A C421 L MAIN LNDG GEAR OVER-CTR DOWN-LOCK FAILED. AS IT RETRACTED; THE L GEAR DOWN MOTOR EXTENDED THE GEAR AGAIN CAUSING THE ACFT TO BOUNCE. ULTIMATELY; A BOLT FAILED; ALLOWING THE L GEAR TO FULLY COLLAPSE.

Narrative

APPROX 5 MI FINAL; LNDG GEAR WAS LOWERED. BOTH PLTS VERIFIED THAT GEAR WAS DOWN AND LOCKED '3 GREEN.' LNDG WAS NORMAL WITH A SLIGHT XWIND. AFTER ALL 3 WHEELS WERE ON THE GND; THE L SIDE OF THE ACFT STARTED BOUNCING. ON THE 4TH OR 5TH BOUNCE; THE L MAIN GEAR COLLAPSED. THE L ENG WAS SHUT DOWN DURING THE COLLAPSE. THE ACFT SLID ON THE RWY APPROX 1000 FT; AND REMAINED ON THE RWY. WHEN THE ACFT CAME TO A STOP; THE R ENG WAS SECURED; AND BOTH PLTS EVACED THE ACFT. THERE WERE NO PAX ON THIS FLT AND THERE WERE NO INJURIES. IT APPEARS THAT THE L MAIN LNDG GEAR DOWN LOCK MECHANISM FAILED AFTER TOUCHDOWN. THIS RESULTED IN THE BOUNCING OF THE L SIDE AND SUBSEQUENT COLLAPSE OF THE L MAIN GEAR. CAUSE OF THE FAILURE IS UNKNOWN AT THIS TIME. THIS ACFT HAS NO PREVIOUS HISTORY OF LNG GEAR PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE OVER-CTR MECHANISM FAILED AND AS THE GEAR DEPARTED THE FULLY EXTENDED SENSING SWITCH; THE ELECTRIC GEAR DOWN MOTOR AGAIN DROVE IT TO THE GEAR DOWN LIMIT (FULLY EXTENDED). AS THE MOTOR MOVED THE GEAR TO THE GEAR DOWN LIMIT; THE GEAR PUSHED THE ACFT L WING BACK UP CAUSING AN APPARENT BOUNCE. AS THE ACFT SETTLED ONTO THE WING; THE RPTR FEATHERED THE ENG THEREBY REDUCING DAMAGE. THE FAA WAS ON THE FIELD THAT DAY AND BECAUSE THE DAMAGE WAS SET AT $100;000 DID NOT CONSIDER IT AN ACCIDENT.