What was found
UPON GEAR EXTENSION FOR LANDING, 3 GREEN GEAR LIGHTS ILLUMINATED AND INNER GEAR DOORS OPEN RED LIGHT REMAINED (ON). CREW CYCLED LANDING GEAR TO NO AVAIL, EXECUTED TOWER FLY BY FOR CONFIRMATION AND WAS INSTRUCTED TO RETURN, WHERE ANOTHER ATC TOWER FLY BY WAS MADE AND LANDED UNEVENTFULLY. MAINTENANCE DETERMINED THAT THE RT MAIN GEAR DOWNLOCK SWITCH TO BE DEFECTIVE. REPLACED SAME, PERFORMED LANDING GEAR RETRACTION AND EXTENSION IAW MM. SYSTEM SERVICED, CHECKED SATISFACTORY, AND AIRCRAFT RETURNED TO SERVICE. (K)