History of the Flight
On 5 June 2019, a Boeing 737-400 cargo aircraft, registration HS-KMC, operated by K-Mile, landed at Singapore Changi Airport at approximately 0407 local time. The flight originated from Bangkok, Thailand, and was the fifth flight after replacement of all three landing gears by a maintenance, repair and overhaul organisation in Jakarta, Indonesia. The flight crew, consisting of a Captain and a First Officer, were accompanied by a travelling Licensed Aircraft Maintenance Engineer. The First Officer was the pilot flying.
During the landing roll, the flight crew experienced unusual vibrations when the thrust reversers were deployed. The aircraft deviated to the left of the runway centreline, requiring greater than normal right rudder pedal input to maintain direction. The Captain then took over and applied brakes, slowing the aircraft. The vibrations subsided by about 60 knots. The aircraft left the runway via Taxiway W7 and taxied to parking bay 308.
Upon arrival at 0415 local time, the travelling engineer conducted a post-flight inspection and found the upper torsion link of the right main landing gear broken. The damper manifold was missing, and a hydraulic return line was disconnected, resulting in a hydraulic fluid leak. The engineer contacted the operator’s headquarters for advice. Later, a duty engineer from a maintenance organisation assessed the damage as serious and reported it to the aerodrome operator. A foreign object debris search later recovered the damper manifold and a hydraulic line guide from the taxiway.
Damage to Aircraft
The damage to the right main landing gear included: a broken upper torsion link with bushings out of alignment; a distorted lower torsion link with a fractured bushing; a detached damper manifold; torn hydraulic line sleeves causing a fluid leak; a bent damper piston; and scuffs and score marks on the two tyres. No one was injured.
Causal Factors Identified
The investigation identified two causal factors: (1) The right main landing gear upper torsion link was in a misaligned/binding state prior to the last flight, which resulted in the fracture of the upper torsion link and subsequent detachment of the damper manifold during the landing roll. (2) The operator’s in-service maintenance and inspection programme was not effective in detecting the misaligned/binding condition.