ENVIRONMENTAL ICE FORMATIONS REQUIRE A B737-700 TO BE TOWED BACK INTO THE GATE FOR DEICING AT SEA; WA.

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DURING PUSHBACK; A FLT ATTENDANT CHIMED THE FLT DECK TO RPT AN OBSERVATION BY A PAX WHO SPOTTED ICE ON THE WING. THE ACFT WAS IMMEDIATELY TOWED BACK TO THE GATE. A WING INSPECTION FOLLOWED; CONDUCTED BY MAINT. THEY CONFIRMED ICE ON BOTH WINGS; WHICH WERE SUBSEQUENTLY DEICED. FOLLOW-UP INSPECTION CONFIRMED WINGS TO BE CLEAN. FLT WAS CONDUCTED WITHOUT FURTHER INCIDENT. AS THE PUSHBACK CREW TOWED US BACK TO THE GATE; I MADE AN INQUIRY TO SEATTLE OPS TO THE ACFT'S PRIOR LEG. IT WAS; THE CASE BECAME CLR; NON-ENVIRONMENTAL WING ICE. WX CONDITIONS AT SEATAC -- INTERMITTENT RAIN; 7 DEGS C; FUEL TANK TEMP WAS -2 DEGS C; WHICH I NOTED AS THE ACFT WAS BEING TOWED BACK. BOTH THE INTERIOR/EXTERIOR INSPECTION WERE MADE BY THE FO AT XL25 LCL; 45 MINS PRIOR TO SCHEDULED PUSH AND APPROX 30 MINS AFTER ACFT HAD BLOCKED IN. SPECIAL ATTN WAS GIVEN TO THE CONDITION OF THE TOP OF THE WINGS AS VIEWED FROM BOTH THE CABIN AND EXTERIOR WALKAROUND. THE WINGS WERE NOTED TO BE CLEAN. WING ICE DEVELOPED SOMETIME BTWN XL25 AND XM13 LCL; BUT WENT UNKNOWN TO THE FLC UNTIL A PAX SPOKE UP DURING THE PUSHBACK. GIVEN THE OUTSIDE WX CONDITIONS AND RESULTANT DISCOVERY; I MADE A PA TO THE PAX TO EXPLAIN NON-ENVIRONMENTAL WING ICE; IT'S PHENOMENON; AND HOW IT CAN FORM. AFTER WE LANDED IN ANC; I ASKED 1 OF THE FLT ATTENDANTS TO HAVE 1 OF THE PAX; WHO MADE THE DISCOVERY IN SEA; TO STOP BY TO CHAT ABOUT THE INCIDENT. WE SPOKE FOR ABOUT 5 MINS ABOUT WHAT HE SAW -- A PATCH OF ICE ON TOP OF THE WING NEAR THE #1 ENG. ANOTHER PAX COMMENTED SEEING WING ICE DURING THE BOARDING PROCESS; BUT FAILED TO RPT IT. THE ASSUMPTION WAS THAT WE HAD KNOWN ABOUT IT; AND WOULD DEICE DURING PUSHBACK (IF WE HAD; THEN WE WOULD HAVE). HE SPOKE UP WHEN IT BECAME CLR THAT WE HAD NOT PLANNED TO DEICE. I THANKED HIM FOR BEING FORTHCOMING WITH THE INFO; AND ASSURED HIM THIS INCIDENT WOULD BE RPTED TO NASA AND THE FAA. I ALSO EXPLAINED THAT WE WERE UNAWARE OF THE ICE DURING THE PUSHBACK. HAD WE KNOWN ABOUT IT THEN; WE WOULD HAVE DEICED EARLIER. THIS WING ICE PHENOMENON HAS BEEN KNOWN TO OCCUR ON THE B737 NG SERIES ACFT WITH THE CORRECT COMBINATIONS OF LCL WX AND WING TEMP. IMPROVED PROCS TO DETECT WING ICE SHOULD BE IMPLEMENTED. THESE INCLUDE A PHYSICAL INSPECTION OF THE UPPER PART OF WING PRIOR TO PUSHBACK FOR ALL FLTS BY A MECH OR OTHER QUALIFIED PERSONNEL.