Narrative:

During climb; a non-revenue pilot in the cabin called on the interphone to report visible fluid leaking from the right wing near the aileron. We monitored the flight instruments and over 20 minutes noticed a 420 pound imbalance between left and right fuel tanks. I handed control of the aircraft and radios to the first officer while I referenced the QRH. The QRH imbalance checklist was referenced; but not exactly applicable to our situation; which was a suspected fuel leak from the wing tank. Due to the ACARS being MEL'd; we called dispatch via radio to discuss the situation. Although we offered to continue on as long as the rate of the leak did not increase; dispatch and maintenance control elected to have us bring the aircraft to ZZZ. We notified the flight attendants and passengers. We coordinated with ATC and made an otherwise uneventful landing in ZZZ. We did not declare an emergency. Maintenance met us at the gate and I made a logbook entry and called dispatch; maintenance control; and the chief pilot on call.it is unusual to receive a call from the cabin from a non-revenue pilot; so I made sure to pay attention to what he was saying. My initial thought was it was probably just some trapped water being released into the slip stream of the wing. However; after monitoring the fuel quantities for 20 minutes; it was apparent we might have a problem. Thus; my only idea for improvement is to encourage all pilots; whether up front at the controls or riding in the back: if you see something that doesn't look right; bring it to the attention of the crew. I am thankful the non-rev pilot decided to inform us.

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Original NASA ASRS Text

Title: B737 Captain reported being informed of a fuel leak by a non-revenue pilot calling from the cabin. Captain reported diverting after noticing a 420 pound imbalance between the left and right fuel tanks.

Narrative: During climb; a Non-revenue Pilot in the cabin called on the interphone to report visible fluid leaking from the right wing near the aileron. We monitored the flight instruments and over 20 minutes noticed a 420 pound imbalance between left and right fuel tanks. I handed control of the aircraft and radios to the First Officer while I referenced the QRH. The QRH Imbalance Checklist was referenced; but not exactly applicable to our situation; which was a suspected fuel leak from the wing tank. Due to the ACARS being MEL'd; we called Dispatch via radio to discuss the situation. Although we offered to continue on as long as the rate of the leak did not increase; Dispatch and Maintenance Control elected to have us bring the aircraft to ZZZ. We notified the Flight Attendants and Passengers. We coordinated with ATC and made an otherwise uneventful landing in ZZZ. We did not declare an emergency. Maintenance met us at the gate and I made a logbook entry and called Dispatch; Maintenance Control; and the Chief Pilot on Call.It is unusual to receive a call from the cabin from a Non-revenue Pilot; so I made sure to pay attention to what he was saying. My initial thought was it was probably just some trapped water being released into the slip stream of the wing. However; after monitoring the fuel quantities for 20 minutes; it was apparent we might have a problem. Thus; my only idea for improvement is to encourage all Pilots; whether up front at the controls or riding in the back: If you see something that doesn't look right; bring it to the attention of the Crew. I am thankful the Non-rev Pilot decided to inform us.

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.