Having performed myself dozens of airliner technical acceptance flights after scheduled heavy maintenance checks and unscheduled maintenance checks after incidents on the line, my sincerest condolences to the crew members, the base maintenance team and their relatives.
Airlines normally assign the technical pilot of the corresponding fleet to plan, execute and analyze technical acceptance flights together with the involved base maintenance team. Normally, the aircraft manufacturer supplies a dedicated acceptance flight program which is basically a streamlined flight program of the initial certification process. Yet, this is NOT a testflight program which certified test pilots of the aircraft manufacturer perform.
Technical acceptance flights in airlines are performed by the technical pilot, specially trained “normal” airline pilots and one acceptance flight engineer on the jump seat in the cockpit. It is normally the flight engineer who leads the program and who is responsible for the acceptance flight protocol. Close cooperation and information flow between the technical pilot and the base maintenance team during the maintenance visit of the aircraft and during the extensive pre-flight briefing is of utmost importance for a safe and successful acceptance flight.
The workload during those flights is extremely high because you do things, you should not do on line.
I am eager to learn what went wrong on this tragic flight.