Date: 201512
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Climb
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Ground Personnel; Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Flying a [widebody cargo Airbus] the aircraft had just had a leak in the center fuel tank fixed the night before. The aircraft preflight was normal and the logbook was signed off and there were no open items or MELs. There was also no maintenance being performed during the preflight and all the doors; panels and gear doors were closed. The takeoff [and] climb was normal. On departure ATC asked if we had hit a bird as Tower had said it looked like we had. We had not seen any birds on takeoff nor heard any impacts. We then discussed the need to write it up so maintenance could inspect the aircraft for damage. Sometime just after reaching cruise dispatch sent us an ACARS message stating it was not a bird strike; but a panel had fallen off the aircraft. We had no unusual indications; noise or vibrations so we decided it was okay to continue. Part of this decision was we were thinking of panel door like the fueling etc. about 12x12 inches. We did look at the external doors on the Configuration Deviation List (CDL) list for potential performance implications and asked Dispatch to confirm the panel was in fact from an Airbus and what panel it was. A short time later they confirmed it was an airbus panel from the left side. I asked dispatch to further identify the panel if possible. Then approximately 80 miles north of airport ZZZ dispatch sent us a message stating '[Maintenance] thinks you should return to [departure airport] due to the possibility of fire. This panel has ductwork as part of it. The concern is the vapor barrier is compromised.'This note was confusing as I personally think it should have stated 'possibility of fire land ASAP at nearest suitable'. We were approximately 120 miles north of ZZZ and our turn would take us directly over ZZZ so we requested a turn towards [departure airport]. I then asked Dispatch for the threat level of a fire and we talked via a phone patch by radio; but dispatch was unable to include [maintenance] to the conversation. After that brief conversation I asked dispatch to have a conference call with the Duty Officer; [maintenance] and Dispatch to find the exact threat level of a fire or worse an explosion because of the vapor statement. We had already gotten and briefed the ZZZ weather and were thinking the safest course of action would be diverting into ZZZ. Approximately 80 miles north of ZZZ we received a ACARS that the Duty Officer was calling the Tech Pilot and we decided we did not want to overfly ZZZ waiting on more info and started a divert into ZZZ. We informed dispatch via ACARS and ATC of our request. We got prepared to fly the approach into ZZZ. Since this was an aircraft being ferried part 91 there were no procedures in the FMS and we had to shoot a raw data approach. We landed uneventfully at ZZZ and Maintenance met us at the gate. After completing the shutdown checklist and crew debrief; I went outside to see what panel was missing. To my surprise the panel was not a small access panel; but an approximately 6x3 foot panel. This panel is not visible to the crew during the preflight. This information should have been passed on to the crew!The panel departed the aircraft when we raised the gear and the gear doors opened and the panel was sucked out.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Widebody Airbus cargo aircraft flight crew reported diverting to an alternate after being advised they had lost a large panel in the left wheel well area.