Date: 201307
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: MD-83
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
ATC / advisory: Center ZZZ
Aircraft component: Fuel Crossfeed
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Human factors: Human-Machine Interface; Troubleshooting; Time Pressure; Confusion
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Weight And Balance; Inflight Event / Encounter Fuel Issue
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Declared Emergency; General Release Refused / Aircraft Not Accepted
Contributing factors: Procedure; Aircraft
Primary problem: Aircraft

Narrative: [It was] a normal flight with myself as pilot flying. I remember making a mental note of the fuel gauges during cruise; specifically remembering left and right tanks were in balance. Some time later; we both noticed a fuel imbalance: left tank roughly 3;100 LBS; right 9;000 LBS and center 12;100 LBS. We referenced our QRH's and attempted to stop what appeared to be a more greater than normal flow from the left tank. It did not fix the issue. Hence; we declared an emergency and I remained the pilot flying as we turned back to an airport we had just passed and time permitting continued to work on the fuel issue as well as communicate with ATC; OPS; etc. It was an overweight landing and uneventful in VMC conditions on Runway 31. I did notice that upon landing the left tank was roughly 1;500 LBS; but by the time we got to the gate it was about 1;200 LBS. Why the rapid decrease even still on the ground? I don't know.My only goal from noticing till landing was getting the aircraft safely on the ground. Upon arrival at the gate; things moved very quickly. There were the issues of maintenance log write-ups; crew duty days; the passenger service agents wanting answers; potential aircraft swaps; etc. The decision was made to switch aircraft and we continued uneventfully on a new aircraft.Aircraft are machines subject to potential malfunction and we pilots who operate them are humans subject to potential human error. I have reviewed the most current electronic maintenance log write-ups and there appears to be some issues regarding this aircraft's fuel system and unwarranted fuel transfers. Moreover; a more quick or accurate log of the fuel numbers may have allowed an earlier decision point with which to deal with the unbalance. However; I am not certain though that a more accurate fuel log would have avoided the more normal than usual left tank decrease or divert. Either way; as a pilot who plans on remaining on the fleet; I have learned a valuable lesson to monitor more closely the fuel system like a hawk; to be cognizant of the age of the airplanes and to avoid what seems to be a problem with some of these aircrafts fuel issues that even other pilots have encountered!


Second narrative: Needless to say things got hectic at the gate; I was talking with the Station Manager; Dispatch; a Mechanic and Maintenance Control almost simultaneously. Dispatch informed me my First Officer was to go illegal within 50 minutes. We ended up continuing to our filed destination in a different aircraft. I was concentrating on fuel total and missed main tank difference. There were some minor distractions; weather etc. Also; QRH stresses overweight landing more than imbalance. After landing I was told by Maintenance Control that no inspection is needed on an MD-83.


Synopsis: A MD-83 fuel system developed a large fuel imbalance in flight; so an emergency was declared and the flight diverted to a nearby airport where it was removed from service.