​We determined that the probable cause of this ​accident was 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.
   

​​As a result of this investigation, we made the following recommendations: 

To the Federal Aviation Administration: 

• Inform domestic and foreign manufacturers of airplanes that are certified under 14 Code of Federal Regulations Parts 23 and 25 about the circumstances of this accident and advise them to consider, when estimating an airplane‟s stall angle of attack in ground effect, the possibility that the airplane‟s maximum lift coefficient in ground effect could be lower than its maximum lift coefficient in free air. (A-12-54) 
• Work with the Flight Test Safety Committee to develop and issue detailed flight test operating guidance for manufacturers that addresses the deficiencies documented in this report regarding flight test operating policies and procedures and their implementation. (A-12-55) 
• Work with the Flight Test Safety Committee to develop and issue flight test safety program guidelines based on best practices in aviation safety management. (A-12-56) 
• After the Flight Test Safety Committee has issued flight test safety program guidelines, include these guidelines in the next revision of Federal Aviation Administration Order 4040.26, Aircraft Certification Service Flight Test Risk Management Program. (A-12-57) 
• Inform 14 Code of Federal Regulations Part 139 airports that currently have (or may have in the future) flight test activity of the importance of advance coordination of high-risk flight tests with flight test operators to ensure that adequate aircraft rescue and firefighting resources are available to provide increased readiness during known high-risk flight tests. (A-12-58)​
  

To the Flight Test Safety Committee:
• ​ ​​In collaboration with the Federal Aviation Administration, develop and issue flight test operating guidance for manufacturers that addresses the deficiencies documented in this report regarding flight test operating policies and procedures and their implementation, and encourage manufacturers to conduct flight test operations in accordance with the guidance. (A-12-59) 
• In collaboration with the Federal Aviation Administration, develop and issue flight test safety program guidelines based on best practices in aviation safety management, and encourage manufacturers to incorporate these guidelines into their flight test safety programs. (A-12-60)
• Encourage members to provide notice of and coordinate high-risk flight tests with airport operations and aircraft rescue and firefighting personnel. (A-12-61) ​