Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 73; Flight Crew Total 4644; Flight Crew Type 4644
Communication Breakdown
Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Aircraft; Human Factors
During Exterior Preflight inspection it was noted that the Number 2 engine was actively leaking fuel.  Maintenance was consulted and it was determined continued operation was possible providing a successful engine run was completed and the fuel leak had stopped.  Maintenance Control was contacted and approval for an engine run was approved.  I briefed the ramp and maintenance personnel on the logistics of the gate engine run.  The engine run was completed at the gate.  Shortly afterwards we received verification from the Maintenance Technician that we were good as the leak had stopped.  We were informed that the maintenance release would be processed shortly.  Two minutes later we noted on the iPad that the deferral mentioned by the Technician was processed.  As the Number 2 engine was running at the gate and I briefed that we would push as soon as Maintenance said we were good we released the brake and commenced the push.  In-flight we received a message to contact the Dispatch.  After landing a Manager mentioned we may have departed without a maintenance release.  We retained the same plane for the subsequent flight with a valid maintenance release with no maintenance action.
Pre-flight planning completed and aircraft had valid maintenance release.  Fuel leak found on Number 2 engine during walk around.  Local maintenance consulted.  Technician told us engine can be started and run at the gate and if fuel leak stopped; issue would be put on deferral; and we could just push out and continue.  Received authorization from Maintenance Control for engine run and all personnel were briefed on procedure.  Number 2 engine was started and run at the gate with Technician on the headset. Technician told us that fuel leak stopped after startup so paperwork would be completed immediately; deferral issued and we were good to go.  Flight pushed back very soon after as briefed; taxied out and took off.  Message was received in flight to contact Dispatch management upon landing ZZZ1.  Called and [was] told apparently flight departed without valid maintenance release.  We received a valid maintenance release and flew same aircraft to ZZZ2 without incident.
B737 flight crew reported they took off without proper maintenance release documentation.