What was found
DURING LANDING GEAR EXTENSION, LT MAIN GEAR LIGHT DID NOT ILLUMINATE AND UNLOCK LIGHT STAYED ON. DID ROTARY TEST AND DETERMINED LT GEAR LIGHT WORKED. CYCLED GEAR UP AND DOWN TWICE AND COULD NOT GET 3 GREEN LIGHTS. UNLOCK LIGHT REMAINED ILLUMINATED. DECLARED EMERGENCY. CONDUCTED (GEAR WILL NOT EXTEND) CHECKLIST. PUT GEAR HANDLE DOWN - GOT 3 GREEN AND NO UNLOCK LIGHT. CONTINUED CHECKLIST AND ACTIVATED NITROGEN BLOW DOWN BOTTLE TO INSURE GEAR WAS DOWN - NORMAL LANDING FOLLOWED. TROUBLESHOT TO LT MAIN ACTUATOR. REMOVED ACTUATOR P/N 9912537-3 S/N 0752. INSTALLED NEW ACTUATOR P/N 9912537-3 S/N 1145. OPS AND LEAK CHECK GOOD. WORK DONE IAW MM CH 32-30-04 SERVICED GEAR N2 BOTTLE. DONE IAW MM 12-13-01.