Narrative:

Passing through 15000 ft the l-hand fire light illuminated. I looked outside and noticed no indication of a fire. I then checked the engine instruments again nothing abnormal indicated. I elected to declare an emergency and return for a landing. I made ATC and my company aware of the situation and requested for the fire trucks to be standing by. I briefed the passenger as to our situation and reviewed emergency procedures with them. I shot the ILS approach and made an uneventful landing. On rollout I shut the l-hand engine down and taxied in on the right engine only. We deplaned uneventfully. As it turns out, the plane had been sitting outside that evening during a hard rain. The mechanics determined that something on the fire sensing system had gotten wet and given a false indication. They said that this is a common problem with that system. We also discussed how discomforting it is to have a fire detection system but no fire extinguishing system. This particular BE200 is an early serial number and does not have the optional extinguishing system. In flight safety training manuals it discusses the problem of the false indications of the photocell system by sunlight (which has been minimized by shrouds to keep sunlight out of the cowling) but not about false indications due to water (moisture). Callback conversation with reporter revealed the following information: the reporter stated the air carrier has 3 aircraft in the fleet configured with no fire extinguisher system. The reporter said the engine fire warning system is an optical system which senses light waves and is a common problem in the fleet with reports of false warnings. The reporter said several corrective steps have been taken including adding interior nacelle shrouds to prevent sunlight from triggering the optical sensor. The reporter stated now the moisture causing false warning only makes a recurring problem worse.

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Original NASA ASRS Text

Title: A BE200 IN CLB AT 15000 FT DECLARED AN EMER AND DIVERTED DUE TO A L ENG FALSE FIRE WARNING CAUSED BY MOISTURE ON THE ENG OPTICAL DETECTOR.

Narrative: PASSING THROUGH 15000 FT THE L-HAND FIRE LIGHT ILLUMINATED. I LOOKED OUTSIDE AND NOTICED NO INDICATION OF A FIRE. I THEN CHKED THE ENG INSTS AGAIN NOTHING ABNORMAL INDICATED. I ELECTED TO DECLARE AN EMER AND RETURN FOR A LNDG. I MADE ATC AND MY COMPANY AWARE OF THE SIT AND REQUESTED FOR THE FIRE TRUCKS TO BE STANDING BY. I BRIEFED THE PAX AS TO OUR SIT AND REVIEWED EMER PROCS WITH THEM. I SHOT THE ILS APCH AND MADE AN UNEVENTFUL LNDG. ON ROLLOUT I SHUT THE L-HAND ENG DOWN AND TAXIED IN ON THE R ENG ONLY. WE DEPLANED UNEVENTFULLY. AS IT TURNS OUT, THE PLANE HAD BEEN SITTING OUTSIDE THAT EVENING DURING A HARD RAIN. THE MECHS DETERMINED THAT SOMETHING ON THE FIRE SENSING SYS HAD GOTTEN WET AND GIVEN A FALSE INDICATION. THEY SAID THAT THIS IS A COMMON PROB WITH THAT SYS. WE ALSO DISCUSSED HOW DISCOMFORTING IT IS TO HAVE A FIRE DETECTION SYS BUT NO FIRE EXTINGUISHING SYS. THIS PARTICULAR BE200 IS AN EARLY SERIAL NUMBER AND DOES NOT HAVE THE OPTIONAL EXTINGUISHING SYS. IN FLT SAFETY TRAINING MANUALS IT DISCUSSES THE PROB OF THE FALSE INDICATIONS OF THE PHOTOCELL SYS BY SUNLIGHT (WHICH HAS BEEN MINIMIZED BY SHROUDS TO KEEP SUNLIGHT OUT OF THE COWLING) BUT NOT ABOUT FALSE INDICATIONS DUE TO WATER (MOISTURE). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACR HAS 3 ACFT IN THE FLEET CONFIGURED WITH NO FIRE EXTINGUISHER SYS. THE RPTR SAID THE ENG FIRE WARNING SYS IS AN OPTICAL SYS WHICH SENSES LIGHT WAVES AND IS A COMMON PROB IN THE FLEET WITH RPTS OF FALSE WARNINGS. THE RPTR SAID SEVERAL CORRECTIVE STEPS HAVE BEEN TAKEN INCLUDING ADDING INTERIOR NACELLE SHROUDS TO PREVENT SUNLIGHT FROM TRIGGERING THE OPTICAL SENSOR. THE RPTR STATED NOW THE MOISTURE CAUSING FALSE WARNING ONLY MAKES A RECURRING PROB WORSE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.