Narrative:

Flight attendant called to report a small flame coming out of the trailing edge of left inboard flap pylon. We suspected de-ice fluid that was applied in [departure airport] that was reflecting in the sunlight. Had first officer go back and a look at fluid. He called back to the cockpit to report that it looks suspicious and could very well be a leak of some sort. With permission from ATC; we had made a series of turns to see if the angle of the sun would make a difference on the appearance of the fluid but it did not. We were burning out of the center tanks and we were not getting any fuel discrepancies at that time. Just prior to the first officer coming back to the cockpit I noticed the left main tank fuel quantity reducing even while there was fuel in center tank and center pumps on. With the combination of that indication and the report of something leaking out of the aft part of the left wing we determined that we may have a fuel leak. Considering an imbalance is not uncommon; a 300 pound imbalance happened very quickly and continuing on would potentially have us at a larger imbalance. Using the QRH lead us to an engine shut down. We concluded that any reason for shutting down the engine would have led us to the same conclusion; diverting. In the interest of safety and being close to [an alternate] airport which had good maintenance as well as great customer service representatives to handle our customers; we [advised ATC] and landed without incident. We had sent a message via ACARS to dispatch in regards to our divert but dispatch did not receive the message. This we are assuming was due to the ACARS not sending all messages as was noticed in a previous write up 2 days earlier. If we only had the de-ice fluid or just the fuel anomaly then I would have pressed on and analyzed the situation a little further. However; since we had both issues happening at the same time on the same side with 2 hours to go till we land in ZZZ we thought it was better to safely land at [the alternate] and have [maintenance] look at the issue.

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Original NASA ASRS Text

Title: B737-800 Captain reported diverting to an alternate for a possible fuel leak.

Narrative: FA called to report a small flame coming out of the trailing edge of left inboard flap pylon. We suspected de-ice fluid that was applied in [departure airport] that was reflecting in the sunlight. Had FO go back and a look at fluid. He called back to the cockpit to report that it looks suspicious and could very well be a leak of some sort. With permission from ATC; we had made a series of turns to see if the angle of the sun would make a difference on the appearance of the fluid but it did not. We were burning out of the center tanks and we were not getting any fuel discrepancies at that time. Just prior to the FO coming back to the cockpit I noticed the left main tank fuel quantity reducing even while there was fuel in center tank and center pumps on. With the combination of that indication and the report of something leaking out of the aft part of the left wing we determined that we may have a fuel leak. Considering an imbalance is not uncommon; a 300 pound imbalance happened very quickly and continuing on would potentially have us at a larger imbalance. Using the QRH lead us to an engine shut down. We concluded that any reason for shutting down the engine would have led us to the same conclusion; diverting. In the interest of safety and being close to [an alternate] airport which had good maintenance as well as great customer service representatives to handle our customers; we [advised ATC] and landed without incident. We had sent a message via ACARS to dispatch in regards to our divert but dispatch did not receive the message. This we are assuming was due to the ACARS not sending all messages as was noticed in a previous write up 2 days earlier. If we only had the de-ice fluid or just the fuel anomaly then I would have pressed on and analyzed the situation a little further. However; since we had both issues happening at the same time on the same side with 2 hours to go till we land in ZZZ we thought it was better to safely land at [the alternate] and have [Maintenance] look at the issue.

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.