UPS MD-11F crash: Ramifications of engine pylon bearing failures not fully realised

Investigative hearing looks to understand why multiple occurrences of fracture were not flagged as flight safety concern.

US investigators have been seeking to understand why a history of bearing failures on Boeing MD-11 pylons did not lead to sufficient corrective action before last November’s fatal UPS freighter crash during take-off from Louisville.

Fatigue in the bearing race housed between two pylon bulkhead lugs — which anchor the aft section of the engine pylon to the wing — could cause the race to split and migrate, leading to abnormal stress on the lugs.

The National Transportation Safety Board believes stress fracture of both lugs, resulting in aft pylon detachment from the wing, caused the UPS MD-11F’s left engine to pivot upwards and separate on rotation.

Multiple instances of similar bearing race failures on MD-11 pylons, including three on FedEx aircraft since June 2020, were detailed on 19 May during the opening day of the safety board’s hearing into the accident.

Three of the 26 UPS MD-11Fs inspected after the crash were also found to be suffering the same problem in one of their pylons.

“The concern I have is…we had three other [UPS] aircraft that potentially were in the same situation eventually,” said NTSB board member John DeLeeuw.

“We don’t want this to go and happen again. We want to prevent this.”

He pointed out that the prior discoveries of bearing race failure had been “signals to us…saying we might have a small problem here”.

“We had something here, we just didn’t do anything about it,” he said.

UPS senior director of engineering John Springer responded to DeLeeuw, stating: “I think the key is, the way our processes are set up, we’re relying on the data that’s out there — and what kind of determination is made on whether it was safety, or not safety.

“In this case it came through the service letter with explicit notation that it would not be a safety-of-flight condition.

“So that started this on the wrong trajectory to begin with.”

The service letter, which referenced bearing failures, had been issued in May 2008 and introduced a new inspection procedure.

NTSB investigator in charge Chihoon Shin told the hearing that the service letter was followed by a 2011 revision which informed of a redesigned bearing which “eliminated” the lubrication groove in the race — the originating point of the bearing fatigue fracture.

“Investigators found UPS reviewed the service letters and determined no further action was required,” he added.

Springer stated, in his remarks to DeLeeuw, that the Boeing service letter made the problem “sound almost benign”, and “not a big deal”.

He added that the full extent of the “collateral damage”, the fact that the bearing race fractures were causing damage to the lugs, was not conveyed.

“The rest of the damage that was being caused was a big deal,” said Springer. “And I think, if we would have known that at UPS, I think we’d have asked a lot of different questions over the years.”

Several occurrences of bearing race failure had been reported to the US FAA’s service difficulty report database.

But Boeing Commercial Airplanes senior director of fleet operations Scott Hirsch informed the hearing that the company was not mining this database.

“We have not looked at [service difficulty reports] for mining in the past,” he said. “We ran a pilot and also found difficulties with the data quality.”

Hirsch added that the company had not been able to find a way to integrate the data with its continued operational safety criteria.

“We are currently exploring what we can do better, and we plan to work transparently with the FAA,” he said.