Narrative:

We had a slight change to our initial departure but this was minimal. We were flying at an initial altitude of fl 350 and going slightly faster than flight plan to make up some of the time. Approaching [our next cruise] intersection we received an FMC message fuel disagree-prog 2. Looking at the page we discovered a discrepancy between the calculated and totalizer fuel reading. I checked the QRH and did not see a listing for fuel disagree-prog 2 in either the index at the front or the index in the fuel section. The totalizer appeared to be the correct amount and I selected that option. At [the intersection] we discovered a 4000# difference between the [calculated] expected fuel and what we had on board. I sent a message to dispatch via ACARS letting her know this. She replied asking if we were flying the flight plan altitude and speed. I replied that were we at flight plan altitude and going slightly faster .80 vs flight plan of .78 mach but that wouldn't account for that large of a discrepancy. She agreed and re-ran the flt plan since our ZFW was down 6000# from the original and it showed us landing with 13.9 of fuel vs the 16.8 on the original fp. This was more in line with what we were seeing. We continued to monitor the fuel and were seeing slightly more fuel used vs the current burn rate. Approx 28 minutes later at the next point we were now down approx 5700# from forecast. Around this time; we had used all of the fuel in the center tanks and had turned off the center tank fuel pumps and were tank to engine. Concerned that we might have a fuel leak; we again brought out the QRH and began reviewing the fuel leak section. It was then that we discovered on 120.3 of the QRH the fuel disagree-prog 2 in the fuel leak section and proceeded with that checklist. At this point we were seeing only a slight disagreement between the 2 main fuel tanks. I decided to contact dispatch via phone patch. [Maintenance control] and the [chief pilot] were both also brought in. I transferred control of the aircraft as well as the radios to the first officer (first officer). While talking with all 3 we began to see a steady decrease in the fuel in the left main tank as opposed to the right. After approximately 30 minutes we had a 1500# difference between the tanks (by the time we landed this had increased to approx 2500# difference). We were also seeing a steady decrease in our expected landing fuel in the FMC. I directed the first officer to call to the back and ask the flight attendant's (flight attendants) to check behind both wings for anything unusual such as vapor or spray coming off of the wings or from the engines. They reported back that it appeared that there was some type vapor coming from the left engine/wing and nothing on the right side. At this point; suspecting a possible fuel leak; I informed all 3 parties ([dispatch; maintenance control; and the chief pilot]) that we were going [divert]. ZZZ was the nearest suitable airport approx 80 miles off our right wing. They were all in agreement. I directed the first officer to [advise] ATC and start coordinating with them for a divert to ZZZ. ATC turned us direct to ZZZ. I ended the call with dispatch with her telling me they would begin coordinating with the ZZZ station to prepare for our arrival. Throughout the entire phone patch we kept having repeated issues with the transmission cutting out and them not being able to hear me or vice versa. This required both parties to have repeat what had been previously said and increased the length of the call.I began getting everything set up for the divert to ZZZ and then briefed the flight attendant's telling them what was going on; approx time to landing and to review the prep for evac but that I anticipated a normal landing. I then told the passengers what was going on. While I was doing this the first officer was getting ATIS and landing data for ZZZ; setting up the FMC and talking with ATC. We briefed the approach and then I took back control of the aircraft as the flying pilot. We continued our descent into ZZZ and were vectored for a visual approach to xxl. We had a normal; smooth touchdown and taxied clear of the runway. It took approx 15 minutes from [deciding to divert] to touching down in ZZZ.emergency vehicles and personnel were standing by and followed us to our stopping point. They reported seeing and smelling fuel coming from the left engine. We shut down the left engine; turned off the left main fuel pumps and started the APU. Emergency personnel still reported seeing fuel coming from the engine and since the APU running had caused one of the left main fuel pumps to turn back on we shut the APU back down. It was determined that they would tow us to the ramp and deplane the passengers there. The fuel leak had slowed considerably. We again started the APU and shut down the right engine. While this was going on we began to smell fuel in the cockpit and were getting reports that they were smelling fuel in the cabin as well. The flight attendant's asked for permission to open a door or two to ventilate the cabin. I directed them to disarm and crack the forward and aft doors on the right side of the aircraft and to keep a flight attendant posted by both doors. I made a PA announcement to the passengers asking them to remain seated and not go near either of the doors. Once we were ready for the tow; the open doors were closed and re-armed and we were towed to a position on the north ramp where an air stairs and buses were waiting for the passengers. With the fuel leak on the left side; I directed that the passengers be deplaned from door 1R and that the remaining doors remain armed in case an evacuation became necessary. Once the last of the passengers were off the aircraft; all remaining doors were disarmed. The passengers and flight attendant's were transported to the terminal to await a rescue flight. We had no reports of any injuries to the passengers or crew. The first officer and I remained at the aircraft to assist in repositioning the plane to another spot where maintenance would be able to work on the aircraft. We finally finished [late at night]. We proceeded with the station manager to the terminal and secured hotel rooms for the night. Once at the hotel; I began making phone calls to dispatch; [maintenance control; and the chief pilot] to brief them on the [situation]. As an FYI; a human factors checklist was not run on either of us.

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Original NASA ASRS Text

Title: B757 Captain reported diverting to an alternate airport after experiencing a fuel leak during cruise.

Narrative: We had a slight change to our initial departure but this was minimal. We were flying at an initial altitude of FL 350 and going slightly faster than flight plan to make up some of the time. Approaching [our next cruise] intersection we received an FMC message FUEL DISAGREE-PROG 2. Looking at the page we discovered a discrepancy between the calculated and totalizer fuel reading. I checked the QRH and did not see a listing for FUEL DISAGREE-PROG 2 in either the index at the front or the index in the fuel section. The totalizer appeared to be the correct amount and I selected that option. At [the intersection] we discovered a 4000# difference between the [calculated] expected fuel and what we had on board. I sent a message to Dispatch via ACARS letting her know this. She replied asking if we were flying the flight plan altitude and speed. I replied that were we at flight plan altitude and going slightly faster .80 vs flight plan of .78 mach but that wouldn't account for that large of a discrepancy. She agreed and re-ran the Flt plan since our ZFW was down 6000# from the original and it showed us landing with 13.9 of fuel vs the 16.8 on the original FP. This was more in line with what we were seeing. We continued to monitor the fuel and were seeing slightly more fuel used vs the current burn rate. Approx 28 minutes later at the next point we were now down approx 5700# from forecast. Around this time; we had used all of the fuel in the center tanks and had turned off the center tank fuel pumps and were tank to engine. Concerned that we might have a fuel leak; we again brought out the QRH and began reviewing the Fuel Leak section. It was then that we discovered on 120.3 of the QRH the FUEL DISAGREE-PROG 2 in the Fuel Leak section and proceeded with that checklist. At this point we were seeing only a slight disagreement between the 2 main fuel tanks. I decided to contact Dispatch via phone patch. [Maintenance Control] and the [Chief Pilot] were both also brought in. I transferred control of the aircraft as well as the radios to the FO (First Officer). While talking with all 3 we began to see a steady decrease in the fuel in the left main tank as opposed to the right. After approximately 30 minutes we had a 1500# difference between the tanks (by the time we landed this had increased to approx 2500# difference). We were also seeing a steady decrease in our expected landing fuel in the FMC. I directed the FO to call to the back and ask the FA's (Flight Attendants) to check behind both wings for anything unusual such as vapor or spray coming off of the wings or from the engines. They reported back that it appeared that there was some type vapor coming from the left engine/wing and nothing on the right side. At this point; suspecting a possible fuel leak; I informed all 3 parties ([Dispatch; Maintenance Control; and the Chief Pilot]) that we were going [divert]. ZZZ was the nearest suitable airport approx 80 miles off our right wing. They were all in agreement. I directed the FO to [advise] ATC and start coordinating with them for a divert to ZZZ. ATC turned us direct to ZZZ. I ended the call with Dispatch with her telling me they would begin coordinating with the ZZZ station to prepare for our arrival. Throughout the entire phone patch we kept having repeated issues with the transmission cutting out and them not being able to hear me or vice versa. This required both parties to have repeat what had been previously said and increased the length of the call.I began getting everything set up for the divert to ZZZ and then briefed the FA's telling them what was going on; approx time to landing and to review the Prep for Evac but that I anticipated a normal landing. I then told the passengers what was going on. While I was doing this the FO was getting ATIS and landing data for ZZZ; setting up the FMC and talking with ATC. We briefed the approach and then I took back control of the aircraft as the flying pilot. We continued our descent into ZZZ and were vectored for a visual approach to XXL. We had a normal; smooth touchdown and taxied clear of the runway. It took approx 15 minutes from [deciding to divert] to touching down in ZZZ.Emergency vehicles and personnel were standing by and followed us to our stopping point. They reported seeing and smelling fuel coming from the left engine. We shut down the left engine; turned off the left main fuel pumps and started the APU. Emergency personnel still reported seeing fuel coming from the engine and since the APU running had caused one of the left main fuel pumps to turn back on we shut the APU back down. It was determined that they would tow us to the ramp and deplane the passengers there. The fuel leak had slowed considerably. We again started the APU and shut down the right engine. While this was going on we began to smell fuel in the cockpit and were getting reports that they were smelling fuel in the cabin as well. The FA's asked for permission to open a door or two to ventilate the cabin. I directed them to disarm and crack the forward and aft doors on the right side of the aircraft and to keep a FA posted by both doors. I made a PA announcement to the passengers asking them to remain seated and not go near either of the doors. Once we were ready for the tow; the open doors were closed and re-armed and we were towed to a position on the North ramp where an air stairs and buses were waiting for the passengers. With the fuel leak on the left side; I directed that the passengers be deplaned from door 1R and that the remaining doors remain armed in case an evacuation became necessary. Once the last of the passengers were off the aircraft; all remaining doors were disarmed. The passengers and FA's were transported to the terminal to await a rescue flight. We had no reports of any injuries to the passengers or crew. The FO and I remained at the aircraft to assist in repositioning the plane to another spot where maintenance would be able to work on the aircraft. We finally finished [late at night]. We proceeded with the Station Manager to the terminal and secured hotel rooms for the night. Once at the hotel; I began making phone calls to Dispatch; [Maintenance Control; and the Chief Pilot] to brief them on the [situation]. As an FYI; a Human Factors Checklist was not run on either of 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.