Narrative:

Approaching southeast il approximately 60 mi form destination I asked for WX at destination cps. Visibility was called to be 3/4 mi in fog. I reduced my speed for remainder of flight because it was ground fog, late morning, and I suspected that because it was due to cool air over warm ground, the undercast over which I was flying had holes (it was not a consistent fog) and that the sun would burn off the fog, I would arrive after the visibility had come up above the 1 mi minimum for the approach. As I was being vectored for the approach, I checked in and found that spirit of st louis airport was above minimums and told the controller I would go there if I missed the approach into cps (it was still reported as 3/4 mi visibility). At 450 ft AGL I broke out of the clouds, saw the approach lights and runway in front of me and to the right (the localizer is offset). I was surprised -- I have shot ILS approachs down to 1/2 and 3/4 mi visibility in the past -- the visibility was better than that. In the time I had to look around, I estimated the visibility to be a mi or better. I gave a call to the tower that I had the runway and visibility was a mile to a mile and a quarter, and then landed. The next morning, on departure, I felt that the tower under-reported the visibility once again. I have had some experience estimating visibilities and also know that towers use landmarks at known distances to report visibility, which is a good procedure. However, from time to time, I have observed controllers who do not understand visibility reported and report erroneous distances.

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Original NASA ASRS Text

Title: SMA MAKES INST APCH BELOW MINIMUMS RPTED.

Narrative: APCHING SE IL APPROX 60 MI FORM DEST I ASKED FOR WX AT DEST CPS. VISIBILITY WAS CALLED TO BE 3/4 MI IN FOG. I REDUCED MY SPD FOR REMAINDER OF FLT BECAUSE IT WAS GND FOG, LATE MORNING, AND I SUSPECTED THAT BECAUSE IT WAS DUE TO COOL AIR OVER WARM GND, THE UNDERCAST OVER WHICH I WAS FLYING HAD HOLES (IT WAS NOT A CONSISTENT FOG) AND THAT THE SUN WOULD BURN OFF THE FOG, I WOULD ARRIVE AFTER THE VISIBILITY HAD COME UP ABOVE THE 1 MI MINIMUM FOR THE APCH. AS I WAS BEING VECTORED FOR THE APCH, I CHKED IN AND FOUND THAT SPIRIT OF ST LOUIS ARPT WAS ABOVE MINIMUMS AND TOLD THE CTLR I WOULD GO THERE IF I MISSED THE APCH INTO CPS (IT WAS STILL RPTED AS 3/4 MI VISIBILITY). AT 450 FT AGL I BROKE OUT OF THE CLOUDS, SAW THE APCH LIGHTS AND RWY IN FRONT OF ME AND TO THE R (THE LOC IS OFFSET). I WAS SURPRISED -- I HAVE SHOT ILS APCHS DOWN TO 1/2 AND 3/4 MI VISIBILITY IN THE PAST -- THE VISIBILITY WAS BETTER THAN THAT. IN THE TIME I HAD TO LOOK AROUND, I ESTIMATED THE VISIBILITY TO BE A MI OR BETTER. I GAVE A CALL TO THE TWR THAT I HAD THE RWY AND VISIBILITY WAS A MILE TO A MILE AND A QUARTER, AND THEN LANDED. THE NEXT MORNING, ON DEP, I FELT THAT THE TWR UNDER-RPTED THE VISIBILITY ONCE AGAIN. I HAVE HAD SOME EXPERIENCE ESTIMATING VISIBILITIES AND ALSO KNOW THAT TWRS USE LANDMARKS AT KNOWN DISTANCES TO RPT VISIBILITY, WHICH IS A GOOD PROC. HOWEVER, FROM TIME TO TIME, I HAVE OBSERVED CTLRS WHO DO NOT UNDERSTAND VISIBILITY RPTED AND RPT ERRONEOUS DISTANCES.

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.