FARS STIPULATE THAT NO PAX CARRYING ACFT WILL TKOF WITH EVIDENT ICE OR SNOW ON IT'S SURFACES. THIS LTT DID; UNKNOWINGLY.

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AFTER REFUELING OUR LTT MAIN FUEL TANKS COMPLETELY; WE TAXIED FOR TKOF. A WINTER TSTM WHICH WAS W OF THE ARPT WAS REACHING THE W EDGE OF THE RWY SYS. SO WE ELECTED TO TKOF TO THE S. AS WE STARTED TO TAXI; THE STRONG GUST FRONT BROUGHT SNOW FLURRIES. THE AIR TEMP HAD BEEN 39 DEGS F WHILE WE REFUELED; SO THE ACFT WAS WARM AND WE WERE NOT WORRIED ABOUT ICING FROM THE SNOW. WHEN WE REACHED THE END OF THE RWY; VISIBILITY HAD DROPPED TO 1/2 MI IN SNOW; AND WE DISCUSSED THE POSSIBILITY OF ICING AS THE TEMP DROPPED RAPIDLY. WE HAD TO WAIT MOMENTARILY WHILE ANOTHER ACFT LANDED; THEN WE WERE CLRED FOR TKOF. VISIBILITY HAD DROPPED TO 1/4 MI; WITHIN APPROVED TKOF MINS; AND ONLY 6 MINS HAD ELAPSED SINCE THE FIRST FLURRY STARTED. BOTH PLTS CHKED THE WINGS; AND WE CONCURRED THAT NO SNOW OR ICE HAD ADHERED AND WE WERE SAFE FOR TKOF SO WE DID. AFTER LNDG AT DEST; 2 OTHER COMPANY PLTS WHO HAD BEEN RIDING AS PAXS CAME FORWARD AND ASKED; DID WE KNOW THAT BOTH AILERONS HAD BEEN ICE-COVERED SINCE TKOF? OBVIOUSLY NOT; OR WE WOULD NOT HAVE TAKEN OFF -- THE PORTIONS OF THE WINGS WE COULD SEE WERE COMPLETELY ICE- FREE! THANKS GUYS. LESSON LEARNED -- THE PART OF THE WINGS VISIBLE FROM THE COCKPIT IS OUTBOARD OF THE ENGS. WHEN THE MAIN FUEL TANKS ARE FULL OF WARMER-THAN-FREEZING FUEL; THE FUEL KEEPS THE WING WARM AND SNOW AND ICE DO NOT STICK. THE AILERONS AND OTHER PARTS OF THE ACFT NOT WARMED BY FUEL COOL RAPIDLY TO AMBIENT TEMP BECAUSE ALUMINUM CHANGES TEMP QUICKLY. WITH A WARM WINDSHIELD AND WARM OUTER WING PANELS; THE VIEW FROM THE COCKPIT IS COZY WHILE THE REST OF THE AIRPLANE IS FROSTY. 6 MINS MADE A LOT OF DIFFERENCE; AND IF THE ACFT HAD BEEN COATED WITH DEICING SOLUTION BEFORE TAXI; THE ICE PROBABLY WOULD NOT HAVE ADHERED; EITHER. WE THOUGHT IT WAS A TSTM; WE REACTED ACCORDINGLY; AND WE WERE WRONG.