program for training its inspectors who examined the fan blades.

The safety board issued its conclusions in a report Tuesday.

The incident happened as the Boeing 777 from San Francisco cruised toward Honolulu. The crew heard a loud bang and felt the plane shake. The pilots were able to land safely, and none of the 374 passengers and crew were injured, although parts of the engine housing were blown off, according to the NTSB.

The safety board said the blade that snapped off had shown signs of metal wear and tear in previous examinations in 2010 and 2015, but Pratt & Whitney inspectors believed it was just a paint imperfection.

Investigators said Pratt & Whitney had developed a new inspection process in 2005, using thermal sensors to inspect fan blades on its PW4000 engines. Because the company classified the process as a new and emerging technology – and didn’t change that description over 13 years — it did not have to develop a formal training program for training and certifying inspectors.

In an emailed statement, Pratt & Whitney said they supported the NTSB investigation and have taken corrective actions in response to the event.

As a result of the United flight, the Federal Aviation Administration in March 2019 ordered inspections of fan blades on all PW4000 engines.

It is rare for a fan blade to break in flight, but there have been several such incidents in recent years. One, involving a CFM International engine on a Southwest Airlines Boeing 737, triggered an engine failure that sent shrapnel crashing into the plane, killing a passenger on a 2018 flight.