Narrative:

Supervised refuel at [FBO] support at ZZZ with an upload of 400 gallons. Total fuel load onboard 5;600 pounds; each wing had 1;400 pounds each; (2;800 pounds) fuselage 2;800. Passengers arrived and flight departed ZZZ normal uneventful. ATC held us down at 3;000 ft. Step climb to 6;000 ft. Radar vectors over ZZZ1. After passing ZZZ1; ATC gave us climb to 14;000 ft. MSL. Out of 10;000 ft. A right red fuel pressure light illuminated. All other indication normal. Engine climb power normal. Fuel balance normal. No other warnings. Reference QRH followed all procedures. To monitor indications - if fuel pressure light does not go out; flight can be continued at altitude if engines are operating properly' we had no additional indications of a malfunction. Climbing up through 30;000 ft. To assigned final fl 400 we noticed a fuel imbalance . This was 20 minutes into the flight. The left wing quantities 1;000 points right wing indication went to 800 pounds. While trying to transfer and balance fuel; we noticed the fuel rate decreases. Crew agreed that the cause of imbalance and red light unknown we should divert to closest airport as a safety precaution. Request ATC to divert due to unknown fuel imbalance. Stopped climb at 390 received descent clearance to fl 360. Started descent. Power idle. Pilot flying hand flew the aircraft and found imbalance or need for trim. Pilot flying duties took over ATC communications while pilot not flying notification of passenger that we had to divert. Continue to troubleshoot fuel loss and reviewed QRH again looking for guidance. Pilot not flying suggests ZZZ2 [as] it was straight ahead 60 miles with long runway. We had a lot of altitude to lose. ATC agreed and assigned heading southbound. Pilot not flying loaded ZZZ2 into FMS. Monitoring fuel balance problem. ATC gave clearance to 3;000 ft. As we approached 17 miles away ATC gave us level off at 14;000 ft. And vectors away from the airport. With the rate of fuel loss increasing and balance not maintained crew agreed it was time to [request priority handling and] did not want to turn way from the airport. ATC acknowledging [request] and gave us descent clearance again and turn back to the airport. Fuel imbalance getting worse. Pilot not flying could not maintain 500 pounds in right wing. Fuselage gauges display over 2;000 pounds. The fuel panel was displaying a fuselage empty light. With transfer we could push fuel to both or wing tanks. The confusion and problem could not be identified. Priority was to keep fuel in the left wing. Get on the ground as soon as possible. ATC tower ZZZ2 transmitted smoke coming from the back of the aircraft in the flare. Made normal landing with left wing was 1;000 pounds quantity; fuselage empty; right wing quantities 20 pounds. Taxing in on one engine. The right engine shut down as a precaution. In summary 5;600 start fuel; 1;241 fuel burn; 1;050 fuel remaining 3;310 fuel loss in flight. Fuel quantities gauge 1;050 left wing; empty fuselage; empty right wing. Cause for leak unknown. Reported to flight control and maintenance at ZZZ3. Landed the airplane with normal trim. Crew confirmed loss of fuel upon landing and opening the door seeing the fuel leak. Total flight duration 41 minutes. First problem revealed 15 minutes into the flight. Diverting from 39;000 ft. To touch down when fuel imbalance discovered was less than 25 minutes from start to finish.

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Original NASA ASRS Text

Title: Learjet Crew reported that a fuel imbalance resulted in a diversion.

Narrative: Supervised refuel at [FBO] support at ZZZ with an upload of 400 gallons. Total fuel load onboard 5;600 pounds; each wing had 1;400 pounds each; (2;800 pounds) fuselage 2;800. Passengers arrived and flight departed ZZZ normal uneventful. ATC held us down at 3;000 ft. step climb to 6;000 ft. radar vectors over ZZZ1. After passing ZZZ1; ATC gave us climb to 14;000 ft. MSL. Out of 10;000 ft. a Right Red Fuel Pressure light illuminated. All other indication normal. Engine climb power normal. Fuel balance normal. No other warnings. Reference QRH followed ALL procedures. TO MONITOR INDICATIONS - IF FUEL PRESSURE LIGHT DOES NOT GO OUT; FLIGHT CAN BE CONTINUED AT ALTITUDE IF ENGINES ARE OPERATING PROPERLY' We had no additional indications of a malfunction. Climbing up through 30;000 ft. to assigned final FL 400 we noticed a fuel imbalance . This was 20 minutes into the flight. The left wing quantities 1;000 points right wing indication went to 800 pounds. While trying to transfer and balance fuel; we noticed the fuel rate decreases. Crew agreed that the cause of imbalance and Red light unknown we should divert to closest airport as a safety precaution. REQUEST ATC TO DIVERT DUE TO UNKNOWN FUEL IMBALANCE. Stopped climb at 390 received descent clearance to FL 360. Started descent. Power idle. Pilot flying hand flew the aircraft and found imbalance or need for trim. Pilot flying duties took over ATC communications while pilot not flying notification of passenger that we had to divert. Continue to troubleshoot fuel loss and reviewed QRH again looking for guidance. Pilot Not flying suggests ZZZ2 [as] it was straight ahead 60 miles with long runway. We had a lot of altitude to lose. ATC agreed and assigned heading southbound. Pilot not flying loaded ZZZ2 into FMS. Monitoring fuel balance problem. ATC gave clearance to 3;000 ft. As we approached 17 miles away ATC gave us level off at 14;000 ft. and vectors away from the airport. With the rate of fuel loss increasing and balance not maintained crew agreed it was time to [request priority handling and] did not want to turn way from the airport. ATC acknowledging [request] and gave us descent clearance again and turn back to the airport. Fuel imbalance getting worse. Pilot not flying could not maintain 500 pounds in right wing. Fuselage gauges display over 2;000 pounds. The fuel panel was displaying a fuselage EMPTY light. With transfer we could push fuel to both or wing tanks. The confusion and problem could not be identified. Priority was to keep fuel in the left wing. Get on the ground as soon as possible. ATC Tower ZZZ2 transmitted smoke coming from the back of the aircraft in the flare. Made normal landing with Left wing was 1;000 pounds quantity; fuselage EMPTY; right wing quantities 20 pounds. Taxing in on one engine. The right engine shut down as a precaution. In summary 5;600 start fuel; 1;241 Fuel Burn; 1;050 Fuel Remaining 3;310 Fuel loss in flight. Fuel quantities gauge 1;050 Left Wing; EMPTY fuselage; EMPTY Right wing. Cause for leak unknown. Reported to flight control and maintenance at ZZZ3. Landed the airplane with normal trim. Crew confirmed loss of fuel upon landing and opening the door seeing the fuel leak. Total flight duration 41 minutes. First problem revealed 15 minutes into the flight. Diverting from 39;000 ft. to touch down when fuel imbalance discovered was less than 25 minutes from start to finish.

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.