NTSB Says Aggressive Test Flight Schedule, Overlooked Errors Led to Stall and Crash

Oct. 10, 2012
The National Transportation Safety Board determined today that the probable cause of the crash of an experimental Gulfstream G650 on April 2, 2011, in Roswell, N.M., was the result of an aerodynamic stall and uncommanded roll during a planned takeoff test flight conducted with only one of the airplane’s two engines operating.
The Board found that the crash was the result of Gulfstream’s failure to properly develop and validate takeoff speeds and recognize and correct errors in the takeoff safety speed that manifested during previous G650 flight tests; the flight test team’s persistent and aggressive attempts to achieve a takeoff speed that was erroneously low; and Gulfstream’s inadequate investigation of uncommanded roll events that occurred during previous flight tests, which should have revealed incorrect assumptions about the airplane’s stall angle of attack in ground effect.

Contributing to the accident, the NTSB found, was Gulfstream’s pursuit of an aggressive flight test schedule without ensuring that the roles and responsibilities of team members were appropriately defined, sufficient technical planning and oversight was performed, and that hazards had been fully identified and addressed with appropriate, effective risk controls.

“In this investigation we saw an aggressive test flight schedule and pressure to get the aircraft certified,” said NTSB Chairman Deborah A.P. Hersman. “Deadlines are essential motivators, but safety must always trump schedule.”

At approximately 9:34 a.m. Mountain Time, during takeoff on the accident flight, the G-650 experienced a right wing stall, causing the airplane to roll to the right with the right wingtip contacting the runway. The airplane then departed the runway, impacting a concrete structure and an airport weather station, resulting in extensive structural damage and a post-crash fire. The two pilots and two flight engineers on board were fatally injured and the airplane was substantially damaged.

The NTSB made recommendations to the Flight Test Safety Committee and the Federal Aviation Administration to improve flight test operating policies and encourage manufacturers to follow best practices and to coordinate high-risk flight tests. And the Board recommended that Gulfstream Aerospace Corporation commission an independent safety audit to review the company’s progress in implementing a flight test safety management system and provide information about the lessons learned from its implementation to interested manufacturers, flight test safety groups and other appropriate parties.

“In all areas of aircraft manufacturing, and particularly in flight testing, where the risks are greater, leadership must require processes that are complete, clear and include well-defined criteria,” said Chairman Deborah A.P. Hersman. “This crash was as much an absence of leadership as it was of lift.”

The preliminary synopsis of the report is below:

NATIONAL TRANSPORTATION SAFETY BOARD
Public Meeting of October 10, 2012
(Information subject to editing)
Aircraft Accident Report:
Crash During Experimental Test Flight
Gulfstream Aerospace Corporation GVI (G650), N652GD
Roswell, New Mexico
April 2, 2011

NTSB/AAR-12/02

This is a synopsis from the National Transportation Safety Board’s report and does not include the NTSB’s rationale for the conclusions, probable cause, and safety recommendations. Safety Board staff is currently making final revisions to the report from which the attached conclusions and safety recommendations have been extracted. The final report and pertinent safety recommendation letters will be distributed to recommendation recipients as soon as possible. The attached information is subject to further review and editing.

Executive Summary

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 Airport, 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.

Conclusions

1. The test team’s focus on achieving the takeoff safety speeds for the flight tests and the lack of guidance specifying precisely when the pitch angle target and pitch limit applied during the test maneuver contributed to the team’s decision to exceed the initial pitch target and the pitch angle at which a takeoff test was to be discontinued.

2. A stall on the right outboard wing produced a right 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.

3. Given the airplane’s low altitude, the time-critical nature of the situation, and the ambiguous stall cues presented in the cockpit, the flight crew’s response to the stall event was understandable.

4. The impact forces from the accident were survivable, but the cabin environment deteriorated quickly and became unsurvivable because of the large amount of fuel, fuel vapor, smoke, and fire entering the cabin through the breaches in the fuselage.

5. The airplane stalled at an angle of attack (AOA) that was below the in ground effect stall AOA predicted by Gulfstream and the AOA threshold for the activation of the stick shaker stall warning.

6. If Gulfstream had performed an in-depth aerodynamic analysis of the cause of two previous G650 uncommanded roll events, similar to the analyses performed for roll events during previous company airplane programs, the company could have recognized that the actual in-ground-effect stall angle of attack for the accident flight test was significantly lower than the company predicted.

7. Gulfstream’s decision to use a takeoff speed development method from a previous airplane program was inappropriate and resulted in target takeoff safety speed values that were too low to be achieved.

8. By not performing a rigorous analysis of the root cause for the ongoing difficulties in achieving the G650 takeoff safety speeds (V2), Gulfstream missed an opportunity to recognize and correct the low target V2 speeds.

9. Before the accident flight, Gulfstream had sufficient information from previous flight tests to determine that the target takeoff safety speeds (V2) could not be achieved with a certifiable takeoff rotation technique and that the V2 speeds needed to be increased.

10. Deficiencies in Gulfstream’s technical planning and oversight contributed to the incorrect speeds used on the day of the accident.

11. Because Gulfstream did not clearly define the roles and responsibilities for on site test team members, critical safety-related parameters were not being adequately monitored and test results were not being sufficiently examined during flight testing on the day of the accident.

12. Gulfstream’s focus on meeting the G650’s planned certification date caused schedule related pressure that was not adequately counterbalanced by robust organizational processes to prevent, identify, and correct the company’s key engineering and oversight errors.

13. Gulfstream’s flight test safety program at the time of the accident was deficient because risk controls were insufficient and safety assurance activities were lacking.

14. The inherent risks associated with field performance flight testing, and minimum unstick speed testing in particular, could be reduced if airplane manufacturers considered the potential for a lower maximum lift coefficient in ground effect when estimating the stall angle of attack in ground effect.

15. Effective flight test standard operating policies and procedures that are fully implemented by manufacturers would help reduce the inherent risks associated with flight testing.

16. Flight test safety management system guidance specifically tailored to the needs of manufacturers would help promote the development of effective flight test safety programs.

17. External safety audits would help Gulfstream monitor the implementation of safety management principles and practices into its flight test operations and sustain long-term cultural change.

18. Flight test safety would be enhanced if manufacturers and flight test industry groups had knowledge of the lessons learned from Gulfstream’s implementation of its flight test safety management system.

19. Advance coordination between flight test operators and airport operations and aircraft rescue and firefighting personnel for high-risk flight tests could reduce the response time to an accident site in the event of an emergency.

Probable Cause

The National Transportation Safety Board determines that the 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.

Recommendations

To the Federal Aviation Administration:

1. 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.

2. 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.

3. Work with the Flight Test Safety Committee to develop and issue flight test safety program guidelines based on best practices in aviation safety management.

4. 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.

5. 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 adequate aircraft rescue and firefighting resources are available to provide increased readiness during known high risk flight tests.

To the Flight Test Safety Committee:

6. 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.

7. 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.

8. Encourage members to provide notice of and coordinate high-risk flight tests with airport operations and aircraft rescue and firefighting personnel.

To Gulfstream Aerospace Corporation:

9. 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.

10. 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.

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