B757-200. PAX CALLED TO THE FLT ATTENDANT'S ATTN; ICE FORMATION ADHERING TO THE UPPER WING SURFACE AT SLC.

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DURING INITIAL TAXI TO THE RWY; A FLT ATTENDANT CALLED THE COCKPIT TO ASK IF WE WERE PLANNING ON DEICING THE ACFT PRIOR TO TKOF. SHE STATED THE PAX SEATED IN SEAT XX OBSERVED SOME ICE ON THE TOP SURFACE OF THE WING. WE COORDINATED WITH GND CTL TO PULL OFF THE ACTIVE TXWY AND STOP THE ACFT. THE CAPT SENT ME BACK TO PERFORM A VISUAL INSPECTION OF THE WINGS FROM INSIDE THE CABIN. I CONFIRMED THERE WERE THIN; PATCHY AREAS OF ICE ADHERING TO THE TOP SURFACE OF BOTH WINGS. I RPTED THIS TO THE CAPT; AND WE IMMEDIATELY COORDINATED TO RETURN TO THE DEICE AREA TO HAVE THE ICE REMOVED. PRIOR TO THIS; WE WERE UNAWARE OF THE NEED TO DEICE THE ACFT THAT DAY. THE FORMATION OF ICE ON THE TOP SURFACE OF THE WINGS WAS A SURPRISE SINCE THE OUTSIDE AIR TEMP WAS WELL ABOVE FREEZING; IT WAS A PARTLY SUNNY AFTERNOON; AND THE ACFT HAD JUST FLOWN IN 2 HRS PRIOR. ALSO; THERE HAD BEEN NO PRECIP AT THE ARPT IN OVER 2 WKS! I REMEMBER THINKING AS I ARRIVED AT THE ACFT WHAT A NICE DAY IT WAS. THE ARPT WAS NOT RUNNING A DEICING OP FOR ANY OTHER ACFT; AND THERE WAS NO WAY DEICING WOULD BE REQUIRED. I PERFORMED THE EXTERIOR PREFLT INSPECTION AND DID OBSERVE A VERY THIN LAYER OF ICE ON THE BOTTOM SURFACE OF THE WINGS. HOWEVER; AT THE TOP OF THE JETWAY STAIRS; I COULD SEE NO INDICATION OF ICE ON THE TOP SURFACE OF THE WINGS. THIS IS ACCEPTABLE PER THE OPS MANUAL WITHOUT DEICING; AS LONG AS THE WING TANKS ARE NOT FULL OF COLD-SOAKED FUEL. AT THAT TIME; I DID NOT REALIZE THAT THE ACFT HAD ARRIVED WITH BOTH WING TANKS COMPLETELY FULL OF FUEL. I ASSUMED THEY HAD ARRIVED LOW ON FUEL AS THEY DO FOLLOWING MOST FLTS. JUST PRIOR TO PUSHBACK; WE RECEIVED THE FUEL SLIP AND THE WT AND BAL PAPERWORK. BY THE TIME I ANALYZED THE FUEL SLIP; THE SIGNIFICANCE OF THE WING TANKS ARRIVING FULL; RESULTING IN POSSIBLE COLD-SOAKED FUEL CONTACTING THE UPPER SURFACE OF THE WINGS AND FREEZING ANY CONDENSATION; DID NOT OCCUR TO ME. IN RETROSPECT; SEVERAL THINGS SHOULD HAVE TIPPED ME OFF TO THE NEED FOR DEICING THAT DAY. FIRST; SEEING THE THIN LAYER OF ICE ON THE UNDER SIDE SHOULD HAVE PROMPTED A CLOSER INSPECTION OF THE TOP SIDE OF THE WINGS FROM INSIDE THE CABIN. SECOND; I SHOULD HAVE COMPREHENDED THE SIGNIFICANCE OF AN ACFT ARRIVING WITH FULL WING FUEL TANKS WHEN THE OUTSIDE AIR TEMP IS LESS THAN 60 DEGS F. AND FINALLY; I SHALL NEVER AGAIN MAKE AN ASSUMPTION ABOUT THE NEED FOR DEICING BEFORE ARRIVING AT THE ACFT; NO MATTER HOW NICE THE WX IS.