PORT ANGELES, WA, USA
N9588X
Cessna 210B
After takeoff, the left main landing gear would not lock in the 'up' position. The pilot was subsequently unable to get the left main gear into either the up-and-locked or down-and-locked positions, despite repeated efforts to do so. After burning down fuel, the pilot performed a successful landing in grass next to the runway at his destination with the left main gear unlocked, sustaining only slight additional damage in the landing. Post-accident examinations disclosed a fatigue fracture with at least three separate origins in the left main landing gear saddle (a structural component that mounts the gear strut to the airframe), and fatigue cracks in the right main gear saddle. Both main gear saddles were dye-penetrant inspected for cracks in accordance with AD 76-14-07R2 approximately 3 weeks/30 flight hours before the accident with no defects found.
On August 13, 1999, approximately 2330 Pacific daylight time, a 1962 Cessna 210B, N9588X, landed in grass adjacent to the runway at Port Angeles, Washington, after the pilot could not get the left main gear down and locked. The commercial pilot-in-command of the aircraft was not injured in the landing. A post-accident examination of the aircraft revealed substantial damage. The 14 CFR 91 personal flight had originated at Paine Field, Everett, Washington. Visual meteorological conditions were reported at Port Angeles at 2253, and no flight plan had been filed. The pilot reported that when he attempted to raise the gear after takeoff, he felt the "thud" of normal gear retraction but did not get a red light (indicating all gear are up and locked on the accident aircraft.) He stated that he tried to recycle the gear to the down position but that he could not get the left main gear in sight, and got no "green light" indication for the gear (note: on the accident aircraft, a single green light indicates all gear are down and locked.) He stated he tried again with both the engine-driven pump and the emergency hand pump, but got no locked indication in either the up or down position. The pilot reported that further attempts to lower the left main gear using pull-up maneuvers were also unsuccessful. He stated that he then confirmed from ground personnel that the left main gear was "dangling" although the nose and right main gear appeared normal. The pilot stated that after burning down the left side fuel for about 1 hour to lighten the load, he elected to land gear-down on the grass adjacent to runway 26. The landing attempt was successful, with only slight additional damage sustained in the landing. FAA inspectors who examined the aircraft after the accident found a fracture in the left main landing gear "saddle block", a structural casting that mounts the main landing gear strut to the airframe and on which the strut pivots. An FAA inspector reported that one of the saddle block's "ears", comprising one of the pivot hinge points for the strut, was fractured. Both saddle blocks were subsequently removed from the aircraft and sent to the NTSB Materials Laboratory in Washington, D.C. for metallurgical examination. The NTSB Materials Laboratory examination report (No. 01-032, March 9, 2001, attached) stated that optical examination of the fracture surfaces of the left saddle found a darkly discolored thumbnail region covering about one-third of the fracture. Close inspection established that the discoloration was consistent with long-term oxidation and corrosion of the fracture surfaces. Further examinations uncovered topographic features in the discolored thumbnail indicative of progressive cracking, according to the Materials Laboratory report. These features, including ratchet marks (small steps in the fracture that separate crack initiation areas on slightly offset planes), indicated that a fatigue crack initiated at multiple locations at the upper aft corner of the cross section. Scanning electron microscope (SEM) examination of the inboard fracture face revealed sufficient detail to establish at least three separate initiation locations on the surface of the saddle. The initiation sites were located at and adjacent to the upper aft corner of the saddle. Further examination uncovered an uncorroded fracture band at the edge of the thumbnail with "clear features of fatigue" (i.e., "parallel bands of micro fissures indicative of high stress fatigue propagation") progressing out of the corroded region. The fatigue region extended about 0.5 inch along the upper surface and 0.4 inch along the aft surface of the saddle. The NTSB Materials Laboratory report stated that on close visual inspection, the right main landing gear saddle was found to be cracked at the same corresponding location as the fatigue area on the left saddle (upper aft corner). The crack appeared to be 0.25 inches long on the upper surface and 0.22 inches long on the aft surface. Upon opening the saddle crack for visual examination (via saw cuts and hammer blows), it was found that the crack faces showed a heavily corroded thumbnail region with many ratchet marks and fracture traces indicating initiation at and progression from the upper aft corner area. Several individual origin sites were visible at the corner and on the upper surface immediately adjacent to the corner. Closer examinations uncovered a narrow uncorroded band at the progression front of the crack that was not associated with the laboratory-induced overstress region. The entire thumbnail region (corroded and uncorroded areas) measured approximately 0.35 inches along the upper surface and 0.25 inches along the aft side of the saddle. SEM examinations of the crack surface found an oxidized surface in the corroded area with little or no fine fracture detail remaining. However, fatigue features and microfissuring were clearly visible in the uncorroded band at the periphery of the crack, according to the Materials Laboratory report. The pilot/operator reported that the fractured/cracked main landing gear saddles (Cessna part numbers 1241423-1 and 1241423-2) were of an "improved" type (as required by an FAA Airworthiness Directive [AD], AD 76-14-07R2), with an estimated time on the parts of 2,000 hours. The operator reported that he was unable to determine the exact time on the parts, due to the original aircraft maintenance logs being destroyed in a hangar fire in 1980. The pilot/operator reported that the aircraft's last annual inspection was on July 21, 1999, approximately 3 weeks and 30 flight hours before the accident. The mechanic who performed the last annual inspection indicated that AD 76-14-07R2 (which requires dye penetrant inspection of "improved" main landing gear saddles at each annual inspection) was complied with at that time with no defects noted, but stated that the area "is extremely difficult to inspect [and] the saddles are so porous that it is virtually impossible to get the dye penetrant cleaned out sufficiently so as to get a good visual indication when the developer is applied to the suspected area to draw the penetrant out of a possible crack." The AD 76-14-07R2 inspection procedure specifies one of two methods of inspecting the saddles: 1) place the airplane on jacks, disconnect the main landing gear doors, retract the landing gear and perform dye penetrant inspection of the saddle fitting from underneath the airplane; or 2) with the airplane in normal ground attitude, remove the inspection cover in the floorboard area of the airplane and perform dye penetrant inspection of the saddle fittings from inside the airplane.
The failure of maintenance personnel to detect cracked main landing gear saddles during required inspection, and subsequent fatigue fracture of the left main landing gear saddle resulting in separation of the saddle and disabling of the left main gear.
Source: NTSB Aviation Accident Database
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