A B727 FREIGHTER CREW PERFORMS A TKOF WHEN VISIBILITY MINIMUMS ARE LESS THAN THE RWY ENVIRONMENT ALLOWS. 2 CREW MEMBERS EXHIBITED CONCERN REGARDING THEIR COMPANY'S REACTION IF THEY DID NOT PERFORM A TKOF.

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WHEN WE ARRIVED AT GRR THE WX WAS VMC. AFTER BEING ON THE GND FOR ABOUT 40 MINS; WE PUSHED BACK AND THE WX HAD DROPPED TO LESS THAN 1 MI VISIBILITY. ON TAXI OUT; WE WERE ADVISED THAT THE VISIBILITY WAS 1/4 MI AND THE RVR WAS 1200 FT IN THE TOUCHDOWN ZONE; 1000 FT ROLLOUT. IT WAS MY UNDERSTANDING OF OUR OPS SPECS THAT A TKOF WAS ALLOWED WITH 1200 FT RVR TOUCHDOWN ZONE AND 1000 FT ROLLOUT AS LONG AS RWY MARKINGS OR LIGHTING PROVIDED ADEQUATE REF TO IDENT THE TKOF SURFACE TO MAINTAIN DIRECTIONAL CTL ON THE CTR OF THE RWY DURING THE TKOF RUN. THAT BEING THE CASE; I DETERMINED THAT WE COULD MAKE A LEGAL AND SAFE DEP AND THE TKOF WAS MADE WITHOUT INCIDENT. AS A CREW; WE DECIDED TO LOOK UP THE SPECIFIC LANGUAGE IN OPS SPECS AND IT APPEARS THAT THE REF TO TKOF MINIMUMS BELOW STANDARD IN PARAGRAPH C56.C(1)(D) WHICH CONTAINS THE AFOREMENTIONED VERBIAGE APPLIES TO RVR'S OF 1600 FT OR GREATER. C56.C(2) REQUIRES CTRLINE LIGHTS. WE WERE SURPRISED BY HOW QUICKLY THE VISIBILITY DETERIORATED AND THAT WE HADN'T NOTICED IT UNTIL AFTER WE HAD BEEN PUSHED BACK AND WERE READY FOR TAXI. AT THIS POINT THE QUESTION WAS ASKED AS TO OUR MINIMUMS REQUIREMENT PER OUR OPS SPECS. THE DISCUSSION WAS BRIEF AND WE WERE CERTAIN THAT OUR INTERP WAS CORRECT. ONLY AFTER THE BUSINESS OF ACCOMPLISHING THE DEP DID WE DECIDE TO CONSULT THE MANUAL DIRECTLY. HUMAN PERFORMANCE CONSIDERATIONS: THIS OCCURRED 9 1/2 HRS INTO OUR DUTY DAY AFTER 5 1/2 HRS OF FLT AT A TIME WHEN WE ANTICIPATED THAT WE WOULD BE FINISHED FOR THE DAY AND BE SENT TO THE HOTEL. WHEN WE ARRIVED AT GRR TO FIND OUT THAT WE WERE INDEED NOT FINISHED; THE NEWS WAS GREETED NEGATIVELY. WHAT WE SHOULD HAVE DONE WAS TO PARK IT ON THE APRON AND CONSULTED THE MANUAL IMMEDIATELY. FACTORS CONTRIBUTING TO THE DECISION TO GO WERE THE BELIEF THAT WE KNEW THE ANSWER TO THE QUESTION; A DESIRE TO FINISH UP AND GET TO BED; AND TO AVOID A FLT WITH OPS OVER THE GO/NO-GO DECISION WHICH INVARIABLY CONTAINS THE THREAT OF BEING FIRED FOR REFUSING TO TAKE THE TRIP. SUPPLEMENTAL INFO FROM ACN 402227: WE SHOULD HAVE PARKED THE PLANE BUT WITH WX DETERIORATING RAPIDLY AND THE POSSIBLE CONFRONTATION WITH THE COMPANY WE CONTINUED. I SHOULD HAVE SPOKEN UP AND CONSULTED MANUAL; INSTEAD OF RELYING ON THE FRONT SEATS' KNOWLEDGE AND EXPERIENCE.