Analysis of a main landing-gear axle fracture on a Korean Air Boeing 777-300 has traced the origin to stress corrosion cracking.
As the aircraft (HL7573) touched down on runway 16L at Tokyo Narita, on 29 June last year, the aft axle on its right-hand main landing-gear assembly snapped.
The aircraft, with 335 occupants, landed at 144kt and the contact impact of 1.41g did not qualify as hard.
Japan Transport Safety Board investigators state that the captain did not sense anything unusual about the touchdown and taxiing.
But another aircraft's crew informed Narita ground controllers that the 777 was emitting smoke from its right-hand main gear, and the twinjet was instructed to halt on taxiway K.
The aft axle, which pivots to assist with steering the six-wheel bogie, had fractured in the centre and collapsed, cutting the hydraulic hoses of the steering and braking system. This sprayed leaking hydraulic fluid onto the hot brakes and caused the smoke.
Parts of the main-gear assembly were found scattered on the runway.
Examination of the axle, says the inquiry, indicates that a region of corrosion on the lower side of the pivot bore was the likely initiator of the stress cracking, which progressed as a result of repetitive loading.
The inquiry believes rotation of bushing and the wearing of seals allowed water penetration, generating the corrosion. Overhaul of the axles had taken place in July 2009, before a revision of the component maintenance manual required application of corrosion inhibitor.
"It is highly probable that the forward side of the pivot bore had fractured first," says the inquiry. "Even thereafter, the aircraft [continued] operations with the aft side of the pivot bore partially cracked."
Korean Air carried out inspections on seven other 777s which had been overhauled before the maintenance manual revision, and replaced all 14 aft main-gear axles on the jets.