MIL PLT RPTS THAT IT IS POSSIBLE TO CONFUSE A PLASI LIGHT WITH A REIL IN MARGINAL WX.

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NO VIOLATION OR INCIDENT IS INVOLVED IN THIS RPT BUT I JUST WANTED TO RELATE AN OCCURRENCE THAT MIGHT NEED SOME EXAMINATION. I WAS FLYING AN NDB APCH AT DUSK WITH WX IN MINS. AS I RECALL THE VISIBILITY WAS JUST ABOVE MINS WITH FOG. I HAD FLOWN INTO THIS ARPT BEFORE AND WAS FAMILIAR WITH THE LIGHTING SYSTEMS. AS WE FLEW IN AT THE MDA I NOTICED ONE WHITE FLASHING LIGHT AND ASSUMED THIS TO BE THE 'ABOVE GS' INDICATION ON THE PULSATING VASI SYS. HOWEVER; OTHER CUES; PRIMARILY GND REF; TOLD ME THAT I WAS NOT ABOVE GS. SURE ENOUGH; MOMENTS LATER WE NOTICED ANOTHER FLASHING WHITE LIGHT WHICH WAS ABEAM THE ONE WE HAD SEEN INITIALLY. THESE LIGHTS WERE NOT THE VASI SYS; BUT THE REILS. WE WERE; IN FACT STILL BELOW GS WHILE AT THE MDA. IF WE HAD RELIED ON THE INITIAL WHITE FLASHING LIGHT TO BEGIN OUR DSCNT; WE MIGHT HAVE GOTTEN DANGEROUSLY LOW. I DON'T KNOW WHY WE COULD NOT SEE BOTH REILS AT THE SAME TIME; BUT WE DID NOT. IT MIGHT HAVE BEEN DUE TO ALIGNMENT OR OBSTRUCTED VISIBILITY. MY POINT IN RPTING ALL OF THIS IS THAT IT SEEMS THAT THE COMBINATION OF REILS WITH A SINGLE LIGHT PULSATING VASI SYS MIGHT NEED FURTHER EXAMINATION. I HAVE SEEN REILS WITH ONLY ONE LIGHT OPERATING. AS I RECALL THE VASI AND THE REIL HAVE DIFFERENT INTENSITIES (AND POSSIBLY FLASH RATES) BUT UNLESS THEY ARE SEEN TOGETHER; IT IS IMPOSSIBLE TO TELL THE DIFFERENCE. I HOPE THAT THIS IS HELPFUL TO YOU. IF YOU HAVE ANY QUESTIONS PLEASE GIVE ME A CALL.