Investigators probing the failure and separation of a Cargolux Boeing 747-400F’s body main landing-gear assembly at Luxembourg last year have found evidence that components in its inner shock-strut cylinder failed.
But while a previous similar occurrence involving a Nippon Cargo Airlines 747-400F at Amsterdam in September 2005 was traced to a heat-treatment error on a batch of these components – known as upper bearing carriers – the Cargolux components were not part of the batch.
As the Cargolux aircraft lifted off from runway 06 on 14 May 2023 its right-hand body main-gear strut over-extended and, as a result, its wheel bogie tilted down rather than adopting the normal slight tilt-up ahead of gear retraction.
The pilots of the 747, bound for Chicago O’Hare, were unable to select gear-up and were presented with landing-gear alerts.
After the crew opted to dump fuel and return to the departure airport, the jet touched down on runway 06 at which point the right-hand body main-gear bogie separated along with part of the inner cylinder, striking the aft fuselage underside several times as well as the right-hand horizontal stabiliser.
While the tilt-down of the bogie on take-off “indicates an over-extension of the shock strut”, says Luxembourg’s investigation authority AET in preliminary findings, there is evidence that a maximum over-extension condition “was not reached”.
AET states that parts of the landing-gear assembly were sent for analysis by Boeing under US National Transportation Safety Board oversight.
Boeing found that the upper bearing carrier had failed, with bearing and piston ring retention lands sheared off. Further examination of an inner cylinder segment is also being conducted.
AET says the 747’s right-hand body main-gear had been installed in June 2016 and the jet (LX-OCV) carried out 4,717 cycles before the accident.
The last shock-strut servicing – which involves deflating the strut, filling it with hydraulic fluid and pressurising it – was performed in January-February 2023.
Subsequent servicing after five to 10 landings was conducted over a two-week period in February 2023 to ensure that strut extension and inflation pressure parameters were within limits.
