Narrative:

WX was below minimums at time of arrival at mfe. Low fog had set in with a visibility of 1/8 mi. Minimum visibility for our operation on this field is 1/2 mi visibility. Holding was initiated at the time of arrival over the OM so when the visibility came up we would be in a position to shoot an immediate approach. The visibility was slowly increasing from 1/8-1/2 mi. We were under valley approach control authority, but were also monitoring mfe's control tower for updated visibility reports. After approximately 1 hour, 10 mins of holding, the tower reported the northeast sector visibility at 1/2 mi. The approach was initiated and successfully completed west/O incident. We were then informed that only 1 sector had a visibility of 1/2 mi and that the prevailing visibility was still 3/8 mi. The fog bank lifted moments later and the field was then VFR. This experience left some confusion as to whether any 1/2 mi visibility report is sufficient to initiate an approach or whether a prevailing visibility report of 1/2 mi is required to initiate an approach. I am currently seeking clarification of the rule.

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Original NASA ASRS Text

Title: COMMUTER ACFT EXPERIENCES SOME CONFUSION ABOUT TWR REPORTED VISIBILITY FOR APCH MINIMA.

Narrative: WX WAS BELOW MINIMUMS AT TIME OF ARR AT MFE. LOW FOG HAD SET IN WITH A VISIBILITY OF 1/8 MI. MINIMUM VISIBILITY FOR OUR OPERATION ON THIS FIELD IS 1/2 MI VISIBILITY. HOLDING WAS INITIATED AT THE TIME OF ARR OVER THE OM SO WHEN THE VISIBILITY CAME UP WE WOULD BE IN A POS TO SHOOT AN IMMEDIATE APCH. THE VISIBILITY WAS SLOWLY INCREASING FROM 1/8-1/2 MI. WE WERE UNDER VALLEY APCH CTL AUTHORITY, BUT WERE ALSO MONITORING MFE'S CTL TWR FOR UPDATED VISIBILITY RPTS. AFTER APPROX 1 HR, 10 MINS OF HOLDING, THE TWR RPTED THE NE SECTOR VISIBILITY AT 1/2 MI. THE APCH WAS INITIATED AND SUCCESSFULLY COMPLETED W/O INCIDENT. WE WERE THEN INFORMED THAT ONLY 1 SECTOR HAD A VISIBILITY OF 1/2 MI AND THAT THE PREVAILING VISIBILITY WAS STILL 3/8 MI. THE FOG BANK LIFTED MOMENTS LATER AND THE FIELD WAS THEN VFR. THIS EXPERIENCE LEFT SOME CONFUSION AS TO WHETHER ANY 1/2 MI VISIBILITY RPT IS SUFFICIENT TO INITIATE AN APCH OR WHETHER A PREVAILING VISIBILITY RPT OF 1/2 MI IS REQUIRED TO INITIATE AN APCH. I AM CURRENTLY SEEKING CLARIFICATION OF THE RULE.

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.