What was found
MIA - FLT 22 - GATE TURN BACK - DURING TAXI TO RUNWAY THE MID COMPARTMENT. SMOKE LIGHT ILLUMINATED ALONG WITH MWP SMOKE LIGHT. DISCHARGED BOTH CARGO FIRE BOTTLES. EMERGENCY EQUIPMENT CALLED OUT. NO EVIDENCE OF SMOKE IN MID CARGO COMPARTMENT. SMOKE GROUND OPERATIONAL CHECKED GOOD. VERIFIED NR 1 AND 2 CARGO COMPARTMENT FIRE BOTTLES BLOWN, REPLACED BOTH BOTTLES. UPON TROUBLE SHOOTING FOUND THE FORWARD CARGO AFT SMOKE DETECTOR INOPERATIVE, PLACARDED FORWARD CARGO AFT SMOKE DETECTOR INOP IAW WITH THE MEL. AIRCRAFT RETURNED TO SERVICE. (M)