Narrative:

On the flight listed above, the aircraft descended through the clearance altitude while approaching the airport. The deviation occurred during a discussion with ATC (geg approach) about an approach clearance. While descending, but before reaching the clearance altitude, the aircraft descended into VMC conditions (5000 ft overcast and 15+ mi visibility). The airport was acquired visually at a distance of 12 mi. The controller refused to issue a visual approach clearance due to the ASOS report of 2 mi visibility at the airport. While this discussion ensued, the aircraft descended below the clearance limit. The descent was not arrested because I, the PIC, anticipated a clearance for a visual approach due to having the field in sight in VFR WX. This has been SOP for the 4 yrs that I have operated out of this airport. When the visual approach clearance was denied, the approach controller requested that the aircraft climb back to the proper altitude. When the flight was 3 mi east of the mso VOR, the flight was handed off to the tower. When the tower was informed of the visual contact with the airport, a clearance for a visual approach was issued. This clearance was canceled when the tower controller was reminded of the 'reported' visibility. When the flight reached the VOR, the controller cleared the flight to enter the pattern and land. After landing, the tower controller discussed the reported visibility with me and stated that the ASOS was often inaccurate. I feel that the blind trust in the ASOS system over the report of experienced pilots and controllers is one contributing factor to this incident. The fact that the report of the ASOS was erroneous should have been considered by the controllers. Another factor in this incident was the past history of having a clearance for a visual approach being issued in this situation.

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Original NASA ASRS Text

Title: ERRONEOUS ASOS PREVENTS THE ISSUANCE OF A VISUAL APCH CLRNC TO AN SMT.

Narrative: ON THE FLT LISTED ABOVE, THE ACFT DSNDED THROUGH THE CLRNC ALT WHILE APCHING THE ARPT. THE DEV OCCURRED DURING A DISCUSSION WITH ATC (GEG APCH) ABOUT AN APCH CLRNC. WHILE DSNDING, BUT BEFORE REACHING THE CLRNC ALT, THE ACFT DSNDED INTO VMC CONDITIONS (5000 FT OVCST AND 15+ MI VISIBILITY). THE ARPT WAS ACQUIRED VISUALLY AT A DISTANCE OF 12 MI. THE CTLR REFUSED TO ISSUE A VISUAL APCH CLRNC DUE TO THE ASOS RPT OF 2 MI VISIBILITY AT THE ARPT. WHILE THIS DISCUSSION ENSUED, THE ACFT DSNDED BELOW THE CLRNC LIMIT. THE DSCNT WAS NOT ARRESTED BECAUSE I, THE PIC, ANTICIPATED A CLRNC FOR A VISUAL APCH DUE TO HAVING THE FIELD IN SIGHT IN VFR WX. THIS HAS BEEN SOP FOR THE 4 YRS THAT I HAVE OPERATED OUT OF THIS ARPT. WHEN THE VISUAL APCH CLRNC WAS DENIED, THE APCH CTLR REQUESTED THAT THE ACFT CLB BACK TO THE PROPER ALT. WHEN THE FLT WAS 3 MI E OF THE MSO VOR, THE FLT WAS HANDED OFF TO THE TWR. WHEN THE TWR WAS INFORMED OF THE VISUAL CONTACT WITH THE ARPT, A CLRNC FOR A VISUAL APCH WAS ISSUED. THIS CLRNC WAS CANCELED WHEN THE TWR CTLR WAS REMINDED OF THE 'RPTED' VISIBILITY. WHEN THE FLT REACHED THE VOR, THE CTLR CLRED THE FLT TO ENTER THE PATTERN AND LAND. AFTER LNDG, THE TWR CTLR DISCUSSED THE RPTED VISIBILITY WITH ME AND STATED THAT THE ASOS WAS OFTEN INACCURATE. I FEEL THAT THE BLIND TRUST IN THE ASOS SYS OVER THE RPT OF EXPERIENCED PLTS AND CTLRS IS ONE CONTRIBUTING FACTOR TO THIS INCIDENT. THE FACT THAT THE RPT OF THE ASOS WAS ERRONEOUS SHOULD HAVE BEEN CONSIDERED BY THE CTLRS. ANOTHER FACTOR IN THIS INCIDENT WAS THE PAST HISTORY OF HAVING A CLRNC FOR A VISUAL APCH BEING ISSUED IN THIS SIT.

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.