Commuter aircraft experiences some confusion about tower reported visibility for approach minima.

Attributes
ACN	141561
Time
Date	199003
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : mfe
State Reference	TX
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent : approach landing other other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 3300
ASRS Report	141561
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	non adherence : published procedure non adherence : far other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation

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.

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.