Narrative:

Upon leaving the gate at phl I noticed a fog bank developing rapidly on the south end of the field where the tower is located. They reported 1/4 mi visibility, which is the takeoff minimum (air carrier) for runway 9L. Traffic was departing off runway 9L. We were #3 in line when the tower said the visibility was now 1/16 mi at runway 9L (we observed better than 1 mi as the fog was only on the south side of the field). They then cleared another air carrier to takeoff, no mention was made of legality by the tower. They then cleared the next carrier into position and for takeoff. Now we were #2. The carrier ahead of me requested runway 9R and was told to hold position. I was then cleared for takeoff. At all times we had better than 1 mi visibility, however tower visibility was reported as 1/16 mi. I feel if this was not a legal departure, then the tower should not have cleared us to depart with no mention of takeoff minimums required. At no time were we asked what we required and the carrier that requested runway 9R was given an indefinite delay to get there. I feel that in the interest of safety and legality that tower controllers should work with flight crews in protecting each other from such mistakes. Supplemental information from acn 98996: it was evident from our position that a light wind was blowing a heavy wall of fog off the delaware river from the southeast. Being that the tower is located approximately 2600' to the south of runway 9L, and only approximately 600' north of the delaware river, it was enshrouded in the fog from the delaware river while the active runway remained in the clear. Tower called tower visibility 1/16 mi. I estimated the visibility to be 1 1/4 mi to the sse and to the east. The visibility in all other quadrants was clear. Expected the tower to inform us of what we already knew and to expect an indefinite delay, but instead asked us if we had access to the runway and to position and hold. After a brief discussion the captain and myself agreed that the visibility for runway 9L was far above minimums and that the takeoff would be completed west/O any compromise to safety.

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Original NASA ASRS Text

Title: FOG BANK MOVES OVER TWR RESULTING IN VISIBILITY BELOW TKOF MINIMUMS. RWY VISIBILITY GREATER THAN 1 MILE.

Narrative: UPON LEAVING THE GATE AT PHL I NOTICED A FOG BANK DEVELOPING RAPIDLY ON THE S END OF THE FIELD WHERE THE TWR IS LOCATED. THEY RPTED 1/4 MI VIS, WHICH IS THE TKOF MINIMUM (ACR) FOR RWY 9L. TFC WAS DEPARTING OFF RWY 9L. WE WERE #3 IN LINE WHEN THE TWR SAID THE VIS WAS NOW 1/16 MI AT RWY 9L (WE OBSERVED BETTER THAN 1 MI AS THE FOG WAS ONLY ON THE S SIDE OF THE FIELD). THEY THEN CLRED ANOTHER ACR TO TKOF, NO MENTION WAS MADE OF LEGALITY BY THE TWR. THEY THEN CLRED THE NEXT CARRIER INTO POS AND FOR TKOF. NOW WE WERE #2. THE CARRIER AHEAD OF ME REQUESTED RWY 9R AND WAS TOLD TO HOLD POS. I WAS THEN CLRED FOR TKOF. AT ALL TIMES WE HAD BETTER THAN 1 MI VIS, HOWEVER TWR VIS WAS RPTED AS 1/16 MI. I FEEL IF THIS WAS NOT A LEGAL DEP, THEN THE TWR SHOULD NOT HAVE CLRED US TO DEPART WITH NO MENTION OF TKOF MINIMUMS REQUIRED. AT NO TIME WERE WE ASKED WHAT WE REQUIRED AND THE CARRIER THAT REQUESTED RWY 9R WAS GIVEN AN INDEFINITE DELAY TO GET THERE. I FEEL THAT IN THE INTEREST OF SAFETY AND LEGALITY THAT TWR CTLRS SHOULD WORK WITH FLT CREWS IN PROTECTING EACH OTHER FROM SUCH MISTAKES. SUPPLEMENTAL INFO FROM ACN 98996: IT WAS EVIDENT FROM OUR POS THAT A LIGHT WIND WAS BLOWING A HEAVY WALL OF FOG OFF THE DELAWARE RIVER FROM THE SE. BEING THAT THE TWR IS LOCATED APPROX 2600' TO THE S OF RWY 9L, AND ONLY APPROX 600' N OF THE DELAWARE RIVER, IT WAS ENSHROUDED IN THE FOG FROM THE DELAWARE RIVER WHILE THE ACTIVE RWY REMAINED IN THE CLR. TWR CALLED TWR VIS 1/16 MI. I ESTIMATED THE VIS TO BE 1 1/4 MI TO THE SSE AND TO THE E. THE VIS IN ALL OTHER QUADRANTS WAS CLEAR. EXPECTED THE TWR TO INFORM US OF WHAT WE ALREADY KNEW AND TO EXPECT AN INDEFINITE DELAY, BUT INSTEAD ASKED US IF WE HAD ACCESS TO THE RWY AND TO POS AND HOLD. AFTER A BRIEF DISCUSSION THE CAPT AND MYSELF AGREED THAT THE VIS FOR RWY 9L WAS FAR ABOVE MINIMUMS AND THAT THE TKOF WOULD BE COMPLETED W/O ANY COMPROMISE TO SAFETY.

Data retrieved from NASA's ASRS site as of August 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.