Inadequate Shot Peening Cited in Two Failures of Left-main Landing Gear On Fokker 100
Author: FSF Editorial Staff
Source: Flight Safety Foundation - Aviation Mechanics Bulletin - September/October
Doc ID: 2001010
Year of Publication: 2001
Abstract:
About 1030 local time July 4, 1999, the crew of a Fokker 100 experienced a severe vibration from the left-main landing gear when they applied the wheel brakes during the landing roll at Norfolk Island Airport after a domestic flight in Australia. The airplane received minor damage; none of the 43 people on board was injured.
Three months later, at 1040 local time Oct. 9, 1999, the same airplane was being landed at the same airport when the crew experienced a severe vibration throughout the airframe. The airplane received substantial damage; none of the 84 people on board was injured.
The incidents were investigated by the Australian Transport Safety Bureau (ATSB), which issued a technical report on the analysis of the airplane's left-main landing-gear failures and air safety occurrence reports on each incident.
ATSB said in its technical report that, in the first incident, the outboard wheel on the left-main landing gear separated from the wheel hub during landing and that the second incident involved the fracture of the left-main landing-gear upper-torque-link attachment lugs.
The repairs included the reduction of the hub diameter by 0.02 inch (0.51 millimeter). The work was conducted in accordance with requirements of repair no. 15 of the Aircraft Braking Systems Corp. maintenance manual of Sept. 27, 1998, and the Aircraft Braking Systems Corp. authorized an increase of the repair tolerance for the hub-diameter reduction from between 6.41 inches and 6.48 inches (16.28 centimeters and 16.46 centimeters) to between 6.39 inches (16.23 centimeters) and 6.48 inches. Instructions for the repairs said that after material was removed for the hub-diameter reduction, the repaired area was to be shot peened. The air safety occurrence report said that shot-peening parameters were to be "adjusted to produce a specific surface quality."
The technical report said that a comparison of the surface of the repaired area and the original surface of the wheel hub revealed a "markedly different" intensity in the shot peening of each area. The intensity of shot peening was lower on the repaired surface, the report said.
"This variation would be expected to lower the resistance of the wheel to fatigue cracking," the air safety occurrence report said. "The lower level of compressive residual stress associated with the less intense shot-peening process applied to the repaired (area) would also increase the likelihood of fatigue failure under normal loading conditions."
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