A B747 MAKES A FUEL STOP AT ANCHORAGE ENRTE TO NARITA. IT ARRIVES AT ANC WITH A LARGE FUEL LOAD; 112000 LBS. AFTER REFUELING IN A LIGHT RAIN THE ACFT DEPARTS. NEAR 14000 FT IN CLB A CABIN ATTENDANT ADVISES THE COCKPIT THAT TWO LARGE PATCHES OF ICE 10 FT BY 15 FT ARE NOTED ON THE TOP PANELS OF THE WINGS. ICE HAD FORMED OVER THE FUEL TANKS. FLT WAS CONTINUED.

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OUR DUTY DAY BEGAN UPON RPTING TO THE FLT PLANNING ROOM AT DETROIT METRO WAYNE INTL ARPT. WE WERE SCHEDULED TO FLY NON-STOP TO TOKYO'S NARITA INTL WITH AN AUGMENTED FLT DECK CREW CONSISTING OF 2 CAPTS; 1 FO AND 2 SO'S. SHORTLY AFTER STARTING THE FLT PLANNING PROCESS; AN OPS AGENT INFORMED US THE COMPANY HAD OPTED FOR AN ANCHORAGE FUEL STOP IN ORDER TO ACCOMMODATE PAYLOAD. WE DEPARTED DETROIT WITH A RELATIVELY LARGE FUEL LOAD AND ARRIVED AT ANCHORAGE WITH 112000 LBS REMAINING; WHICH IS NEARLY 1/3 OF THE ACFT'S CAPACITY. THE WX ON ARR WAS OVCST; TEMP 40 DEGS F; AND LIGHT RAIN. WE SPENT 41 MINS AT THE GATE REFUELING; FLT PLANNING AND ACCOMPLISHING PREFLT DUTIES. THE OTHER CAPT OCCUPIED THE L SEAT ON THE FIRST LEG AND I TOOK COMMAND OF THE ACFT FOR THE LEG TO NARITA. ALL 5 PLT CREW MEMBERS WERE SEATED ON THE FLT DECK DURING THE DEP. AS WE CLBED THROUGH ABOUT 14000 FT A FLT ATTENDANT ENTERED THE FLT DECK AND ADVISED US THE WINGS HAD ICE ON THEM. THE OFF-DUTY CAPT WENT DOWNSTAIRS TO HAVE A LOOK. HE RETURNED A FEW MINS LATER AND RPTED THERE WERE TWO SMOOTH PATCHES OF ICE ON THE TOP OF EACH WING; EACH MEASURING ABOUT 10 FT BY 15 FT WITH A THICKNESS OF 1/4- 1/2 OF AN INCH. THE SO WHO ACCOMPLISHED THE EXTERIOR PREFLT ON THE GND IN ANCHORAGE STATED HE LOOKED AT THE L WING FROM THE TOP OF THE LOADING BRIDGE STAIRS AND OBSERVED IT TO BE CLEAN. HOPEFULLY THIS WAS THE CASE; BUT I THINK THE ICE MAY HAVE ACCUMULATED DURING OUR REFUELING STOP AND ESCAPED DETECTION DUE TO A POOR VIEWING ANGLE. THE UNIQUE SET OF CONDITIONS PRESENT AT THE TIME COULD MAKE THIS PLAUSIBLE. A LARGE VOLUME OF COLD-SOAKED WING TANK FUEL; COOL OUTSIDE AIR TEMP; LIGHT RAIN AND A SHORT GND STOP MAY HAVE COMBINED TO PERMIT A GRADUAL FORMATION OF ICE ON THE WINGS. THE FO MADE THE TKOF AND RPTED THE ACFT HANDLED NORMALLY IN ALL RESPECTS; SO WHETHER OR NOT ICE WAS PRESENT; ACFT PERFORMANCE WAS SATISFACTORY. HOWEVER; THIS IS NOT TO SAY I WOULD KNOWINGLY FLY AN ACFT IN THIS CONDITION AGAIN. DURING MY 20 YRS OF SCHEDULED AIRLINE FLYING; I HAVE NEVER SEEN ICE FORM ON THE TOP OF A WING WHEN THE TEMP WAS 40 DEGS F. FURTHERMORE; NONE OF THE FOUR OTHER HIGHLY TRAINED AND EXPERIENCED AVIATORS INVOLVED HAD EVER ENCOUNTERED THIS PHENOMENON BEFORE. WE ALL LEARNED A VALUABLE LESSON THAT DAY. IF I AM FACED WITH A SIT LIKE THIS AGAIN; I'LL HAVE THE DEICING EQUIP STANDING BY FOR OUR ARR.