The plane that crashed into flames in Kentucky in November had a structural flaw that Boeing had identified in similar planes 15 years ago, according to investigators.
An MD-11F cargo plane operated by UPS crashed when one of its engines separated from the wing as it was preparing to take off from Louisville.
The plane briefly skidded off the runway before going out of control in an industrial area. Fifteen people died as a result, including three crew and 12 people on the ground.
In an updated report, the US National Transportation Safety Board (NTSB) revealed that the cracks found in the engine mounting assembly had previously occurred in several other aircraft.
The manufacturer responsible for the aircraft at the time, Boeing, concluded that the issue would not result in “safe flight conditions”.
The MD-11 is a relatively old 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 continues to provide parts and service support.
Following the Kentucky disaster, the NTSB issued a preliminary report that drew attention to cracks in the engine attachment mechanism. Its latest update goes further, describing a fracture caused by evidence of “fatigue” – or repeated stress – in a key bearing, as well as the mountings it sits on.
It said Boeing had previously found defects in the same part on four occasions, affecting three different aircraft. In 2011, the company sent a “service letter” to operators warning them about its findings. This is a non-legally-binding document used to alert operators to important safety or maintenance information.
In this case, Boeing recommended that the part be included in a routine visual inspection every five years. It also pointed to changes to the inspection procedure contained in the aircraft maintenance manual, and drew attention to a modified bearing assembly that could be fitted – although it was not mandatory.
Tim Atkinson, a former air crash investigator who now works as an aviation safety consultant, said the NTSB’s update makes the study troubling.
“The associated structure is not decorative, it is an essential part of the mechanism that connects the engine to the wing, and carries loads such as thrust and drag,” he explained.
He claimed, “It is extraordinary that Boeing concluded that the failure of this part would have no safety consequences.”
Boeing’s internal processes have come under criticism on several occasions in recent years.
Criticism has focused on how the design of its 737 Max included flawed software that was implicated in two crashes in 2018 and 2019 that killed a total of 346 people.
Quality control at its factories has also come under scrutiny after a door panel on a new 737 Max fell off shortly after takeoff in early 2024.
“We will continue to support the investigation led by the NTSB. Our deepest sympathies go out to the families who lost loved ones, and our thoughts are with all those affected,” Boeing said in a statement.
The NTSB investigation continues. It has not yet issued any firm findings about the cause of the accident, and is unlikely to do so until it publishes its final report.