Date: 201402
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Check Pilot; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Detector: Person Passenger
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Aircraft; Human Factors; Weather
Primary problem: Ambiguous

Narrative: I was deadheading on this flight.  Approximately 1 hour into the flight I was asked by a flight attendant if I would look at our wing because some passengers were concerned that it might have ice on it.  I walked back and confirmed that there was; in fact; ice on our right wing.  I appeared to be an area 2 to 3 feet wide and about 4 feet long.  The ice was wavy in appearance; as if water had frozen on the wing while moving along the surface slowly; and varied in thickness from zero to approximately 1/4' in thickness.  It appeared to be sublimating slowly at altitude.  I also checked the left wing; but was unable to determine if it had ice on it due to the low sun angle which put that wing in shade.  I decided that this was something the flight crew needed to know about; so I took photos of the wing.  I was in uniform; and identified myself as a check airman to the #1 Flight Attendant and asked permission to enter the flight deck to speak with the crew.   Once in the cockpit; I advised them of the situation and showed them the photos I took.  Although the ice looked like a classic 'cold shoulder' event from rain in contact with a cold soaked wing; the First Officer said that he was quite certain that there was no frost on the underside of the wing during his preflight.  I did not ask him if he checked the fuel tank temperature after fueling.  The Captain stated that there appeared to be no control issues on takeoff or climbout; but was considering dropping down a few thousand feet to obtain an even better buffet margin; and adding 5 knots or so to his approach speed for the same reason.  I felt that this was prudent; and with the ice sublimating and a landing temperature of 50 deg F; I felt that it would probably be gone by landing anyway.  Pressing on seemed the safest way to go to me.  I advised the crew that they should fill out a report; and I later emailed copies of the photos to the Captain for inclusion with his report.  I went back to the cabin to re-check the wings; but by this time it was too dark to see much of anything.  A couple of the passengers in the exit row were rather nervous; and I spent about 10 minutes answering their questions and calming their fears.  I remained in the cabin for the remainder of the flight; which was uneventful.  I'm at a loss to explain this.  I have never heard of ice being deposited on top of a wing like this in flight.  Yet the First Officer was adamant that he never saw any frost on the underside of the wing during pre-flight.  Perhaps there was no frost; but the wing tank remained below freezing after refueling and nobody caught it.  The cold rainy conditions that day were certainly conducive to this kind of ice on a cold soaked wing.




Synopsis: Deadheading Captain reports being asked to examine the right wing for ice while airborne. Clear ice is detected on the inboard portion and the flight crew is informed. Flight continues to destination where the air temperature is 50 degrees F. Rain on a cold soaked wing was thought to be the source of the ice.