AN ATX PLT PERFORMS A TKOF DURING WX CONDITIONS THAT MADE THAT OP ILLEGAL AT DBQ; IA.

Narrative

WHEN I STARTED ENGS FOR A TRIP UNDER FAR PART 135; THE PLT'S SIDE HORIZON WAS SLOW TO ERECT. IN THE R SEAT THERE WAS A PLT RATED NON REVENUE/NON PAX THAT WAS MONITORING AND TUNING RADIOS. THIS IS PART OF THE TRAINING IN THE FLT PROGRAM. WHILE THAT PERSON PERFORMS SOME OF THE COMS; IT IS STILL THE DIRECT RESPONSIBILITY OF THE PIC. WE OBTAINED THE CURRENT ATIS WHICH INDICATED 100 FT OVER WITH 1/2 MI VISIBILITY. AS WE TAXIED OUT; I WAS CONCENTRATING ON THE GYRO AND THINKING ABOUT WHETHER OR NOT THE FLT COULD CONTINUE. THE GYRO HAD STABILIZED; BUT I BRIEFED THE R SEAT PAX ON THE ACTIONS WE WOULD TAKE IF THE GYRO SUBSEQUENTLY FAILED. DURING THIS TIME; A NEW ATIS WAS ISSUED AND THE GND CTLR CAME ON THE FREQ AND SAID SOMETHING TO THE EFFECT 'THE NEW ATIS IS IN EFFECT; ALTIMETER XXXX; EVERYTHING ELSE IS THE SAME.' WE THEN WERE CLRED FOR TKOF AND DEPARTED WITHOUT INCIDENT. AS WE WERE CLBING OUT; THERE WAS A NAGGING NOTION IN MY MIND THAT I MAY HAVE MISSED SOMETHING IN THE XMISSIONS. I ASKED THE R SEAT PAX WHAT THE CTLR HAD SAID ABOUT VISIBILITY; AND SHE REPLIED THAT THE MESSAGE HAD BEEN 'VISIBILITY HOLDING STEADY AT 1/4; NEW ALTIMETER IS XXXX; EVERYTHING ELSE IS THE SAME.' BECAUSE OF MY ATTN BEING DIRECTED TO THE SLOW GYRO; LOOKING AT THE OPTIONS; AND DECIDING ON A COURSE OF ACTION BOTH WITH THE GYRO OPERATIONAL AND IF IT SHOULD FAIL; I WAS NOT REALLY 'LISTENING' TO THE CTLR. THEN; WHEN I HEARD THE ALTIMETER SETTING AND THE PHRASE THAT 'EVERYTHING ELSE IS THE SAME;' I SET THE ALTIMETER AND PRESSED ON. ONLY AFTER EVERYTHING WAS STABLE AND ROUTINE DID I PLAY BACK THE PRECEDING EVENTS AND REALIZE I HAD MISSED THE DECREASE IN VISIBILITY; WHICH WAS BELOW OUR TKOF MINIMUMS.