B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Troubleshooting
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Aircraft; Human Factors; Manuals; Weather
I showed up at the gate to a jet that was parked overnight with no visible signs of frost or ice on the wings or fuselage. the Captain and I talked about my walk around and if I saw any ice on the underside of the wings; fuselage and tail. The only ice was a small thin line of ice under the wing root of the right wing that when I touched it; it slid off as it was melting. The weather was overcast at about 3000 feet; visibility was greater than 10; winds out of the north and just above freezing. I did the walk around and told the Captain what I saw. We both agreed that since no ice was present on the wings fuselage or tail that we did not need to deice. We configured the aircraft and began to taxi. As we were taxing the Captain got a call from the cabin from a pilot that asked if we noticed the ice. He didn't say where; and we just assumed he was talking about what I saw earlier. We said we saw it and he didn't say another word and hung up. Takeoff cruise and landing were uneventful. After taxi in the deadheading pilot said the ice was under the slats and had some choice words about our decision to take off. The weather the day before was light snow; and the plane had sat for a few days. I had never considered there would be ice under the slats; so I didn't consider moving them prior to my walk around. There was no circumstantial evidence for me to even consider the above facts. Nor even deicing. In the future; if it is near or below freezing with known previous weather phenomena and a jet that has sat overnight or longer; I will drop the slats and do a further investigation prior to declining to deice. And once a Deadheading pilot says something; I should be more proactive in getting the data the that pilot has on hand. I do not think any additional training is needed; just another lesson learned.
B737 First Officer reported a deadheading pilot advised that there was ice on the aircraft prior to departure; but the crew took off anyway.