Dhaka, May 12 -- FedEx has begun a series of test flights with its McDonnell Douglas MD-11 freighters as the delivery giant moves toward the aircraft's operational return following a fatal UPS crash in Louisville last year.

According to ADS-B data from a flight tracking platform, two FedEx MD-11Fs - N621FE and N521FE - have undertaken four test flights so far, with a partial fleet return expected by the end of May.

The first test flight saw N621FE depart Memphis International Airport on May 9, with the aircraft returning approximately an hour and 15 minutes later. A second flight on May 10 traveled from Memphis to Miami International Airport, lasting just under two hours.

Meanwhile, N521FE completed two flights on May 10 - one returnin...