Narrative:

When we left dtw to jax the WX was clear with good visibility. En route, dispatch sent an ACARS that jax was 1/4 in fog and added an alternate. We checked jax WX 2-3 more times en route and noticed it went from below minimums to VFR every few mins. The ATIS said the visibility was 3 mi. We could see the airport from 20 mi away and noticed some patchy ground fog in the area. We could have easily accepted a visual approach. When approach turned us onto the localizer, in the same transmission as the ILS clearance, he advised us that the touchdown RVR was 1200 ft with mid and rollout better than 6000 ft. I asked the controller to repeat the RVR, which he did. I advised the captain that it was below CAT I minimums and we would need 1800 ft RVR. He said that he could see the runway clearly. I again advised him that RVR has priority over the prevailing visibility and it was illegal to conduct the approach. His response was 'gear down.' a third time I advised him that 'technically' we can't shoot this approach because of the touchdown RVR. I suggested we circle to a runway without RVR. He declined. At the OM, I advised the tower we had 'a good visual with the runway' and were cleared to land. I could see full length and width of the runway and it was obvious there was a patch of fog over the transmissometer site -- it was visible. Although I knew we were technically illegal to conduct the approach, in my judgement we did not conduct an unsafe operation. We continued visually and made a visual landing without incident. On about a 3 mi final, the tower advised that the ATIS had changed, obviously the RVR had increased. I'm not sure what else I could have done short of taking the aircraft and I really didn't feel that was warranted. I see the first officer's job as that of a safety valve -- I provide input and ideas, and take over if safety is compromised. I made a decision that this was safe, albeit technically illegal. I have gone over it 100 times and decided if it happens again to be more forceful in my input, but again, I doubt I would take the aircraft unless safety were compromised.

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Original NASA ASRS Text

Title: CAPT OF A DC9 CONTINUED APCH AFTER BEING ADVISED OF THE LATEST WX RPT WHICH INDICATED BELOW CAT I LNDG MINIMUMS, TOGETHER WITH THE RPTING FO'S CAUTION TO THE CAPT OF BELOW THEIR MINIMUMS FOR LNDG. A SUCCESSFUL LNDG WAS MADE IN GOOD VISIBILITY AND THE ARPT WX RPT WAS SUBSEQUENTLY UPDATED TO REFLECT SAME.

Narrative: WHEN WE LEFT DTW TO JAX THE WX WAS CLR WITH GOOD VISIBILITY. ENRTE, DISPATCH SENT AN ACARS THAT JAX WAS 1/4 IN FOG AND ADDED AN ALTERNATE. WE CHKED JAX WX 2-3 MORE TIMES ENRTE AND NOTICED IT WENT FROM BELOW MINIMUMS TO VFR EVERY FEW MINS. THE ATIS SAID THE VISIBILITY WAS 3 MI. WE COULD SEE THE ARPT FROM 20 MI AWAY AND NOTICED SOME PATCHY GND FOG IN THE AREA. WE COULD HAVE EASILY ACCEPTED A VISUAL APCH. WHEN APCH TURNED US ONTO THE LOC, IN THE SAME XMISSION AS THE ILS CLRNC, HE ADVISED US THAT THE TOUCHDOWN RVR WAS 1200 FT WITH MID AND ROLLOUT BETTER THAN 6000 FT. I ASKED THE CTLR TO REPEAT THE RVR, WHICH HE DID. I ADVISED THE CAPT THAT IT WAS BELOW CAT I MINIMUMS AND WE WOULD NEED 1800 FT RVR. HE SAID THAT HE COULD SEE THE RWY CLRLY. I AGAIN ADVISED HIM THAT RVR HAS PRIORITY OVER THE PREVAILING VISIBILITY AND IT WAS ILLEGAL TO CONDUCT THE APCH. HIS RESPONSE WAS 'GEAR DOWN.' A THIRD TIME I ADVISED HIM THAT 'TECHNICALLY' WE CAN'T SHOOT THIS APCH BECAUSE OF THE TOUCHDOWN RVR. I SUGGESTED WE CIRCLE TO A RWY WITHOUT RVR. HE DECLINED. AT THE OM, I ADVISED THE TWR WE HAD 'A GOOD VISUAL WITH THE RWY' AND WERE CLRED TO LAND. I COULD SEE FULL LENGTH AND WIDTH OF THE RWY AND IT WAS OBVIOUS THERE WAS A PATCH OF FOG OVER THE TRANSMISSOMETER SITE -- IT WAS VISIBLE. ALTHOUGH I KNEW WE WERE TECHNICALLY ILLEGAL TO CONDUCT THE APCH, IN MY JUDGEMENT WE DID NOT CONDUCT AN UNSAFE OP. WE CONTINUED VISUALLY AND MADE A VISUAL LNDG WITHOUT INCIDENT. ON ABOUT A 3 MI FINAL, THE TWR ADVISED THAT THE ATIS HAD CHANGED, OBVIOUSLY THE RVR HAD INCREASED. I'M NOT SURE WHAT ELSE I COULD HAVE DONE SHORT OF TAKING THE ACFT AND I REALLY DIDN'T FEEL THAT WAS WARRANTED. I SEE THE FO'S JOB AS THAT OF A SAFETY VALVE -- I PROVIDE INPUT AND IDEAS, AND TAKE OVER IF SAFETY IS COMPROMISED. I MADE A DECISION THAT THIS WAS SAFE, ALBEIT TECHNICALLY ILLEGAL. I HAVE GONE OVER IT 100 TIMES AND DECIDED IF IT HAPPENS AGAIN TO BE MORE FORCEFUL IN MY INPUT, BUT AGAIN, I DOUBT I WOULD TAKE THE ACFT UNLESS SAFETY WERE COMPROMISED.

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.