An aircraft that crashed in flames in Kentucky in November had a structural flaw that had been identified by Boeing on similar planes 15 years ago, according to investigators.

The MD-11F freighter operated by UPS, crashed after one of its engines separated from the wing as it was preparing to take off from Louisville.

The plane briefly lifted off from the runway, before hurtling out of control into an industrial area. Fifteen people were killed including three crew and 12 on the ground.

In an update, the US National Transportation Safety Board (NTSB) revealed that cracks found in the engine mounting assembly had previously occurred on several other aircraft.

At the time the manufacturer responsible for the aircraft, Boeing, concluded that the issue “would not result in a safety of flight condition”.

The MD-11 is a relatively elderly design that was originally produced by McDonnell Douglas. Boeing acquired the company in 1997.

The last MD-11 came off the production line in 2001, but Boeing has continued providing parts and service support.

In the aftermath of the Kentucky disaster, the NTSB issued a preliminary report which drew attention to cracks in the engine attachment mechanism. Its latest update goes further, describing fractures due to “fatigue” – or repeated stresses – in a critical bearing, as well as the mounting it is meant to sit in.

It points out that Boeing had previously found failures of the same part on four occasions, affecting three different aircraft. In 2011, the company sent a “service letter” to operators warning them of its findings. This is a non legally-binding document used to alert operators about important safety or maintenance information.

In this case, Boeing recommended that the part be included in a general visual inspection every five years. It also pointed out changes to the inspection procedure contained in the aircraft maintenance manual, and drew attention to a revised bearing assembly that could be fitted – although this was not mandatory.

Tim Atkinson, a former air accident investigator who now works as an aviation safety consultant, said the NTSB’s update made disturbing reading.

“The structure concerned is not decorative, it’s an essential part of the mechanism that attaches the engine to the wing, and carries loads such as thrust and drag,” he explained.

“It’s extraordinary that Boeing concluded that a failure of this part would not have safety consequences.”

Boeing’s internal processes have come under fire on a number of occasions in recent years.

Criticisms have focused on how the design of its 737 Max included flawed software that was implicated in two accidents, in 2018 and 2019, that together cost 346 lives.

Quality controls in its factories have also come under scrutiny, after a door panel fell off a brand new 737 Max shortly after take-off in early 2024.

In a statement, Boeing said: “We continue to support the investigation led by the NTSB. Our deepest condolences go out to the families who lost loved ones and our thoughts remain with all those affected.”

The NTSB’s investigation is continuing. It has not yet issued any firm conclusions about the cause of the accident, and is unlikely to do so until it publishes its final report.