GAINESVILLE, FL, USA
N3237U
CESSNA 421C
DURING TAKEOFF, THE AIRCREW HEARD AN UNUSUAL 'THUD' SOUND AS THE GEAR WAS RETRACTING, BUT ALL GEAR INDICATIONS WERE NORMAL. PRIOR TO LANDING AT THE DESTINATION, 3 GREEN LIGHTS WERE OBSERVED WHEN THE LANDING GEAR SWITCH WAS MOVED TO EXTEND THE GEAR. A NORMAL TOUCHDOWN WAS MADE, BUT WHEN THE NOSE WAS LOWER, THE ACFT SETTLED ON ITS NOSE. AN INVESTIGATION REVEALED THAT THE ROD END, PN MS21242S4K, WHICH CONNECTS THE NOSE GEAR TO THE NOSE GEAR ACTUATOR, HAD FAILED. A METALLURGICAL EXAM REVEALED SIGNS OF HIGH STRESS FATIGUE CRACKING OVER 1/2 OF THE FRACTURE SURFACE. THE FATIGUE ORIGINATED IN THE CURVE OF A THREAD RUNOUT THAT WAS CORRODED. THE RADIUS OF THE CURVED SURFACE MEASURED 0.056 INCH. THE SPECIFICATION, MS-21242, REQUIRED A MINIMUM RADIUS OF 0.063 INCH. SINCE THE DOWN POSITION SWITCH WAS CONNECTED THE ACTUATOR, IT HAD INDICATED A SAFE INDICATION WHEN THE ACTUATOR HAD OPERATED TO THE EXTENDED POSITION. HOWEVER, WITH THE ROD END FAILURE, THE NOSE GEAR WAS HANGING IN AN LOCKED POSITION.
Source: NTSB Aviation Accident Database
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