Narrative:

While in cruise we received a call from a flight attendant stating a passenger observed a flame coming from the left wing. We had the flight attendants up front secure the area outside the cockpit allowing me to investigate the situation. The passenger was sitting directly over the left wing and observed what she thought was a very small fire. At first glance it actually did have the appearance of a low flame; however after taking a closer look I determined it was most likely residue from the type I fluid used which is orange in color. The airflow over the wing was causing the fluid to ripple giving it the appearance of an orange flicker. This was also an early morning flight heading south toward ZZZ so the left wing was directly in the path of the rising sun from the east adding to the intensity of the orange glow. I had the captain turn away from the sun to see if the illusion would remain and to my surprise it did not change. I still felt the bright orange color of the type I fluid was giving the appearance of the flame; however I originally thought the sun shining through it was causing it to be as bright as it was. When the intensity did not change after turning away from the sun I would have to admit it caused me to question my theory.while I was in the back of the aircraft the captain noticed the fuel in the left side was quickly dropping. It is important to note that the aircraft was still burning out of the center tank. We were not yet in the tank to engine configuration. Also; the fuel flow was consistent around 2;500 lbs/hr per engine at cruise and in balance. Within a few minutes we were experiencing close to a 300 lb imbalance between the left and right engine. Normally; this wouldn't be much of an issue. Of course we see this at times when on the ground for an excessive period with the APU burning requiring the need to balance in-flight. This situation; however; was completely different in that the APU was not running; we were burning from the center tank and we were in balance just before the situation began. The captain was monitoring the engine parameters closely while I was in the back and noticed the quick change. At this point there were a few narratives; but they all led to the same conclusion which was to land. We felt strongly that the fluid was most likely type I; but could not explain the immediate loss of fuel from the left engine. We asked ourselves all the questions... Was there actually a leak or was fuel being moved through the common manifold somewhere else? Was it a pump pressure issue and were we 100% sure the it was just an illusion and not a flame. We were also already 30% through our 1;000 lb imbalance limitation with 2 hours remaining. With all these indications it was clear that some action had to be taken. We also realized another important variable was that we still had a healthy aircraft with 2 good engines and we were close to a good station. One with maintenance; facilities for our passengers and personnel to assist with any ticketing; rebooking; etc. It felt almost reckless not to take the safest path and land where we had incredible resources. If we had not had what we theorized was not a flame I believe we would have run the QRH for a suspected fuel leak all the way through. The issue with that is that any leak seemed to lead to an engine shutdown which in the end would still lead to landing. If we were able to isolate a leak we still weren't 100% positive about the fluid on the wing and felt we were in no position to jeopardize the safety of the passengers; crew and aircraft with a facility below us.the last variable was weather. This was the winter storm all through the north and northeast and weather moving through ZZZ. We actually were rerouted from our original arrival route to come over the west coast of florida because of thunderstorms. With all this in mind; we were in a phase of cruise which had us in clear skies for a short period of our flight. With overall safety of our passengers and the health of our aircraft ZZZ1 seemed like the best option.

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Original NASA ASRS Text

Title: B737-800 First Officer was informed by a Flight Attendant that a passenger believed there is a small flame on the left wing. The First Officer went back to investigate and thought it was sunlight on orange deicing fluid; but couldn't be sure. At the same time the Captain detects a possible fuel leak from the left wing and the crew elects to divert.

Narrative: While in cruise we received a call from a flight attendant stating a passenger observed a flame coming from the left wing. We had the flight attendants up front secure the area outside the cockpit allowing me to investigate the situation. The passenger was sitting directly over the left wing and observed what she thought was a very small fire. At first glance it actually did have the appearance of a low flame; however after taking a closer look I determined it was most likely residue from the type I fluid used which is orange in color. The airflow over the wing was causing the fluid to ripple giving it the appearance of an orange flicker. This was also an early morning flight heading South toward ZZZ so the left wing was directly in the path of the rising sun from the East adding to the intensity of the orange glow. I had the Captain turn away from the sun to see if the illusion would remain and to my surprise it did not change. I still felt the bright orange color of the type I fluid was giving the appearance of the flame; however I originally thought the sun shining through it was causing it to be as bright as it was. When the intensity did not change after turning away from the sun I would have to admit it caused me to question my theory.While I was in the back of the aircraft the Captain noticed the fuel in the left side was quickly dropping. It is important to note that the aircraft was still burning out of the center tank. We were not yet in the tank to engine configuration. Also; the fuel flow was consistent around 2;500 lbs/hr per engine at cruise and in balance. Within a few minutes we were experiencing close to a 300 lb imbalance between the left and right engine. Normally; this wouldn't be much of an issue. Of course we see this at times when on the ground for an excessive period with the APU burning requiring the need to balance in-flight. This situation; however; was completely different in that the APU was not running; we were burning from the center tank and we were in balance just before the situation began. The Captain was monitoring the engine parameters closely while I was in the back and noticed the quick change. At this point there were a few narratives; but they all led to the same conclusion which was to land. We felt strongly that the fluid was most likely type I; but could not explain the immediate loss of fuel from the left engine. We asked ourselves all the questions... Was there actually a leak or was fuel being moved through the common manifold somewhere else? Was it a pump pressure issue and were we 100% sure the it was just an illusion and not a flame. We were also already 30% through our 1;000 lb imbalance limitation with 2 hours remaining. With all these indications it was clear that some action had to be taken. We also realized another important variable was that we still had a healthy aircraft with 2 good engines and we were close to a good station. One with maintenance; facilities for our passengers and personnel to assist with any ticketing; rebooking; etc. It felt almost reckless not to take the safest path and land where we had incredible resources. If we had not had what we theorized was not a flame I believe we would have run the QRH for a suspected fuel leak all the way through. The issue with that is that any leak seemed to lead to an engine shutdown which in the end would still lead to landing. If we were able to isolate a leak we still weren't 100% positive about the fluid on the wing and felt we were in no position to jeopardize the safety of the passengers; crew and aircraft with a facility below us.The last variable was weather. This was the winter storm all through the North and Northeast and weather moving through ZZZ. We actually were rerouted from our original arrival route to come over the west coast of Florida because of thunderstorms. With all this in mind; we were in a phase of cruise which had us in clear skies for a short period of our flight. With overall safety of our passengers and the health of our aircraft ZZZ1 seemed like the best option.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.