B747-400
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 60; Flight Crew Total 10000; Flight Crew Type 1800
Troubleshooting
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Inflight Event / Encounter Fuel Issue
Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Aircraft
Climbing out of 12;000 to 23;000 I (monitoring pilot) heard a sound like a CB popping. I looked up and noticed 'MAIN CABIN SMOKE' light on the overhead panel; I informed the First Officer (fly pilot) and directed him to level off while I talked with ATC and called the dead heading pilot (aircraft qualified) up to the cockpit. I asked dead heading crew member to look in the main cargo area for smoke or fire he grabbed portable O2 bottle and went downstairs. While rider was checking; the First Officer (FO) turned back and I talked with ATC. When jump seat pilot returned reporting no smoke or fire visible we (jump seat pilot and I) ran the 'MAIN CARGO FIRE' checklist while FO handled communications with ATC. 'MAIN CARGO LIGHT' extinguished before we depressurize aircraft so checklist was stopped. We as a crew discussed and agreed we had no visual indication of smoke or fire to dump fuel and return without [advising ATC of an] emergency. We advised ATC we would need to dump fuel and it would be 24 mins to dump 117k pounds. On completing dumping of fuel we had a fuel imbalance of 3000. We asked ATC for more vectors to run checklist. When all checklist complete; FO flew a coupled approach ILS 15 to a full stop.
[Report narrative contained no additional information.]
B747-400 flight crew reported experiencing a 'MAIN CABIN SMOKE' light shortly after takeoff. Crew elected to return to departure airport.