SF340B ACFT EXPERIENCED CARGO SMOKE FIRE WARNINGS; INTERMITTENT AT FIRST THEN STEADY. FLC USED FIRE BOTTLE IN ACCORDANCE WITH THEIR GENERAL OPS MANUAL; DECLARED EMER AND RETURNED TO DEP ARPT. FLT ATTENDANT RPTED NO SMOKE OR HEAT IN VICINITY OF CARGO AREA. RPTR CAPT SAYS THE SAAB 340 IS PRONE TO FALSE ALARMS AND HE ATTRIBUTES IT TO OPERATING AT HIGH ALTS AND THEN LNDG AT HOT HUMID ARPTS RESULTING IN CONDENSATION ON THE FIRE SMOKE DETECTORS.

Narrative

DURING CLB; A SERIES OF CARGO SMOKE WARNINGS CAME ON AND OFF. IT WAS ALSO NOTED THAT PREVIOUSLY FLTS HAD EXPERIENCED THE SAME PROBS. ACCORDING TO OUR GENERAL OPS MANUAL; WE'RE REQUIRED TO TREAT THIS SIT AS A REAL FIRE; EVEN IF WE THINK IT IS A FALSE ALARM. I INSTRUCTED THE FLT ATTENDANT TO CHK THE LAVATORY AND ADJACENT WALL/CRACK/DOOR JAMB THAT IS NEXT TO THE CARGO COMPARTMENT FOR ANY SMOKE OR HEAT. SHE CONTACTED US AND RPTING NO FIRE WAS PRESENT. THE ALARM WENT STEADY AFTER A MIN OR 2 OF INTERMITTENT WARNINGS; SO I FIRED THE FIRE BOTTLE (CARGO COMPARTMENT) AND NOTED THE TIME. WE CONTACTED ATC FOR A RETURN BACK TO MEM IN AN EXPEDITIOUS MANNER. WE HAD CFR UNITS STANDING BY IN CASE THERE REALLY WAS A FIRE. UPON LNDG; I SHUT DOWN THE #1 ENG AND HAD THE AFT AND L SIDE OF THE ACFT INSPECTED FOR ANY INDICATIONS OF SMOKE OR FIRE. THERE WASN'T ANY; SO WE TAXIED SINGLE ENG BACK TO THE GATE WITH NO FURTHER INCIDENT. I BELIEVE THE PROB LIES IN THE SAAB 340 FIRE DETECTION SYS WHICH IS PRONE TO GIVE A FALSE ALARM IF THE ACFT IS OPERATED AT HIGH ALTS (ABOVE FL180) AND THEN LANDS AT A VERY HOT AND HUMID ARPT. CONDENSATION FORMS ON THE DETECTOR GIVING OFF A FALSE ALARM.