Fatigue cracking led bearing race to split and put abnormal stress on lugs anchoring pylon to wing.
While an investigative hearing has opened to determine the circumstances of the fatal UPS Boeing MD-11F accident at Louisville in November last year, the inquiry has revealed that bearing fractures in the aft anchor point of the type’s engine pylons have occurred several times within the last 25 years.
This anchor point — the rearmost of three which attach the pylon to the wing — comprises a bulkhead featuring two adjacent lugs, housed within which is an outer race containing a spherical bearing.
The bearing can rotate slightly to relieve operational load stresses from engine weight and thrust. A bolt passing through the lugs, anchoring the bulkhead to the wing clevis, provides a path for greasing the bearing and the race, which has a circumferential groove to distribute the grease.
On the MD-11F involved in the accident, says the National Transportation Safety Board, this circumferential groove in the race was the source of fatigue cracks which split the race in two.
“With the outer race separated in to two pieces, loads were no longer evenly distributed across the lugs,” says the inquiry. The two pieces of the race could migrate out of the lugs, resulting in abnormal loads on the lugs.
This caused the lugs to crack on their inboard and outboard sides. The inboard side failed first on the Louisville aircraft, bending the outboard side and causing it to fail as well — which meant the aft pylon attachment was no longer anchored to the wing.
Aerodynamic forces as the aircraft rotated led the engine to pivot upwards around the forward pylon attachment point, and the engine broke away over the wing.
The spherical bearing for the MD-11 pylon was developed to replace a dry-film lubricant bearing on the McDonnell Douglas DC-10, which had encountered lubrication failures and corrosion problems.
According to the structures group participating in the accident inquiry, Boeing received the first report of a fractured spherical bearing on an MD-11 in July 2002. The entire outer race had broken in half and one of the lugs had deformed.
Another outer race fracture was reported, by FedEx to the US FAA, in August 2007 — the aft half of the race was found to have migrated on the right-hand pylon — and a different operator reported a similar fracture, and race migration, to Boeing the following month.
Analysis of this subsequent occurrence found circumferential cracks coincident with the greasing groove. The damage was considered a Continued Operational Safety Process event, as defined by a Boeing-FAA working agreement, and reported to the FAA.
The COSP report into this occurrence stated that it was the first reported bearing failure, and was deemed “not a safety issue”, because the bearing failure would not cause a lug joint failure and the lugs showed no crack growth.
According to the structures group, Boeing received six reports of spherical bearing failures, after that of July 2002.
Examination of an FAA database on service difficulty reports found four reports, two of which had not been reported to Boeing, while FedEx informed the National Transportation Safety Board of an additional failure that was reported neither to Boeing nor the FAA. The latest occurrence listed by the structures group was in December 2022.
After the Louisville crash, which grounded MD-11 fleets, Boeing requested that operators perform a visual inspection of pylon aft bulkheads for bearing migration.
Fifty-nine MD-11s were inspected, including all 26 of UPS’s aircraft — of which 13 were powered by the GE Aerospace CF6 and 13 had Pratt & Whitney PW4000 engines. Finding were discovered on three of the UPS aircraft — N250UP, N255UP and N257UP — all with CF6 engines.
All three exhibited bearing race migration in one of their wing pylons. The aircraft had respectively undergone detailed visual inspection in July 2022, January 2025 and June 2022.
Boeing undertook an inspection of the pylon aft bulkhead lugs of N250UP, and found the bearing had suffered a full circumferential fracture of the outer race, while a surface eddy-current examination of the lug bores confirmed the presence of inboard and outboard cracks in the aft lug.
The post UPS MD-11F crash hearing details multiple prior pylon bearing failures first appeared on FlightGlobal.