Narrative:

When number two for departure with light snow; fog; and limited visibility the cabin crew hit the alert chimes multiple times. I answered the interphone and F/a (flight attendant) told us that the left wing was on fire. He further stated that a passenger had informed him of this and he too saw a fire. I had no indications of a left engine fire; I saw no evidence of a fire out of the window; but out of an abundance of caution I shut down the left engine. I called ground control and requested the fire trucks telling him we had a report of a fire on the left wing. I asked the tower to tell me if they saw anything. They could not see us due to the poor visibility.since I could not see the fire; I asked the F/a to tell me exactly where the fire was. He said that the wingtip was on fire and that smoke was pouring out of where the fire was. I looked carefully at the left wingtip. The red position light appeared as if it were a flame; due to the blowing snow and fog. Furthermore it appeared as if white smoke was coming off the position light. I turned off the light and asked the cabin crew whether there was still a fire? They said 'no'. I turned the position light on and they reported a fire again. Off; and they said the fire was out.I have to say that it did look like a flame and smoke. It was obviously the position light but there was a possibility that it was an electrical fire that popped up each time the light came on. Although that was unlikely; I felt it had to check it out. I; of course; kept the customers informed once I was pretty sure what the situation actually was. They remained calm. After twice asking the status of the fire truck; eventually one truck meandered out and parked 200 feet in front of the aircraft and sat there doing nothing. I instructed (we had early on confirmed with ground that the truck would be on ground frequency) the fire crew precisely what to check. They eventually got out of the truck pulled on their gear and wandered over to the wingtip with an ir (infrared) camera.I turned on and off the position light and they confirmed it was all normal and there was no heat (ir camera) and no flame/smoke; although it still appeared as a fire to the cabin. I thanked the fire crew and contacted dispatch. I told my dispatcher the situation and told him I thought we should go. He agreed. In coordination with the cabin crew; I went to the back; talked to the passengers who saw the flame and told them exactly what was going on. We then re-did our checklists; got new data for brake condition 2; and did the departure plan checklist. We then departed uneventfully approximately 40 minutes late.it really did look like a fire; especially to untrained eyes. I have to commend the cabin crew for keeping me informed and the passengers calm. They did the right thing. Unfortunately it caused a big delay.

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Original NASA ASRS Text

Title: B737 Captain reported being informed of a possible fire on the wingtip by a Flight Attendant which turned out to be an optical illusion from the red position light due to the weather.

Narrative: When number two for departure with light snow; fog; and limited visibility the cabin crew hit the alert chimes multiple times. I answered the interphone and F/A (Flight Attendant) told us that the left wing was on fire. He further stated that a passenger had informed him of this and he too saw a fire. I had no indications of a left engine fire; I saw no evidence of a fire out of the window; but out of an abundance of caution I shut down the Left engine. I called Ground Control and requested the fire trucks telling him we had a report of a fire on the left wing. I asked the Tower to tell me if they saw anything. They could not see us due to the poor visibility.Since I could not see the fire; I asked the F/A to tell me exactly where the fire was. He said that the wingtip was on fire and that smoke was pouring out of where the fire was. I looked carefully at the left wingtip. The red position light appeared as if it were a flame; due to the blowing snow and fog. Furthermore it appeared as if white smoke was coming off the position light. I turned off the light and asked the cabin crew whether there was still a fire? They said 'No'. I turned the position light on and they reported a fire again. Off; and they said the fire was out.I have to say that it did look like a flame and smoke. It was obviously the position light but there was a possibility that it was an electrical fire that popped up each time the light came on. Although that was unlikely; I felt it had to check it out. I; of course; kept the customers informed once I was pretty sure what the situation actually was. They remained calm. After twice asking the status of the fire truck; eventually one truck meandered out and parked 200 feet in front of the aircraft and sat there doing nothing. I instructed (we had early on confirmed with Ground that the truck would be on Ground frequency) the fire crew precisely what to check. They eventually got out of the truck pulled on their gear and wandered over to the wingtip with an IR (Infrared) Camera.I turned on and off the position light and they confirmed it was all normal and there was no heat (IR Camera) and no flame/smoke; although it still appeared as a fire to the cabin. I thanked the fire crew and contacted Dispatch. I told my Dispatcher the situation and told him I thought we should go. He agreed. In coordination with the cabin crew; I went to the back; talked to the passengers who saw the flame and told them exactly what was going on. We then re-did our checklists; got new data for Brake Condition 2; and did the Departure Plan Checklist. We then departed uneventfully approximately 40 minutes late.It really did look like a fire; especially to untrained eyes. I have to commend the cabin crew for keeping me informed and the passengers calm. They did the right thing. Unfortunately it caused a big delay.

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.