Date: 201511
Local time of day: 0001-0600
Location: SEA.Airport, WA
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Training / Qualification
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Detector: Person Observer
When detected: Taxi
Result: Flight Crew Returned To Gate
Contributing factors: Human Factors; Weather; Procedure
Primary problem: Human Factors

Narrative: Ice check inadequate; returned to gate for de-ice and re-inspection.Prior to push; requested Ice Check from maintenance per company policy. This check was provided by maintenance at scheduled ETD. Pilots observed maintenance technician with a wand and ladder proceeding back to the left wing for the check.   He returned back from the area of the right wing about 5 minutes later.  He informed us personally; as did line maintenance via radio; that the ice check was complete and wings were clean of contaminants.  We re-started the departure process with that information. However; during taxi-out we were informed by a deadheading (DH) pilot that the wings had frost and contamination. We immediately stopped. After conversing with the DH pilot over inter phone; we returned to the gate and were de-iced prior to departure according to company policy.I attempted to contact line Maintenance by phone for an explanation; but was unable to get adequate information.  I felt continued pursuit of how ice check was accomplished at that point was unnecessary and a potential conflict with safety as a distraction; so I terminated my attempt and returned to managing the current flight sequence.  So now I really don't know how the wing was checked during that ice check and why contaminant was missed and relayed as clear.Not having a chance to talk to the maintenance tech who checked the wings; I assume he accomplished a visual inspection rather than a tactile inspection.  With no falling precipitation a visual inspection is all that is required; so even though he had the ladder and wand; possibly he didn't use them.  For my purposes; I had full confidence in the check since I saw the ladder and wand taken to both sides; for the expected amount of time; (5 minutes approx.); and that a tactile check was accomplished.  After seeing the picture of the wing; that is unlikely.  It is possible only a visual check was done from behind the wing that could not see the necessary surfaces.  Bottom line; I don't know why or how it was missed and I wish I did.My suggestion is that an upper wing check is required whenever an ice check is called for.  It seems to me that there are flaws in seeing the upper surface from the ground as the sole means of inspection.




Synopsis: A B737 Captain reported returning to the gate for de-ice when a deadheading pilot in the cabin noticed the wings were not clear. The reporter expressed concern that the pre-departure Ice check was not performed properly by ground personnel.