The National Transportation Safety Board determines the probable cause(s) of this accident as follows: an aerodynamic stall and subsequent uncommanded roll during a one engine-inoperative takeoff flight test, which were the result of (1) Gulfstream’s failure to properly develop and validate takeoff speeds for the flight tests and recognize and correct the takeoff safety speed (V2) error during previous G650 flight tests, (2) the G650 flight test team’s persistent and increasingly aggressive attempts to achieve V2 speeds that were erroneously low, and (3) Gulfstream’s inadequate investigation of previous G650 uncommanded roll events, which indicated that the company’s estimated stall angle of attack while the airplane was in ground effect was too high. Contributing to the accident was Gulfstream’s failure to effectively manage the G650 flight test program by pursuing an aggressive program schedule without ensuring that the roles and responsibilities of team members had been appropriately defined and implemented, engineering processes had received sufficient technical planning and oversight, potential hazards had been fully identified, and appropriate risk controls had been implemented and were functioning as intended.