Narrative:

Sea WX reported as clear, 15 mi visibility, fog restricting visibility to north and south. Boeing field, just north of sea was 0-0 in fog. However, private aircraft were still attempting ILS approachs with no luck landing and all were making missed approachs. First officer was flying the aircraft on autoplt. Flight conditions were VMC above large areas for ground fog. Approach control cleared us for an ILS runway 16R approach, maintain 3200' to anvil, call the tower at parkk. However the part, 'maintain 3200' to anvil' didn't sink in. First officer thought it was to maintain 3200' till established on the ILS localizer. I just didn't give it much thought. He's a sharp pilot. First officer descended to 2000' per the approach plate. Approach called to check our altitude. He showed us at 2000'. I advised that we were at 2000' and in visual flight conditions. Approach told us to climb immediately to 3200' as he had a missed approach at boeing field. He then directed us to climb quickly and then turned us 100 degrees to the left and climb to 5000' for vectors to another 16R ILS. Next approach was normal other than having to sidestep to runway 16L in marginal visibility just north of the airport. How to prevent this? 1) fly the clearance exactly, 2) change the far's to require all aircraft to operate under the same rules re: initiating IFR approachs into below minimums WX conditions.

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Original NASA ASRS Text

Title: FAILED TO MAINTAIN ASSIGNED ALT TO ANVIL RESULTING IN LESS THAN STANDARD SEPARATION WITH ACFT ON MISSED APCH BFI.

Narrative: SEA WX RPTED AS CLEAR, 15 MI VIS, FOG RESTRICTING VIS TO N AND S. BOEING FIELD, JUST N OF SEA WAS 0-0 IN FOG. HOWEVER, PVT ACFT WERE STILL ATTEMPTING ILS APCHS WITH NO LUCK LNDG AND ALL WERE MAKING MISSED APCHS. F/O WAS FLYING THE ACFT ON AUTOPLT. FLT CONDITIONS WERE VMC ABOVE LARGE AREAS FOR GND FOG. APCH CTL CLRED US FOR AN ILS RWY 16R APCH, MAINTAIN 3200' TO ANVIL, CALL THE TWR AT PARKK. HOWEVER THE PART, 'MAINTAIN 3200' TO ANVIL' DIDN'T SINK IN. F/O THOUGHT IT WAS TO MAINTAIN 3200' TILL ESTABLISHED ON THE ILS LOC. I JUST DIDN'T GIVE IT MUCH THOUGHT. HE'S A SHARP PLT. F/O DSNDED TO 2000' PER THE APCH PLATE. APCH CALLED TO CHK OUR ALT. HE SHOWED US AT 2000'. I ADVISED THAT WE WERE AT 2000' AND IN VISUAL FLT CONDITIONS. APCH TOLD US TO CLB IMMEDIATELY TO 3200' AS HE HAD A MISSED APCH AT BOEING FIELD. HE THEN DIRECTED US TO CLB QUICKLY AND THEN TURNED US 100 DEGS TO THE LEFT AND CLB TO 5000' FOR VECTORS TO ANOTHER 16R ILS. NEXT APCH WAS NORMAL OTHER THAN HAVING TO SIDESTEP TO RWY 16L IN MARGINAL VIS JUST N OF THE ARPT. HOW TO PREVENT THIS? 1) FLY THE CLRNC EXACTLY, 2) CHANGE THE FAR'S TO REQUIRE ALL ACFT TO OPERATE UNDER THE SAME RULES RE: INITIATING IFR APCHS INTO BELOW MINIMUMS WX CONDITIONS.

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.