Roswell, New Mexico
April 2,2011
NTSB Number: AAR-12-02
NTIS Number: PB2012-910402
Adopted: October 10,2012
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On April 2, 2011, about 0934 mountain daylight time, an experimental Gulfstream Aerospace Corporation GVI (G650), N652GD, crashed during takeoff from runway 21 at Roswell International Air Center, Roswell, New Mexico. The two pilots and the two flight test engineers were fatally injured, and the airplane was substantially damaged by impact forces and a postcrash fire. The airplane was registered to and operated by Gulfstream as part of its G650 flight test program. The flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time of the accident.
The accident occurred during a planned one-engine-inoperative (OEI) takeoff when a stall on the right outboard wing produced a rolling moment that the flight crew was not able to control, which led to the right wingtip contacting the runway and the airplane departing the runway from the right side. After departing the runway, the airplane impacted a concrete structure and an airport weather station, resulting in extensive structural damage and a postcrash fire that completely consumed the fuselage and cabin interior.
The National Transportation Safety Board's (NTSB) investigation of this accident found that the airplane stalled while lifting off the ground. As a result, the NTSB examined the role of "ground effect" on the airplane's performance. Ground effect refers to changes in the airflow over the airplane resulting from the proximity of the airplane to the ground. Ground effect results in increased lift and reduced drag at a given angle of attack (AOA) as well as a reduction in the stall AOA. In preparing for the G650 field performance flight tests, Gulfstream considered ground effect when predicting the airplane's takeoff performance capability but overestimated the in ground effect stall AOA. Consequently, the airplane's AOA threshold for stick shaker (stall warning) activation and the corresponding pitch limit indicator (on the primary flight display) were set too high, and the flight crew received no tactile or visual warning before the actual stall occurred.
The accident flight was the third time that a right outboard wing stall occurred during G650 flight testing. Gulfstream did not determine (until after the accident) that the cause of two previous uncommanded roll events was a stall of the right outboard wing at a lower than expected AOA. (Similar to the accident circumstances, the two previous events occurred during liftoff; however, the right wingtip did not contact the runway during either of these events.) If Gulfstream had performed an in-depth aerodynamic analysis of these events shortly after they occurred, the company could have recognized before the accident that the actual in-ground-effect stall AOA was lower than predicted.
During field performance testing before the accident, the G650 consistently exceeded target takeoff safety speeds (V2). V2 is the speed that an airplane attains at or before a height above the ground of 35 feet with one engine inoperative. Gulfstream needed to resolve these V2 exceedances because achieving the planned V2 speeds was necessary to maintain the airplane's 6,000-foot takeoff performance guarantee (at standard sea level conditions). If the G650 did not meet this takeoff performance guarantee, then the airplane could only operate on longer runways. However, a key assumption that Gulfstream used to develop takeoff speeds was flawed and resulted in V2 speeds that were too low and takeoff distances that were longer than anticipated.
Rather than determining the root cause for the V2 exceedance problem, Gulfstream attempted to reduce the V2 speeds and the takeoff distances by modifying the piloting technique used to rotate the airplane for takeoff. Further, Gulfstream did not validate the speeds using a simulation or physics-based dynamic analysis before or during field performance testing. If the company had done so, then it could have recognized that the target V2 speeds could not be achieved even with the modified piloting technique. In addition, the difficulties in achieving the target V2 speeds were exacerbated in late March 2011 when the company reduced the target pitch angle for some takeoff tests without an accompanying increase in the takeoff speeds.
Gulfstream maintained an aggressive schedule for the G650 flight test program so that the company could obtain Federal Aviation Administration (FAA) type certification by the third quarter of 2011. The schedule pressure, combined with inadequately developed organizational processes for technical oversight and safety management, led to a strong focus on keeping the program moving and a reluctance to challenge key assumptions and highlight anomalous airplane behavior during tests that could slow the pace of the program. These factors likely contributed to key errors, including the development of unachievable takeoff speeds, as well as the superficial review of the two previous uncommanded roll events, which allowed the company's overestimation of the in-ground-effect stall AOA to remain undetected.
After the accident, Gulfstream suspended field performance testing through December 2011 while the company examined the circumstances of the accident. In March 2012, Gulfstream reported that company field performance testing had been repeated and completed successfully. In June 2012, the company reported that FAA certification field performance testing had been successfully completed. Gulfstream obtained FAA type certification for the G650 on September 7, 2012.
The NTSB determines that the probable cause of this accident was an aerodynamic stall and subsequent uncommanded roll during an OEI 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 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 AOA 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.
In its party submission for this accident investigation, Gulfstream stated that it accepted "full responsibility" for the accident and, in response, implemented corrective actions to preclude such an accident from recurring. One of these actions was to integrate safety management system principles and practices into the company's flight test operations. As a result of this investigation, the NTSB is issuing two recommendations to Gulfstream to commission an audit to evaluate the status of the company's safety management program before the start of its next major certification program and share lessons learned with aircraft manufacturers and flight test industry groups.
Additional actions to help improve the management and safety of flight test programs include providing aircraft manufacturers with flight test operating guidance and flight test safety guidelines based on best practices in aviation safety management. The NTSB is issuing two safety recommendations to the Flight Test Safety Committee (an independent flight test safety organization) and two recommendations to the FAA regarding the development of this guidance. The NTSB is also issuing one recommendation to the FAA to incorporate the flight test safety guidelines in an agency document.
In addition, the NTSB is issuing three other recommendations as a result of its investigation of this accident. One of these recommendations, addressed to the FAA, discusses the potential for domestic and foreign airplane manufacturers to overestimate an airplane's stall AOA in ground effect. The other two recommendations, addressed to the FAA and the Flight Test Safety Committee, discusses advance coordination of high risk flight tests among manufacturers, airport operators, and aircraft rescue and firefighting personnel.
As a result of this investigation, the National Transportation Safety Board makes 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)
To Gulfstream Aerospace Corporation:
Commission an audit by qualified independent safety experts, before the start of the next major certification flight test program, to evaluate the company's flight test safety management system, with special attention given to the areas of weakness identified in this report, and address all areas of concern identified by the audit. (A-12-62)
Provide information about the lessons learned from the implementation of its flight test safety management system to interested manufacturers, flight test industry groups, and other appropriate parties. (A-12-63)
