Shearer, ID, USA
N104DE
Cessna A185F
The left main landing gear separated during the landing roll. Post accident metallurgical examination showed that the fracture occurred due to fatigue cracks that emanated from the chamfer areas at two bolt hole locations on the inboard side of the strut. The period of time that was required to cause propagation of the fatigue crack regions was not determined, but most of the brown region fractured in a single event. Because this was in existence for very few flight cycles, a visual inspection of the lower surfaces of the spring strut would have to have been conducted just before the accident flight in order to detect the crack, and even then, it would have been difficult to see. All of the small fatigue cracks at the origin areas were covered by the washers and would not have been visually detectable. Although the Safety Board issued previous recommendations about this issue, the Federal Aviation Administration reported that the visual inspections called out in the maintenance manuals are, "...adequate to detect cracks in the main landing gear struts and that additional airworthiness action is not warranted." On March 25, 2002, the NTSB responded by stating, in part: "The Safety Board continues to believe that a visual inspection alone will not detect cracks in the Cessna main landing gear spring struts. However, the Safety Board acknowledges that the statistical evidence does not warrant issuance of ADs at this time as called for in the Board's recommendation." The safety recommendation was then classified as: "Closed-Reconsidered."
On May 30, 2007, at 1630 Pacific daylight time, a Cessna A185F, N104DE, sustained substantial damage following a separation of the left main landing gear, while landing at a private ranch airstrip approximately 5 statute miles south of Shearer, Idaho. Osprey Aviation LLC was operating the airplane as an on-demand 14 CFR Part 135 passenger flight. The airline transport pilot sustained minor injuries. One passenger was not injured, and one passenger sustained serious injuries. The flight departed Missoula International Airport, Missoula, Montana at 1545, and was landing at the intended destination when the accident occurred. Visual meteorological conditions prevailed and a visual flight rules (VFR) flight plan was filed. According to the pilot, the touchdown was normal. During the rollout, the left main gear separated above the left wheel axle, through the upper two bolt holes. The pilot noted corrosion and cracking in the area of the separation. An inspector from the Spokane Federal Aviation Administration Flight Standards District Office responded to the accident site on May 31. The inspector examined the fracture surfaces and indicated that corrosion and cracking were present. The components were retained for further examination. The NTSB Materials Laboratory examined the fractured lower portion of the left main landing gear spring strut. The bottom end of the spring strut contained four bolt holes for the axle attachment bolts. The fracture intersected the upper two bolt holes. Visual examination of the spring strut portion revealed that the fracture face contained a brown oxidized region at the inboard side between the two bolt holes. The brown region extended through approximately 80 percent of the wall thickness. Bench binocular examination of the fracture face revealed that the fracture features within the brown region emanated from a small area that contained crack arrest marks typical of fatigue cracking. Additional small discolored regions with curved boundaries, also typical of fatigue cracking, were found at the transition between the chamfer and bore. The length of each fatigue crack region was in the range between 0.02 and 0.04 inch. The bore surface adjacent to the chamfer area in both holes exhibited impressions from the threads of the bolt and fretting damage. The fracture face also contained a fatigue striation band that was located inside and adjacent to the curved outer boundary of the brown region. The length of the fatigue striation band measured approximately 0.03 inch. The fracture area that extended between the fatigue region at the origin area and the fatigue striation band contained matte features typical of overstress separation. Visual inspection of the landing gear strut is required at prescribed intervals by 14 CFR Part 43. Removal of the gear for visual inspection, or the use of nondestructive inspection techniques (NDT), is not required. Neither the manufacturer nor the FAA, have established a life limit (hours or cycles) for the main landing gear strut. On March 16, 2001, the NTSB recommended that the Federal Aviation Administration issue an airworthiness directive for initial inspection of Cessna 170, 180, 185, 190, and 195 series airplane main landing gear spring struts, involving the removal of the spring struts from the fuselage attachment clamp and axle assembly and the use of nondestructive inspection techniques to examine the upper and lower ends of the spring struts for corrosion and cracks, at the next 100-hour or annual inspection, whichever occurred first. In addition, the NTSB recommended that the FAA issue an AD requiring repetitive inspections of the inspection as outlined above. The Safety Recommendation letter is A-01-01 and -02. In response to Safety Recommendation A-01-01 and -02, the FAA issued an Airworthiness Concern Sheet on May 23, 2001. Then, they reviewed the history of the 9,300 registered airplanes affected by the safety issue. In their response to the safety recommendation, the FAA concluded, "That the main landing gear spring struts may become damaged at the upper end due to hard landings or other abuse, which overstress the landing gear. The review revealed that airplanes that acquire damage at the lower end of the spring struts have had skis or oversized wheels installed. Skis cause a twisting motion that over time can initiate cracks. If the cracks are not detected, failures can occur. In any case, proper maintenance is essential, especially for airplanes that operate in harsh environments or experience hard landings...the Cessna Maintenance Manual specified that the main landing gear spring struts be inspected every 50 hours and during annual inspections. The inspections are especially important for airplanes operating in harsh environments because they would detect surface damage. The FAA has determined that these inspections are adequate to detect cracks in the main landing gear struts and that additional airworthiness action is not warranted." The FAA also published a General Aviation Alert in Advisory Circular 43-16 to reiterate that visual inspections of the spring strut should be performed every 50 hours and during annual inspections. On March 25, 2002, the NTSB responded to the FAA by stating, in part: "The Safety Board continues to believe that a visual inspection alone will not detect cracks in the Cessna main landing gear spring struts. However, the Safety Board acknowledges that the statistical evidence does not warrant issuance of ADs at this time as called for in the Board's recommendation." The Safety Board subsequently classified Safety Recommendations A-01-01 and A-01-02 as "Closed, Reconsidered."
A fracture and separation of the left main landing gear strut during the landing roll due to fatigue.
Source: NTSB Aviation Accident Database
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