Date: 201507
Local time of day: 1201-1800
Location: BFI.Airport, WA
Flight conditions: VMC
Light: Daylight
Aircraft: HS 125 Series
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Flight phase: Takeoff / Launch
Airspace: Class D BFI
ATC / advisory: Tower BFI
Reporter organization: Fractional
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Weight And Balance; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We arrived at the aircraft and noted in the logbook there had been maintenance done on the fuel tanks for a fuel indication issue. I looked at the fuel quantity and noticed the right side (which had been the problem tank) indicated more fuel than the left. I remember it showing about 1;600 in the left tank and about 2;200 in the right. We needed 6;800 per release so I added 3;000 LBS of fuel. I closed the refuel valve on the right tank briefly to let the left tank fill to balance with the right. After fueling was done we were showing 7;000 on board; equally balanced. Before takeoff we had to once again transfer fuel to balance the tanks; but I don't remember which direction we had to transfer at that time. When we were cleared for takeoff we had about 3;500 per side. During the takeoff roll I noticed the aircraft pulling to the left when I transferred controls to the SIC. He rotated for takeoff and noticed the plane banking left like the fuel as out of balance. We then both noticed the right tank quantity had dropped to about 3;200 LBS and was fluctuating. He then re-trimmed the plane to compensate for the perceived imbalance. As we continued the departure both tanks indicated as balanced but it was still flying like it was out of balance. We elected to level at 21;000 feet to troubleshoot. I transferred fuel from the left to the right even though it indicated an imbalance. This seemed to help the control imbalance but it was now indicating an opposite imbalance. I attempted to contact the company on multiple iPhones but could not get the Wi-Fi to work. Eventually it connected and I called the Assistant Chief Pilot (ACP) to explain our issue. At this time I was unsure how much fuel we had in the right tank for sure; and thought we probably had less than indicated. I informed the ACP we would probably need to stop short of our destination and thought maybe airport ZZZ would work and I would call him back. We made some more calculations with the fuel and decided we didn't want to go all the way to ZZZ either. We were thinking of ZZZ1 but when I called the ACP again he asked if we could go to ZZZ2. We decided that would work as well and climbed to 28;000 feet and got a new release and clearance to ZZZ2. As the flight continued the right tank quantity seemed to correct itself and decreased closer to the same amount as the left. We landed at ZZZ2 with about 2;200 LBS indicating in the tanks and with our calculated fuel burn it should have been closer to 3;000 LBS remaining. If the landing fuel was correct we were about 700 LBS less than we should have been. Had we continued to [our destination] I estimate we would have landed with about 1;300 remaining (our min reserve fuel) so I feel we made the right choice to divert not knowing the exact quantity of fuel in our right tank.I should have been more concerned with the existing imbalance of the fuel when I arrived at the aircraft. I dismissed it because maintenance had been working on the fuel system and signed it off as released. Also; if I were to encounter this again I would request topping off the wings so I can personally verify the accuracy of the fuel load following maintenance. I do not know the procedure the mechanic used to confirm the amount was accurate so I would like to verify it myself from now on.


Second narrative: [Report Narrative Contained No Additional Information.]


Synopsis: HS-125-800XPC flight crew reported fuel quantity and imbalance issues in flight that caused them to divert.